Single isocenter stereotactic irradiation for multiple brain metastases: current situation and prospects

The prognosis of patients with brain metastases has dramatically improved, and long-term tumor control and reduction of the risk of late toxicities, including neurocognitive dysfunction, are important for patient quality of life. Stereotactic irradiation for multiple brain metastases, rather than whole-brain radiotherapy, can result in high local control rate with low incidence of neurocognitive deterioration and leukoencephalopathy. Recent advances in radiotherapy devices, treatment-planning systems, and image-guided radiotherapy can realize single isocenter stereotactic irradiation for multiple brain metastases (SI-STI-MBM), in which only one isocenter is sufficient to treat multiple brain metastases simultaneously. SI-STI-MBM has expanded the indications for linear accelerator-based stereotactic irradiation and considerably reduced patient burden. This review summarizes the background, methods, clinical outcomes, and specific consideration points of SI-STI-MBM. In addition, the prospects of SI-STI-MBM are addressed.

Collection and analysis of photon beam data for Varian C-series linear accelerators: a potential reference beam data set

This study aimed to collect and analyze photon beam data for the Varian C-series linear accelerators (Varian Medical Systems, Palo Alto, CA, USA). We evaluated the potential of the average data to be used as reference beam data for the radiotherapy treatment planning system commissioning verification. We collected 20 data sets for 4 and 6 MV photon beams, and 40 data sets for a 10 MV photon beam generated by the Varian C-series machines, which contained the percent depth dose (PDD), off-center ratio (OCR), and output factor (OPF) from 20 institutions. The average for each of the data types was calculated across the 20 machines. Dose differences from the average for PDD at the dose fall-off region were less than 1.0%. Relative differences from the average for the OPF data were almost within 1.0% for all energies and field sizes. For OCR data in the flat regions, the standard deviation of the dose differences from the average was within 1.0%, excluding that of the 30 × 30 mm² field size being approximately 1.5%. For all energies and field sizes, the distance to agreement from the average in the OCR penumbra regions was less than 1.0 mm. The average data except for the small field size found in this study can be used as reference beam data for verifying users' commissioning results.

Systematic Analysis of Clinical Outcomes Following Stereotactic Radiosurgery for Central Neurocytoma

Central neurocytoma (CN) typically presents as an intraventricular mass causing obstructive hydrocephalus. The first line of treatment is surgical resection with adjuvant conventional radiotherapy. Stereotactic radiosurgery (SRS) was proposed as an alternative therapy for CN because of its lower risk profile. The objective of this systematic analysis is to assess the efficacy of SRS for CN. A systematic analysis for CN treated with SRS was conducted in PubMed. Baseline patient characteristics and outcomes data were extracted. Heterogeneity and publication bias were also assessed. Univariate and multivariate linear regressions were used to test for correlations to the primary outcome: local control (LC). The estimated cumulative rate of LC was 92.2% (95% confidence interval: 86.5-95.7%, p<0.001). Mean follow-up time was 62.4 months (range 3-149 months). Heterogeneity and publication bias were insignificant. The univariate linear regression models for both mean tumor volume and mean dose were significantly correlated with improved LC (p<0.001). Our data suggests that SRS may be an effective and safe therapy for CN. However, the rarity of CN still limits the efficacy of a quantitative analysis. Future multi-institutional, randomized trials of CN patients should be considered to further elucidate this therapy.

Patient-related quality assurance with different combinations of treatment planning systems, techniques, and machines : A multi-institutional survey

PURPOSE

This project compares the different patient-related quality assurance systems for intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) techniques currently used in the central Germany area with an independent measuring system.

MATERIALS AND METHODS

The participating institutions generated 21 treatment plans with different combinations of treatment planning systems (TPS) and linear accelerators (LINAC) for the QUASIMODO (Quality ASsurance of Intensity MODulated radiation Oncology) patient model. The plans were exposed to the ArcCHECK measuring system (Sun Nuclear Corporation, Melbourne, FL, USA). The dose distributions were analyzed using the corresponding software and a point dose measured at the isocenter with an ionization chamber.

RESULTS

According to the generally used criteria of a 10 % threshold, 3 % difference, and 3 mm distance, the majority of plans investigated showed a gamma index exceeding 95 %. Only one plan did not fulfill the criteria and three of the plans did not comply with the commonly accepted tolerance level of ±3 % in point dose measurement.

CONCLUSION

Using only one of the two examined methods for patient-related quality assurance is not sufficiently significant in all cases.

Automatically gated image-guided breath-hold IMRT is a fast, precise, and dosimetrically robust treatment for lung cancer patients

BACKGROUND

High-dose radiotherapy of lung cancer is challenging. Tumors may move by up to 2 cm in craniocaudal and anteroposterior directions as a function of breathing cycle. Tumor displacement increases with treatment time, which consequentially increases the treatment uncertainty.

OBJECTIVE

This study analyzed whether automatically gated cone-beam-CT (CBCT)-controlled intensity modulated fast deep inspiration breath hold (DIBH) stereotactic body radiation therapy (SBRT) in flattening filter free (FFF) technique and normofractionated lung DIBH intensity-modulated radiotherapy (IMRT)/volumetric-modulated arc therapy (VMAT) treatments delivered with a flattening filter can be applied with sufficient accuracy within a clinically acceptable timeslot.

MATERIALS AND METHODS

Plans of 34 patients with lung tumors were analyzed. Of these patients, 17 received computer-controlled fast DIBH SBRT with a dose of 60 Gy (5 fractions of 12 Gy or 12 fractions of 5 Gy) in an FFF VMAT technique (FFF-SBRT) every other day and 17 received conventional VMAT with a flattening filter (conv-VMAT) and 2-Gy daily fractional doses (cumulative dose 50-70 Gy).

RESULTS

FFF-SBRT plans required more monitor units (MU) than conv-VMAT plans (2956.6 ± 885.3 MU for 12 Gy/fraction and 1148.7 ± 289.2 MU for 5 Gy/fraction vs. 608.4 ± 157.5 MU for 2 Gy/fraction). Total treatment and net beam-on times were shorter for FFF-SBRT plans than conv-VMAT plans (268.0 ± 74.4 s vs. 330.2 ± 93.6 s and 85.8 ± 25.3 s vs. 117.2 ± 29.6 s, respectively). Total slot time was 13.0 min for FFF-SBRT and 14.0 min for conv-VMAT. All modalities could be delivered accurately despite multiple beam-on/-off cycles and were robust against multiple interruptions.

CONCLUSION

Automatically gated CBCT-controlled fast DIBH SBRT in VMAT FFF technique and normofractionated lung DIBH VMAT can be applied with a low number of breath-holds in a short timeslot, with excellent dosimetric accuracy. In clinical routine, these approaches combine optimally reduced lung tissue irradiation with maximal delivery precision for patients with small and larger lung tumors.

Measurement and comparison of head scatter factor for 7 MV unflattened (FFF) and 6 MV flattened photon beam using indigenously designed columnar mini phantom

AIM

To measure and compare the head scatter factor for 7 MV unflattened and 6 MV flattened photon beam using a home-made designed mini phantom.

BACKGROUND

The head scatter factor (Sc) is one of the important parameters for MU calculation. There are multiple factors that influence the Sc values, like accelerator head, flattening filter, primary and secondary collimators.

MATERIALS AND METHODS

A columnar mini phantom was designed as recommended by AAPM Task Group 74 with high and low atomic number material for measurement of head scatter factors at 10 cm and d max dose water equivalent thickness.

RESULTS

The Sc values measured with high-Z are higher than the low-Z mini phantoms observed for both 6MV-FB and 7MV-UFB photon energies. Sc values of 7MV-UFB photon beams were smaller than those of the 6MV-FB photon beams (0.6-2.2% (Primus), 0.2-1.4% (Artiste) and 0.6-3.7% (Clinac iX (2300CD))) for field sizes ranging from 10 cm × 10 cm to 40 cm × 40 cm. The SSD had no influence on head scatter for both flattened and unflattened beams. The presence of wedge filters influences the Sc values. The collimator exchange effects showed that the opening of the upper jaw increases Sc irrespective of FF and FFF.

CONCLUSIONS

There were significant differences in Sc values measured for 6MV-FB and unflattened 7MV-UFB photon beams over the range of field sizes from 10 cm × 10 cm to 40 cm × 04 cm. Different results were obtained for measurements performed with low-Z and high-Z mini phantoms.

Poster - Thur Eve - 72: Conversion of helical tomotherapy plans into clinically favourable step-and-shoot IMRT plans deliverable on a c-arm linac

The treatment planning software SharePlan is designed to convert dose distributions generated by the TomoTherapy planning station into step-and-shoot IMRT plans deliverable on a c-arm linear accelerator. Five anal canal patients who were planned for TomoTherapy treatments were exported into a SharePlan system and plans were generated for delivery on an Elekta Synergy unit. A total of 80 plans were generated for those five patients, with either seven, nine, eleven or twenty-one gantry angles and different priorities between focusing on matching either the target doses or healthy tissue sparing of the TomoTherapy plan. The plans generated by SharePlan, while often not matching target coverage at prescription, matched well the TomoTherapy coverage at 95% and 105% of the prescription dose. Organ at risk dose, when heavily emphazied in the SharePlan calculations matched or bettered the TomoTherapy dose due to the placement of the beams and the sharper sup-inf fall off of the dose distribution on a linac. For one of the patients, it was possible to produce a better DVH with SharePlan than the original TomoTherapy plan for those reasons. The TomoTherapy plans boasted significantly shorter delivery times than the plans generated with SharePlan.

Poster - Thur Eve - 02: Regulatory oversight of the robotic radiosurgery facilities

Following a recent review of the Class II Nuclear Facilities and Prescribed Equipment Regulations and regulatory oversight of particle accelerators, the Canadian Nuclear Safety Commission (CNSC) has changed its policy concerning the regulation of particle accelerators. In November 2011, the CNSC began to exercise its regulatory authority with respect to all particle accelerators operating at a beam energy of 1 (one) MeV or greater. The CNSC already licences and inspects particle accelerators capable of operating at or above 10 MeV. The decision to now include low energy particle accelerators (i.e., those operating at or above 1 MeV) ensures adequate, uniform and consistent regulatory oversight for all Class II accelerators. The CNSC expects these facilities to comply with CNSC requirements by December 2013. Besides conventional linear accelerators of lower energy (6 MeV or below) typically found in cancer clinics, two types of equipment now fall under the CNSC's regulatory oversight as a result of the above change: robotic radiosurgery and tomotherapy equipment and facilities. A number of clinics in Canada already operates these types of equipment and facilities. The safety aspects of radiosurgery equipment differ slightly from those for conventional linear accelerators. This poster aims to present an approach taken by the CNSC to regulate robotic radiosurgery equipment and facilities. The presentation will explain how to meet regulatory requirements of the Class II Nuclear Facilities and Prescribed Equipment Regulations by licensees operating or planning to acquire these types of equipment and facilities.

Sci-Thur AM: Planning - 07: A fast and accurate source model for energy and intensity modulated electron beams

The purpose of this study is to develop a highly accurate and fast method for calculating electron beam dose distributions in Modulated Electron Radiation Therapy (MERT). An algorithm has been developed for creating phase-space files at the exit of a linear accelerator for any arbitrary intensity and energy electron beam without the need of full Monte Carlo simulations. The model assigns each particle to one of the 3 following sources: primary, secondary collimator and electron collimator scatter. The primary component is derived by fast MC transport in air. The scatter components are derived by the use of MC pre-calculated leaf kernels. Each kernel includes the fluence distribution, energy distribution and scatter probability of generating an electron from a leaf. The original position is sampled from tunable Gaussian or uniform distributions. The direction is estimated by geometrical means. According to the projection of the direction a particle is rejected if it is expected to suffer a leaf-hit. A leaf-hit counter is used to calculate the output of scatter particles based on the pre-calculated scatter probabilities. To account for multiple coulomb scattering in air a MC-corrected version of the Fermi-Eyges scattering theory was implemented. Depth and profile dose distributions were derived for the largest and smallest square field sizes, as well as for irregular and off-axis fields. The model agreed with full MC dose distributions within 3 % in all cases. Output at the depth of maximum dose exhibited discrepancies less than 2.6 % in all cases. The model was 16-22 times faster in generating a phase-space file than a full MC simulation with the BEAMnrc code.

Poster - Thur Eve - 17: Control point analysis comparison of three different treatment planning and delivery complexity levels using a commercial three dimensional diode array

PURPOSE

To use Control Point Analysis (Sun Nuclear Corporation, Melbourne, Florida, USA) to analyze and compare delivered VMAT plans for three different treatment planning complexity levels.

METHODS

Nineteen patients were chosen and fully anonymized for the purpose of this study. Ten SBRT, six H&N, one breast and two prostate VMAT plans were generated on Pinnacle³ and delivered on a Varian LINAC. The delivered dose was measured using ArcCHECK™. Each plan was analyzed using SNC Patient 6 and Control Point Analysis. Gamma passing percentage was used to assess the differences between the measured and planned dose distributions and to assess the role of various control point binning scenarios.

RESULTS

The prostate cases reported the highest gamma passing percentages for SNC Patient 6 (99.3%-99.5%,3%/3mm) and Control Point Analysis (99.1--99.3%,3%/3mm). The mean percentage of passing control point sectors for the prostate cases increased from 48.9±3.1% for individual control points to 69.5 ± 3.9% for 5 control points binned together to 100±0% for 10 control points binned together. Over all, there was a trend in the percentage of sectors passing gamma analysis increasing with the increase of the number of control points binned together in one sector for both passing criteria considered (48.9±3.1% for individual control points to 69.5±3.9% for 5 control points binned together in one sector to 100±0% for 10 control points binned together in one sector for the prostate).

CONCLUSION

The delivery accuracy per control point depends on the MU/control point (SBRT) and the plan degree of modulation (H&N).

Sci-Sat AM: Brachy - 04: Neutron production around a radiation therapy linac bunker - monte carlo simulations and physical measurements

Photoneutrons are a major component of the equivalent dose in the maze and near the door of linac bunkers. Physical measurements and Monte Carlo (MC) calculations of neutron dose are key for validating bunker design with respect to health regulations. We attempted to use bubble detectors and a ³ He neutron spectrometer to measure neutron equivalent dose and neutron spectra in the maze and near the door of one of our bunkers. We also ran MC simulations with MCNP5 to measure the neutron fluence in the same region. Using a point source of neutrons, a Clinac 1800 linac operating at 10 MV was simulated and the fluence measured at various locations of interest. We describe the challenges faced when measuring dose with bubble detectors in the maze and the complexity of photoneutron spectrometry with linacs operating in pulsed mode. Finally, we report on the development of a userfriendly GUI for shielding calculations based on the NCRP 151 formalism.

SU-E-T-150: The Basic Dosimetric Properties of NIPAM Polymer Gel Dosimeter

PURPOSE

To asses a series of basic dosimetric properties such as reproducibility, linearity, tissue equivalency, dose rate and energy independency for NIPAM polymer gel dosimeter.

METHODS

The NIPAM gel was manufactured according to the method, described by senden et al (2006). The gels were irradiated approximately 2 h after manufacturing and MR images of the gel were made 24 h after irradiation. Transverse relaxation rates (R2=T2?¹ ) were obtained from the signal decay data using the proper data analyzer. In order to investigate the absorbed dose response reproducibility, the experiment was repeated three times using the same batch of monomer, irradiation method, scanning parameters and conditions, also with analyzing two set of the gel with different batches of chemical the effect of different batches were investigated . For assessing if the NIPAM gel dosimeter response is dependent on different photon energies, two sets of NIPAM gel were irradiated using a 9 MV linear accelerator and a 60co. The effect of different dose rate on gel response was studied in SSD of 80, 90, 100, 110 and 120 and radiation beam were calibrated to give 5Gy in each SSD. To investigate the linearity of the gel, the vials were irradiated from 1 to 35 Gy. In order to verify tissue equivalency, effective atomic number and relative electron density of NIPAM dosimeter were calculated using CT number, and compared with tissue.

RESULTS

This polymer gel found to be tissue equivalent. The results showed that the dose response of NIPAM polymer gel is highly reproducible in same and different batches of chemical and its response was linear up to 26 Gy. Energy and dose rate had no effect on NIPAM gel response.

CONCLUSIONS

NIPAM gel dosimeter appears to be a promising dosimeter in all aspects of dosimetric properties which were assessed in this study.

SU-E-T-165: Protocol for Simplified Radiochromic Film Dosimetry

PURPOSE

Radiochromic film provides dose measurement at high spatial resolution, but often is not selected for routine evaluation of patient-specific IMRT plans owing to ease-of-use factors. We have developed a simplified protocol that avoids complications encountered in commonly used methods.

METHODS

We evaluated the simplified protocol by collecting dose-response data from six production lots of EBT3 film at doses up to 480 cGy. In this work, we used eight different scanners of two different models - Epson 10000XL and V700; post-exposure times before scanning from 30 minutes to 9 days; ambient temperatures for scanning spanning 23°F and two film orientations. Scanning was in 48-bit rgb format at 72 dpi resolution. Dose evaluation was conducted using a triple-channel dosimetry method. To validate the simplified protocol, patient specific IMRT QA was performed using a Varian Trilogy Linac to expose EBT3 films. Scanning and film analysis was done following the protocol.

RESULTS

The results indicated that the dose-response data could be fit by a set of related rational functions leading to the description of a universal calibration curve. A simplified protocol was established where dose-response data for a specific film lot, scanner, and scanning conditions could be derived from no more than two films exposed to known doses. In most cases only one calibrated exposure was required. Using the Gamma test criterion of 2%/2mm to evaluate the measurements, passing rates ranged between 95% and 99%.

CONCLUSIONS

We have demonstrated a simplified protocol to measure doses delivered by an IMRT treatment plan using only the patient film, one calibration film, one unexposed film, and applying a single scan to acquire a digital image for calculation and analysis. The simplification and time-savings provide a practical solution for using radiochromic film for routine IMRT QA without sacrificing spatial resolution for convenience. David Lewis, Andre Micke and Xiang Yu are all employed by Ashland Specialty Ingredients the manufacturer of GAFCHROMIC EBT3 radiochromic film that is the subject of the work presented in the Abstract.

TU-E-BRA-07: Post-Operative Eye Plaque Imaging Using Tomotherapy MVCT

PURPOSE

Intra-operative ultrasound is used to verify the positioning of episcleral eye plaques used to treat ocular melanoma. Ultrasound can be ambiguous because of image artifacts, and plaques may shift position after surgery. Ultrasound verification is particularly challenging for anterior tumors. Post-operative imaging could be used to trigger interventions that would prevent local treatment failure. We investigated if, and under what conditions, the Tomotherapy megavoltage computed tomography (MVCT) system could be used to perform post-implantation verification of eye plaque positioning.

METHODS

Plaques were placed on a preserved cow's eye, and imaged with the megavoltage CT of a Tomotherapy linear accelerator (Accuray, Sunnyvale, CA). The images were visually and quantitatively assessed to determine if they were of sufficient quality to verify tumor coverage and plaque tilt with respect to the sclera. We used the visibility of the lens as a proxy for visibility of a tumor. To test the utility of hypothetical higher beam current Tomotherapy images, we averaged sequential images of the same setup.

RESULTS

The plaque, the lens of the eye, and the globe are visible in the images. The CNR of the lens with respect to the vitreous was 5.6 for a single image. For 10 images averaged, the CNR was 9.2. Estimated dose from a single image was 1.3 cGy (body CTDIvol); even 10 times this dose would be an acceptable image-guidance dose for radiotherapy patients. One limitation of the imaging procedure is the long scan time (up to 240 seconds), during which time any significant patient motion would lead to image artifacts. Human trials on eye plaque patients are planned.

CONCLUSIONS

Tomotherapy MVCT imaging could be used to verify tumor coverage and plaque tilt after episcleral plaque implantation. Tumors should be visible in standard Tomotherapy images but higher beam current images would be preferred if available.

TU-E-BRB-06: Best in Physics (Therapy) - Development and Experimental Validation of EPID-Based 4D Dose Reconstruction

PURPOSE

To develop and validate an EPID-based 4D patient dose reconstruction framework accounting for linac delivery uncertainties, interfractional and intrafractional motions, and interplay effect.

METHODS

Patients with fiducial markers were scanned with 4D-CT for SBRT planning. Before treatment, in-room 4D-CT was performed. Both the MLC and the tumor movements were tracked by continuously acquiring EPID images during treatment. Instead of directly using the heterogeneous transit photon fluence measured by the EPID, this method reconstructed the incident beam fluence based on the MLC apertures measured by the EPID and the delivered MU recorded by the linac. To account for the time-dependent-geometry, the incident fluence distributions were sorted into their corresponding phases based on the tumor motion pattern detected by the EPID and accumulated as the incident fluence map for each phase. Together with 4D-CT, it was then used for Monte Carlo dose calculation. Deformable registration was performed to sum up the phase doses for treatment assessment. The feasibility of using the transit EPID images for incident fluence reconstruction was evaluated against EPID in-air measurements. The accuracy of 3D- and 4D-dose reconstruction was validated by a motordriven cylindrical diode array for six clinical SBRT plans.

RESULTS

The average difference between the measured and reconstructed fluence maps is within 0.16%. The reconstructed 3D-dose shows 1.4% agreement in the CAX-dose and >98.5% gamma-passing-rate (2%/2mm) in the peripheral-dose. A distorted dose distribution is observed in the measurement for the moving ArcCheck-phantom. The comparison between the measured and the reconstructed 4D-dose without considering interplay fails the gammaevaluation (59%-88.9% gamma-passing-rate). In contrast, when the interplay is considered, the dose distortion phenomena is successfully represented in the reconstructed dose (>97.6% gamma-passing-rate).

CONCLUSIONS

The experimental validation demonstrates that the proposed method provides a practical way to reconstruct the fractional 4D-doses received by the patient and enables adaptive SBRT strategy.

SU-E-T-130: IMAT Patient Specific Quality Assurance Using ArcCHECK Diode Array Detector

PURPOSE

To evaluate the IMAT patient specific quality assurance (QA) performed using ArcCHECK detector array in reference with standard ion chamber for routine clinical use.

METHODS

Twelve patient plans having different tumor sites chosen for this study. On Eclipse planning system,IMAT patient plans were calculated on ArcCHECK phantom inserted with Ion chamber using superposition algorithm. ArcCHECK is a cylindrical phantom with a three-dimensional array of 1386 diode detectors, arranged in a spiral pattern, with 10 mm diode spacing. These plans delivered from Clinac-iX linac equipped with 120 MLC. Point dose and Dose/fluence map were measured simultaneously with ion chamber (IC-15) and ArcCHECK diode array detector respectively. Point doses, dose/fluences map and dose at central axis (CAX) on ArcCHECK phantom were compared with their respective TPS calculated values.

RESULTS

The ion chamber measurements are in good agreement with TPS calculated doses. Mean difference between them is 0.50% with standard deviation is 0.51%. Concordance correlation coefficient (CCC) obtained for ion chamber base absolute dose measurements is 0.9996. These results demonstrate a strong correlation between the absolute dose predicted by our TPS and the measured dose. The precision of the TPS software was 0.9999, and its accuracy was 0.9997.The agreement between ArcCHECK doses and TPS predictions on the CAX, shown CCC of 0.9978 (the mean difference in the central axis dose is 2.11%). The 95% Confidence Interval is from 0.9932 to 0.9995. In gamma analysis of dose/fluence map the mean passing rate was 98.53% for 3% dose difference and 3mm distance to agreement.

CONCLUSIONS

The IMAT patient specific QA with Ion chamber and ArcCHECK phantom are consistent with the TPS calculated dose. Statistically good agreement observed between ArcCHECK measured and TPS calculated. Hence it can be used for routine IMAT QA.

SU-E-T-400: SBRT Dose Verification Using Monte Carlo Simulation

PURPOSE

To verify the SBRT plans on CMS Xio treatment planning system using the Monte Carlo simulation and investigate the related issues.

METHODS

The SBRT plans with 6 MV were created on CMS Xio treatment planning system with superposition algorithm. The same patient's CT, beam geometry and MUs were used in the Monte Carlo simulation (MC) on MCSIM. MCSIM is an EGS4-based MC dose calculation system for photon and electron beams. The Monte Carlo plans were compared with the Xio plans to verify Xio superposition algorithm for SBRT. The electron disequilibrium was particularly investigated by comparing the DVHs for a 2-mm thick peel of the GTV. The beam energy was changed from 6 MV to 10 MV for MC to test energy effect on SBRT dosimetry.

RESULTS

Six SBRT lung plans created on Xio and delivered on Varian 21 EX linac were included in this study. The tumor GTV ranged from 1.4 cc to 11 cc and the dose ranged from 1950 cGy to 5400 cGy. The comparisons were made in terms of DVHs, mean doses, minimal doses, and maximal doses for GTV. The results showed all the dose values of Xio plans agreed with MC to within 2% with only two exceptions of 3% and 5%. The dose distribution in the peel of GTV followed the same pattern as the whole GTV. This indicated the Xio superposition algorithm has well accounted for electron disequilibrium. The 10-MV beams had both hot and cold spots from DVH comparison. This may be due to the large build-up region for high energy beams.

CONCLUSIONS

The Xio superposition algorithm has adequately accounted for electron disequilibrium and can perform accurate dose calculation for SBRT. Compared to high energy beams, 6 MV is preferable in terms of the GTV coverage and dose homogeneity.

SU-E-T-207: Flatness and Symmetry Threshold Detection Using Statistical Process Control

PURPOSE

AAPM TG-142 guidelines state that beam uniformity (flatness and symmetry) should maintain a constancy of 1 % relative to baseline. The focus of this study is to determine if statistical process control (SPC) methodology using process control charts (PCC) of steering coil currents (SCC) can detect changes in beam uniformity prior to exceeding the 1% constancy criteria.

METHODS

SCCs for the transverse and radial planes are adjusted such that a reproducibly useful photon or electron beam is available. Transverse and radial - positioning and angle SCC are routinely documented in the Morning Check file during daily warm-up. The 6 MV beam values for our linac were analyzed using average and range (Xbar/R) PCC. Using this data as a baseline, an experiment was performed in which each SCC was changed from its mean value (steps of 0.01 or 0.02 Ampere) while holding the other SCC constant. The effect on beam uniformity was measured using a beam scanning system. These experimental SCC values were plotted in the PCC to determine if they would exceed the predetermined limits.

RESULTS

The change in SCC required to exceed the 1% constancy criteria was detected by the PCC for 3 out of the 4 steering coils. The reliability of the result in the one coil not detected (transverse position coil) is questionable because the SCC slowly drifted during the experiment (0.05 A) regardless of the servo control setting.

CONCLUSIONS

X-bar/R charts of SCC can detect exceptional variation prior to exceeding the beam uniformity criteria set forth in AAPM TG-142. The high level of PCC sensitivity to change may result in an alarm when in fact minimal change in beam uniformity has occurred. Further study is needed to determine if a combination of individual SCC alarms would reduce the false positive rate for beam uniformity intervention. This project was supoorted by a grant from Varian Medical Systems, Inc.

SU-E-T-454: Dosimetric Comparison between Pencil Beam and Monte Carlo Algorithms for SBRT Lung Treatment Using IPlan V4.1 TPS and CIRS Thorax Phantom

PURPOSE

To compare measured and calculated doses using Pencil Beam (PB) and Monte Carlo (MC) algorithm on a CIRS thorax phantom for SBRT lung treatments.

METHODS

A 6MV photon beam generated by a Primus linac with an Optifocus MLC (Siemens) was used. Dose calculation was done using iPlan v4.1.2 TPS (BrainLAB) by PB and MC (dose to water and dose to medium) algorithms. The commissioning of both algorithms was done reproducing experimental measurements in water. A CIRS thorax phantom was used to compare doses using a Farmer type ion chamber (PTW) and EDR2 radiographic films (KODAK). The ionization chamber, into a tissue equivalent insert, was placed in two position of lung tissue and was irradiated using three treatments plans. Axial dose distributions were measured for four treatments plans using conformal and IMRT technique. Dose distribution comparisons were done by dose profiles and gamma index (3%/3mm).

RESULTS

For the studied beam configurations, ion chamber measurements shows that PB overestimate the dose up to 8.5%, whereas MC has a maximum variation of 1.6%. Dosimetric analysis using dose profiles shows that PB overestimates the dose in the region corresponding to the lung up to 16%. For axial dose distribution comparison the percentage of pixels with gamma index bigger than one for MC and PB was, plan 1: 95.6% versus 87.4%, plan 2: 91.2% versus 77.6%, plan 3: 99.7% versus 93.1% and for plan 4: 98.8% versus 91.7%. It was confirmed that the lower dosimetric errors calculated applying MC algorithm appears when the spatial resolution and variance decrease at the expense of increased computation time.

CONCLUSIONS

The agreement between measured and calculated doses, in a phantom with lung heterogeneities, is better with MC algorithm. PB algorithm overestimates the doses in lung tissue, which could have a clinical impact in SBRT lung treatments.

SU-E-J-20: Evaluation of Image Qualities and Registration of Varian KV-CBCT Images Reconstructed from the Reduced Number of Projections

PURPOSE

To quantitatively investigate image qualities of the kilo-voltage cone-beam CT images reconstructed with reduced number of projections on the Varian OBI system. Evaluate the registration accuracy using those CBCT images against the reference CT images.

METHODS

CBCT images were obtained from Varian OBI system using standard dose head, pelvis and pelvis spotlight modes. CBCT reconstructions were performed with full, 1/2, 1/4, 1/6 and 1/8 of the full set of projections. Catphan® 504 phantom was used to evaluate high-contrast spatial resolution, low-contrast visibility and uniformity. Rando phantom was imaged for rigid registration study. Rando was set up on the linac couch deliberately shifted by 1cm in vertical, lateral, and longitudinal (no rotational) directions from the reference position. Automatch followed by manual adjustment was conducted 5 times to obtain the average shifts. The same method of analysis in the Rando study was used for the clinical registration study. One patient was imaged with pelvis mode and two patients were imaged with pelvis spotlight mode.

RESULTS

The Catphan study indicates that high-contrast spatial resolution and uniformity are virtually not affected by the lowest projection-number (1/8) reconstruction scheme. However low-contrast visibility degrades when the projection number used for reconstruction is as low as 1/6. Rando study shows that registration accuracy can be achieved with images reconstructed with 1/6 of the full set of projections. Patient study shows similar results exhibited in Rando study. However, noisy images and streak artifacts are more pronounced with fewer projections (approximate 1/6), which decreases viewer's ability to visualize soft tissues in pelvic sites.

CONCLUSIONS

This study shows that KV-CBCT reconstructed with fewer number (approximately as low as 1/6) of regular projections can be used for registration against the reference CT. Although the results are encouraging, more clinical cases should be evaluated in the future. None.

SU-E-T-104: Commissioning and Dosimetric Characteristics of TrueBeam System: Composite Data of Three TrueBeam Machines

INTRODUCTION

A TrueBeam linear accelerator (TB-LINAC) is designed to deliver standard flattened and flattening-filter-free (FFF) beams. In our institute, three TB-LINAC units are installed. In this work, composite data of the three units and multi-unit comparison are presented.

METHODS

Each TB-LINAC can deliver photon beams from 4MV to 15MV, electron beams from 6MeV to 22MeV, and 6MV-FFF and 10MV-FFF. Dosimetric characteristics are systematically measured for commissioning including percent depth dose (PDD), beam profile, relative scatter factor, dynamic leaf shift, output factor and MLC leakage. Critic considerations of Pion of FFF photon beams and dosimetric penumbra are investigated.

RESULTS

All measured PDDs and profiles of photon and electron matched well across the three machines. Beam data were quantitatively compared and combined through average to yield composite beam data. The discrepancies among the machines were quantified using standard deviation (SD). For example, the mean SD of the PDDs among the three units is 0.12%, and the mean SD of the profiles is 0.40% for 10MV-FFF open fields. The variations of Pion of the chamber CC13 is 1.2±0.1% under 6MV-FFF and 2.0±0.5% from dmax to the 18cm-off-axis point at 35cm depth under 40×40cm² . The measured relative output factors range from 0.866 to 1.141 with the mean discrepancy of 0.06±0.04% among the three units. The measured wedge factors range from 0.863 to 1.254 with the mean overall discrepancy of 0.04±0.04%. The mean MLC transmission and dynamic leaf shift were measured from 1.0% to 1.5% and from 0.77mm to 0.96 mm from 4MV to 15MV. The mean penumbra of various photon beams are measured from 5.88±0.09mm to 5.99±0.13mm from 4MV to 15MV at 10cm depth of 10×10 cm² .

CONCLUSIONS

Dosimetric data demonstrated that the three units could and had been matched well. The systematically measured data might be useful for future reference.

SU-E-T-163: Thin-Film Organic Photocell (OPV) Properties in MV and KV Beams for Dosimetry Applications

PURPOSE

To characterize dosimetric properties of low-cost thin film organic-based photovoltaic (OPV) cells to kV and MV x-ray beams for their usage as large area dosimeter for QA and patient safety monitoring device.

METHODS

A series of thin film OPV cells of various areas and thicknesses were irradiated with MV beams to evaluate the stability and reproducibility of their response, linearity and sensitivity to absorbed dose. The OPV response to x-rays of various linac energies were also characterized. Furthermore the practical (clinical) sensitivity of the cells was determined using IMRT sweeping gap test generated with various gap sizes. To evaluate their potential usage in the development of low cost kV imaging device, the OPV cells were irradiated with kV beam (60-120 kVp) from a fluoroscopy unit. Photocell response to the absorbed dose was characterized as a function of the organic thin film thickness and size, beam energy and exposure for kV beams as well. In addition, photocell response was determined with and without thin plastic scintillator.

RESULTS

Response of the OPV cells to the absorbed dose from kV and MV beams are stable and reproducible. The photocell response was linearly proportional to the size and about slightly decreasing with the thickness of the organic thin film, which agrees with the general performance of the photocells in visible light. The photocell response increases as a linear function of absorbed dose and x-ray energy. The sweeping gap tests performed showed that OPV cells have sufficient practical sensitivity to measured MV x-ray delivery with gap size as small as 1 mm.

CONCLUSIONS

With proper calibration, the OPV cells could be used for online radiation dose measurement for quality assurance and patient safety purposes. Their response to kV beam show promising potential in development of low cost kV radiation detection devices.

SU-E-T-11: LINAC Dose Profiling Using Cherenkov Emission Imaging

PURPOSE

To demonstrate the potential for fast 3D dose profile imaging of a LINAC beam using images of the induced Cherenkov radiation in a water tank. A specialized time-gated imaging system was developed as a prototype to quantify and compare with Monte Carlo, to illustrate the concept.

METHODS

Images were acquired from a water tank during irradiation from a 6 MV Varian-2100C linear accelerator beam using a time-gated CCD-based imaging system. The camera was placed normal to the tank wall to minimize parallax reflections, and resultant images were produced by evaluating the median of each pixel in a stack of 2000 images taken at a rate of 60 Hz with an exposure time of 10 ms. Experimental data was compared to images obtained from GEANT4 simulations of the optical setup.

RESULTS

Examination of the scored quantities for dose and generated Cherenkov photons indicates that there is a strong similarity, which can be explained by considering the electron energy losses per unit path length. However, due to the complex convolution of the Cherenkov emission directionality and camera lens angular field of view, this relationship is distorted. These errors can be calibrated using the GEANT4 simulations to more accurately reflect the intrinsic dose in the water volume.

CONCLUSIONS

This work demonstrates dose profiling using the induced Cherenkov radiation signal for the first time. These preliminary results serve as a proof of concept of imaging at one azimuthal angle. Analogous to SPECT, the technique could easily be translated to multiple angles yielding full dose reconstructions following filtered back projection. Further refinement of this technology could be the first step in a paradigm shift towards an alternative method for fast radiation field analysis. Advantages would include increased speed, as well as the ability to profile dynamic beam shapes within transparent solid anthropomorphic phantoms. This work has been financially supported by NIH grant R01CA109558.

WE-G-BRB-02: MU-EPID an EPID Based Tool for IMRT Quality Assurance

PURPOSE

A software program (MU-EPID) has been developed to perform patient specific IMRT pre-treatment QA verification using an electronic portal imaging device.

METHODS

The software converts measured images of intensity modulated beams delivered to an EPID, into fluence maps that can be imported in the treatment planning system. The dose can then be calculated in the patient anatomy and compared against the patient's treatment plan for QA purposes. We first benchmarked the software using as a patient a cylindrical phantom. An aSi-1000 EPID mounted on a Varian Novalis linear accelerator was used for the image acquisition. Finally, IMRT plans from different treatment sites were used to further validate this in- house software. QA analysis was performed by evaluation of isodose distributions, DVH comparison and 2D gamma analysis.

RESULTS

The validation study with the cylindrical phantom showed that the dose to the ion chamber measurement point was in good agreement with both the original treatment plan and the MU-EPID reconstructed dose. Similar results were found for the clinical cases that we studied. A gamma analysis of the dose to the isocenter plane was performed for each plan. Using 3% and 3 mm as the evaluation criteria, resulted in an average of 97.44% of pixels passing the analysis (gamma<1). Good agreement was also observed for the DVH, isodose and profile comparisons between the clinically delivered IMRT plan and the MU-EPID derived dose calculation.

CONCLUSIONS

The results of the present investigation suggest that MU-EPID can be used in a clinical environment and can be used for patient specific QA for IMRT plans. This work has been supported by the SCOA.

MO-F-213AB-05: Commissioning of Gated RapidArc Radiotherapy for Treatment of Moving Targets

PURPOSE

To commission and evaluate gated RapidArc radiotherapy of a linear accelerator (Varian TrueBeam) for treatment of moving targets using a programmable dynamic phantom.

METHODS

The phantom used had different dosimetry inserts for measurement of dose and dose distribution. It could be programmed to move in the anterior-posterior and superior-inferior axes with different motion patterns, amplitudes and frequencies to simulate lung motions of patients. A set of 4D CT images was acquired with the aid of a Varian RPM system. Images acquired at the 40, 50 and 60% of the motion cycle were selected and transferred to a treatment planning system (Varian Eclipse) for planning. A two-arc RapidArc treatment plan was generated for a C-shaped target volume with a conformity index of 1.49 and transferred to the TrueBeam for treatment delivery. Dose and dose distribution measurements were performed using a 0.057 cc ionization chamber and radiochromic films, respectively and compared with the TPS calculations. Five treatment fractions were given in three days with two different target motion patterns to assess the consistency of the dose delivery.

RESULTS

Agreement between TPS calculation and measurement were within 1.64% for dose and 3% or 3mm in distance to agreement for dose distribution. Repeatability of dose delivery between treatments was within 0.1% (1SD) in the five treatment fractions delivered in three days. The time required to deliver a dose of 2 Gy to a moving C-shaped target using gated RapidArc technique with two gantry rotations was about 15 minutes.

CONCLUSIONS

The geometric and dosimetric accuracy and consistency of gated RapidArc radiotherapy had been verified. Our study indicated that the accuracy and consistency of the treatment modality were acceptable for clinical implementation.

SU-E-I-08: KV XVI Cone Beam-CT Dose Measurement Using Gafchromic XRQA2 Film

PURPOSE

To study the effect of different filters on the dose response curves of the Gafchromic XRQA2 film. To measure the kV XVI cone-beam CT (CBCT) surface dose received during 3D and 4D imaging protocols in three body regions (head and neck, chest and pelvis).

METHODS

GafChromic XR- QA2 film (International Specialty Products, Wayne, NJ) dose response curves were generated for three irradiation settings: 100 kVp S20/F0; 120 kVp S20/F0 and 120 kVp S20/F1(F1 is a Bowtie filter). Film pieces were irradiated in air by the X-ray Volume Imager (XVI) mounted on the Elekta Synergy linear accelerator (Elekta, Crawley, UK) and their responses were correlated to air kerma measurements. To measure the CBCT surface dose, film pieces were taped on the surface of a male Alderson Rando Phantom (Alderson Research Laboratories, Inc., Long Island City, New York) at four different places (Anterior, Posterior, Right Lateral, Left Lateral).

RESULTS

The dose response curves of XRQA2 film generated with F1 and F0 filters were found to differ by 5 to 7% when the air kerma changed between 2 and 5 cGy. This was less than the observed difference (more than 15%, especially at low air kerma) in the dose response curves when different energies (100 and 120 kVp) and same filter were used. Surface dose ranged between 0.02 cGy and 4.99 cGy. The lowest average surface dose (0.05 cGy) was observed when the fast head and neck protocol was used, whilst the highest average surface dose (3.06 cGy) was noticed when the chest m² 0 protocol was used.

CONCLUSIONS

Filters seem to have less effect on the dose response of the film compared with energy. Gafchromic XRQA2 film was used successfully to measure the XVI CBCT surface dose. The dose was found to vary from one imaging protocol to another, with 4D protocols not necessarily delivering more doses.

WE-G-217BCD-07: Implementation and Evaluation of Helical On-Board CBCT and Exact Image Reconstruction

PURPOSE

Longitudinal coverage of CBCT, which is 17 cm for head scan and 15.5 cm for body scan, is not enough to cover the entire PTV for over 90% of head/neck and gastrointestinal/genitourinary/gynecologic patients if lymph nodes are involved. Helical CBCT, which was accomplished using external beam LINAC in its research mode, is one promising way to extend the CBCT longitudinal coverage. Aim of this study is to compare Katsevich's exact algorithm with traditional FDK algorithm for helical CBCT image reconstruction.

METHODS

CBCT projection raw data were acquired on a TrueBeam LINAC machine (Varian Medical Systems) in the research mode in helical trajectories that encompass a 360 degree rotation, 25 cm pitch, 100 kVp, 80 mA, and 25 ms, with a Catphan 600. Reconstruction was done with Katsevich's exact and FDK approximate algorithms. Scatter correction, beam-hardening correction, and non-uniform gantry angle correction, are performed on projection data to reduce artifacts and noise. Image qualities (CT number accuracy, uniformity, SNR) were evaluated and compared between the reconstruction algorithms.

RESULTS

Images reconstructed by Katsevich's algorithm show better qualities, compared to ones by FDK algorithm and HU numbers have higher uniformity and accuracy. The HU-density calibration curve closely conforms to the manufacturer recommended values. The level of noise computed as the standard deviation in the phantom uniform region is 28.07 for the Katsevich algorithm, compared to 44.64 for the FDK algorithm.

CONCLUSIONS

Katsevich's exact reconstruction algorithm provided better image qualities than FDK for helical CBCT scans. This result will very useful for our ongoing investigation of helical CBCT, which would lead to improvement of CBCT longitudinal coverage of PTV and would be essential for future image-guided adaptive radiation therapy applications. Varian Research Agreement with Washington University in St. Louis.

SU-E-T-228: The Beauty and the Beast: Transition from Film/paper Charts to Paperless Environment with a New TrueBeam/ARIA System in a Small Community Hospital on a Tight Budget

PURPOSE

To review the issues a physicist may encounter in a community hospital during the transition from film/paper charts to a paperless environment with ARIA and a TrueBeam LINAC. With a lean budget, it was necessary for the physics group to take on the project management responsibilities in order save costs. This work highlights the lessons learned during the planning and execution of our project.

METHODS

Like many hospitals around the county, our hospital was caught in the economic downturn and was unable to provide all of the capital necessary to upgrade to the radiation oncology department. However, with the support of the hospital foundation, a total of $6M was secured for new LINAC, ARIA and CT simulator. To save costs on facilities and computers, it was necessary for the physics group to be involved in creating architectural drawings for shielding calculations, finding a vendor to remove the old linac, assisting the foundation to raise money, submission of the 'Certificate of Need' approval with the state, negotiation with vendors, IT infrastructure, reviews with the general contractor and vendor's project team, and ultimately writing the commissioning reports for the new systems as well as developing new policies and procedures.

RESULTS

During a period of 4 months, the old LINAC was removed, facility renovations made, the TrueBeam linac was installed, accepted, and commissioned and first patients were treated. In addition, we transitioned from a film/paper environment to a paperless environment. However, this was very stressful for staff and it may be advisable to stage such a project over a longer period of time. There was also significant lost revenue (∼$2M) during downtime of construction, installation, and commissioning.

CONCLUSIONS

The radiation oncology department was upgraded (The Beauty) on a tight budget but at the cost of added stress (The Beast) to the staff.

SU-D-213CD-06: Workflow and Safety Systems of a Linac-MR Sim-Brachytherapy MRgRT™ Facility

PURPOSE

To develop the operational workflow and safety systems of a magnetic resonance-guided radiotherapy system (MRgRT™), which comprises an MR scanner on rails that travels between a linac vault, MR simulation room and brachytherapy suite.

METHODS

To develop a safe and streamlined clinical workflow, we conducted a comprehensive process review based on a layered approach to overall MRgRT safety that included i) facility design, (ii) workflow iii) system design and interlocks and iv) policies and procedures. We applied existing guidelines for MR and radiation safety, and employed system-level failure modes and effects analyses to design the MRgRT facility and clinical procedures.

RESULTS

In the MRgRT system configuration, the MR and treatment systems are physically decoupled and used independently requiring novel administration of existing MR and radiation guidelines. A key element for the safe operation of the moving MR unit is the concept that all three rooms represent zone 4 areas (American College of Radiology guidelines). Using this concept, we applied MR guidelines to develop safe procedures for the overall suite, including screening of all persons entering the suite in zone 2 and control of ferromagnetic materials. We generated a clinical workflow that ensures expedient and safe transition between MR imaging and treatment delivery in both the linac and brachytherapy rooms. In addition, we designed emergency protocols for MRgRT, which helped drive requirements for the facility and system design, e.g., need for an accessible MR-safe stretcher.

CONCLUSIONS

We designed the first comprehensive description of the MRgRT workflow, interlocking systems and safety procedures. With this layered approach to safety, we addressed critical aspects regarding safe operation and workflow for the system and provided multiple redundancies for key processes. Coupled with customized staff training, the proposed design ensures the safe operation of the MRgRT facility. This work has received research personnel support from IMRIS.

SU-E-T-19: Fitting a Multiple Source Photon Model for Monte Carlo Treatment Plan Verification

PURPOSE

To assess and reduce the difficulty of fitting a multiple source photon model for monte carlo treatment plan verification.

METHODS

The EGS4 user code MCSIM, from Fox Chase Cancer Center, was chosen for its support of a multiple source photon model, of which the point and secondary (extrafocal) photon sources were utilized. A described method of fitting the secondary source to in-air output factors was implemented. Additionally, a method to fit the point source to a single large field dose distribution was explored. The point source fitter utilizes a database of pre-simulated mono-energetic fanlines to build distributions from arbitrary spectra. Perturbations are made to fanline spectra to reduce the errors along them. In this study the energy spectrum for each fanline has been limited to the log-normal distribution, which reduces the number of parameters for each to two.

RESULTS

It was found that one spectral parameter could be set to a constant for all fanlines and the other restricted to linearity with respect to off-axis position. The model matched the outputs and distributions in non-superficial areas to within 2% for 6MV and 15MV Varian iX field sizes between 4 and 40 cm. Various types of treatment plans were then successfully verified, including 3D, VMAT, IMRT, and an iPlan Monte Carlo stereotactic lung to within 3% (tumor dose).

CONCLUSION

With such tools it is practical for a non-research physicist to fit a two source photon model for the purpose of monte carlo treatment plan verification. The only commissioning data needed are in-air output factors, a single large field dose distribution, and the usual machine parameters provided by LINAC vendors for clinical second check programs. Even when only photons are simulated and spectra are greatly simplified it is possible to achieve acceptable results for non-superficial tumors. Furthermore, this is achieved without proprietary machine specifications.

SU-E-T-188: Evaluation of a 3D Patient Relevant Dose QA Tool: Multiple Institutional Studies

PURPOSE

To evaluate 3DVHTM as a patient dose-verification and analysis tool through multiple institutional studies. Virtual patient doses were measured and compared among different vendors' treatment planning systems (TPS) and delivered by different vendors' LINACS so that we better understand the uncertainty of entire process within a patient undergone radiotherapy.

METHODS

One head-and-neck (H&N) and one lung patient were selected in this study. The DICOM images/RT structures along with clinical protocols including prescription doses (59.4Gy for H&N and 70.2Gy for lung) and normal&hyphen;tissues tolerances were distributed to six institutions. Based on the same criteria, each institution generated their IMRT plans for the patients. Four different TPS and six different LINACS were used. The conventional per&hyphen;beam IMRT QA using MapCHECK was performed by all participants. All the measured and calculated data were sent back to one institution for 3DVH analysis. Through the use of planned-dose-perturbation (PDP)TM algorithm (Sun Nuclear Corp.), the 'actual-DVHs' were generated and then compared to the 'reference-DVHs' from plans. Their differences represented errors induced from the combination of TPS dose-calculation algorithm and beam-delivery systems.

RESULTS

All plans in the study have met the clinical criteria. The 3D matching rates for 3%global/3mm (DD/DTA) ranged from 95.8-99.9% for H&N and 93.5-100% for lung. The dose-difference-histogram for PTV had a mean of 0.67% [0-2%] for H&N cases and 1% [0.6-2.8%] for lung cases. The QA tool was able to spot the doses outside 3%/3mm criteria for critical structures much easier than conventional planar QA methods. In addition, the hot/cold spots at the boundaries of collimators are attributed to the uncertainty of collimator-positioning greater than 1-mm.

CONCLUSIONS

The analysis of IMRT plans in this study has shown that 3DVH is a vital QA tool for assessing clinically relevant doses as well as diagnosing potential systematic errors from both TPS and delivery systems.

SU-E-T-121: Investigating the Optimal Scanning Resolution for Radiochromic EBT-2 Films Using an Epson 10000XL Flatbed Scanner

PURPOSE

The purpose of this work is to determine the optimal scanner resolution of an Epson 10000XL scanner for the analysis of radiochromic EBT-2 films. Using Fourier analysis and the Nyquist-Shanon sampling theory, the highest frequency component required to sufficiently reproduce a previously measured step dose profile was investigated.

METHODS

A setup was created, in which one half of a 6×6cm² EBT-2 film was shielded on exposure using a 15×5×10cm³ lead block to obtain sharp step dose profiles. The film itself was placed between two 6cm RW3 stacks on top of which the lead block was placed. Using a Siemens Primus linear accelerator operating at 6/15MV nominal energies, the setup was exposed to 400MUs at 6MV and 500MUs at 15MV respectively. Preliminary investigations were performed without RW3 between the lead and film. Initial image acquisition was performed at 600dpi to minimize information loss. Using the average of five line profiles, a uniformity correction algorithm provided by the manufacturer was implemented prior to the Fast Fourier Transform (FFT) operation. In an iterative process, all frequency components above a cut-off frequency wcut were successively removed and the original image reconstructed with the inverse FFT operation. The goodness of fit was evaluated by comparing the change in penumbra width on image reconstruction.

RESULTS

The minimum scanning resolution required to analyze the step dose profiles created without build-up material was 52dpi for 6MV and 30dpi for 15MV. By adding build-up material, in the areas of secondary electron equilibrium the required resolution reduces to 12dpi for 6MV and 8dpi for 15MV.

CONCLUSIONS

For sufficient image reproduction within any information loss, resolutions as low as 52dpi at 6MV and 30dpi at 15MV are sufficient for evaluating EBT-2 films. This is in compliance with 50dpi recommended by the manufacturer.

SU-E-T-410: Spine Radiosurgery Imaging Guidance Using ExacTrac and CT on rails

PURPOSE

To evaluate the patient setup accuracy and effectiveness using ExacTrac and CT_on_rails systems.

METHODS

We used Brainlab's Exactrac system and Varian/GE's CT_on_rails for spine radiosurgery patient setup. Once the patient was setup using the ExacTrac and couch position was recorded, fiducially markers were placed on stable surfaces based on the room laser to indicate the linac iso for the CT images. CT images were acquired using the on-rail CT with the couch rotated 180 degrees. The couch was returned to 0 degree position, and verification X-ray images were taken and corrections were made by ExacTrac. The treatment CT images were registered with the planning CT using the in-house CAT software and it displays the correct couch position based on CT which can be compared with ExacTrac setup. The corrected couch positions from CT registration are compared to those from ExacTrac. The translational discrepancies needed to be within 2 mm for confirmation. If a discrepancy was greater than 2 mm, investigation or re-setup was required. The rotation deviations were also evaluated by ExacTrac and confirmed by the treatment CT images. We would also re-setup patient if Exactrac detected more than 3 degree rotation, or treatment CT images showed significant target rotation compared to planning CT. The use of the CTonrails took little extra time, but make the overall evaluation process easier, faster and with more confidence.

RESULTS

for 171 treatment sessions using this approach, the mean discrepancies between CTonrails and ExacTrac setup is: x=0.0±1.0 mm, y=-0. 1±0.9 mm, z=0.2±0.9 mm; for rotations, about 3% of the cases required re-setup patient due to significant rotation displayed by the treatment CT on the CAT system.

CONCLUSIONS

The combined use of ExacTrac and CT_on_Rails systems can improve the overall setup accuracy and increase the confidence in setup for spine radiosurgery treatments.

SU-E-E-01: Commissiong of Linear Accelerator and Beam Modeling in Treatment Planning Systems

PURPOSE

Sooner or later every medical physicist is involved with commissioning and beam modeling of a new linear accelerator (linac) and a new treatment planning system (TPS). In spite of all instructions and training offered by the vendors, at the time a new linac is being purchased and added to the present ones the outside help is not so complete. The physicist who has to perform the commissioning job may not even be the one who was trained for that. What we are missing is a good comprehensive set of information and instructions on how to do's. From shielding calculation verifications, surveys, to collecting the beam data, modeling, entering the data into the TPS, and verifications of the goodness of the data we need a lot of support and we don't have it. I will provide a step by step description of the required work with the results we are looking for.

METHODS

Presentation of the shielding calculations, survey required, tools needed to perform them. Detailed beam data collections, scanning system needed, machine set of specs needed, applicator details needed. Importing beam data from the scanning system and beam calculations. Algorithms used in dose calculation, IMRT optimization, heterogeneity corrections presented to be understood before modeling the beam data.

RESULTS

At the completion of this course the medical physicist will be able to commission a linear accelerator and a treatment planning system with confidence and very little help from the outside.

CONCLUSIONS

This compendium of detailed instructions on commissioning a linear accelerator will provide good uidance to every physicist who will be involved with the installation and bringing into safe use for treatment of a new linear accelerator.

SU-E-T-581: Planning Evaluation of Step-And-Shoot IMRT, RapidArc and Helical TomoTherapy for Hippocampal-Avoidance Whole Brain Radiotherapy (HA-WBRT)

PURPOSE

Several planning strategies are available for hippocampal- avoidance whole-brain radiotherapy (HA-WBRT) following RTOG protocol 0933, but have yet to be compared on a common set of patient data. In this inter-institutional investigation, we evaluate three modalities likely to be employed by protocol participants; step-and-shoot IMRT, volumetric modulated arc therapy, and helical tomotherapy. A common set of patients is used for comparison, including credentialing and successfully accrued patients.

METHODS

Eight patient datasets were selected and de-identified prior to planning. Structures were contoured by physicians per protocol using fused MRI datasets. Three plans were generated for each dataset: Philips Pinnacle 9-field non-coplanar IMRT using protocol recommended beam parameters, Varian's RapidArc using two coplanar arcs, and Accuray's TomoTherapy using a 1cm jaw width. With the goal of meeting the compliance criteria outlined in RTOG 0933 (target coverage and dose limits to the hippocampus and optic structures), three planners independently planned each modality without prior knowledge of the patient's other plans to reduce bias. The three plans for each patient were compared according to the protocol's dosimetric compliance criteria. A homogeneity index was also computed to compare target dose uniformity.

RESULTS

All plans achieved the protocol dose criteria, except for one RapidArc plan with slightly inferior dose to the optic chiasm. TomoTherapy offered superior dose homogeneity for all patients. For the two linac based methods, RapidArc was found to provide dose homogeneity at least as good as, and in most cases superior to, 9-field step-and-shoot IMRT.

CONCLUSIONS

Helical TomoTherapy offers superior dose homogeneity for HA-WBRT following RTOG 0933. Compared to step-and-shoot IMRT, volumetric modulated arc techniques, such as RapidArc, can offer improved homogeneity for HA- WBRT and are generally more efficient/expeditious to deliver than the noncoplanar 9-field arrangement recommended by the protocol, which uses 7 separate couch angles.

SU-E-J-14: Evaluation of Mechanical Accuracy of Electronic Portal Imaging Devise on Its Use in Patient Specific IMRT QA

PURPOSE

Electronic portal imaging devices (EPID) have been used for both in vivo dosimetry and in vitro dose verification in intensity modulated radiotherapy (IMRT). This study is to investigate the effect of EPID mechanical precision on the accuracy of measured dose distribution.

METHODS

EPID energy fluences (dicom images) of H&N IMRT fields were collected daily on two Varian LINACs (Clinac-iX & Trilogy) over 4-week period. The energy fluences were converted to doses using EPIDoseTM (Sun Nuclear Corp). Mechanical deviations of EPIDs could be divided into two components: one with inherent detector center misalignment from the beam central axis, another caused by the 'sagging effect' from gantry rotation. The first component was detected by 'best matching' of the measured and calculated dose at zero gantry angle (G=0). The second component was computed by 'best matching' the 10×10cm field defined by MLC at G=0, 90,180, and 270, separately. A 'shift' was generated by the combination of these two components and then applied to correct the measured dose at the corresponding gantry angle for the IMRT field.

RESULTS

Inherent misalignment of the detector's center and the 'sagging' deviation were found to be 1-2 mm and 1-5 mm, respectively for both LINACs. Each component was found very stable (change < 1mm) over the 4-week observational period. Using a Gamma index of 2%/2mm (DD/DTA), the 'shift' increased the average passing rate from 59% to more than 92%. On the other hand, blindly applying 'auto-shift' from commercially available software to obtain the best match would compound true QA issues with units' misalignments.

CONCLUSIONS

A false 'mismatch' between measured and calculated dose distribution caused by mechanical inaccuracies of EPID could be avoided by measuring the two components identified in this study. One should examine the mechanical precision of equipment prior to clinical use of EPID dosimetry.

SU-E-I-94: External Beam Radiation Cherenkov Emission in Tissue Used for Tissue Oxygen Sensing

PURPOSE

To show that Cherenkov emission is generated by external radiotherapy beam in tissue, and could serve as optical source to excite an oxygen sensitive phosphor, Oxyphor G4, within tissue. The intensity and lifetime of the phosphorescence was measured with a time-gated system and reveals the oxygenation levels in the tissue phantom.

METHODS

A tissue phantom made with PBS, 1% v/v Intralipid-20% (Sigma Aldrich), 1% v/v whole blood and Oxyphor G4 in 1 μM concentration is irradiated by 18MeV external radiotherapy electron beam at a dose rate of 4 Gy/min generated by a medical linear accelerator (Varian LINAC 2100C, Varian Medical Systems). On one side of the phantom, a fiber bundle is used to conduct optical signal to a spectrometer connected to a fast gating ICCD (PI-MAX3, Princeton Instruments). For each oxygenation level, a series of spectrum of phosphorescence at different time points is measured by the time domain gating technique. Lifetime of phosphorescence is analyzed by exponential fitting and is validated by comparison to an independent analysis by frequency domain phosphorimetry. Monte Carlo simulations using GEANT4, of the fiber optic collection of Cerenkov light were performed to decide the sensitivity of the optical system for a range of specified geometries and beam types. Simulation results identify the effective depth within the phantom that is sampled by the optical collection of the Cerenkov signal.

RESULTS

Simulations show that we can detect the Cherenkov signals comes from an approximately 5 mm depth from within the tissue phantom. Lifetime of the phosphorescence and pO2 of the phantom could be measured and calculated correctly by the time domain gating system.

CONCLUSIONS

This work indicates time domain gating techniques combined with an oxygen sensitive phosphor are capable of accurately monitoring tissue oxygenation from a reasonable sampling depth in tissue in vivo during external beam radiotherapy. NIH grant R01CA109558.

TU-E-BRA-03: Real-Time Fiducial Detection and Prostate Movement Assessment with Cine MV Images in RapidArc Treatments

PURPOSE

To develop an algorithm for detection of metallic fiducial markers in cine MV images, and to assess the prostate movement during RapidArc treatment.

METHODS

A Varian TrueBeam linear accelerator (LINAC) was used to deliver RapidArc treatment for prostate patients. Cine images were acquired with the onboard electronic portal imaging device (EPID) using the MV therapeutic beam. Three metallic fiducial markers were implanted inside the prostate. To detect the fiducial position, we explicitly account for the possible marker blockage by MLC during beam modulation. If the marker is not blocked, we employ the planning coordinates of the marker centroids projected onto the cine MV images and perform template matching in the vicinity of its projection to localize the actual position of the marker. Displacements of the fiducial markers are assessed by comparing the actual and planned positions.

RESULTS

We analyzed ∼280 cine MV images acquired during a 55-sec RapidArc treatment for a prostate patient. The three markers were visible in about 46%, 52%, and 48% of the images, and at least one fiducial was visible during almost entire treatment (97% of the time). The marker detection algorithm agrees well with manual detection (< 0.2 mm). The mean displacement for each fiducial was 0.40 ± 0.42, 0.27 ± 0.29, and 0.46 ± 0.34 mm. The maximum displacement was 2.33, 1.75, and 2.23 mm.

CONCLUSIONS

An algorithm for automatic detection of fiducial markers in cine MV images has been developed. The prostate movement during a RapidArc treatment has been analyzed for a patient with implanted markers. Accurate target positioning is achieved at all times during treatment. In light of the random nature of intrafraction prostate motion, this work represents an important step toward real-time image-guided prostate radiation therapy.

WE-E-BRB-09: A GPU-Based Monte Carlo QA Tool for IMRT and VMAT

PURPOSE

To develop a GPU-based Monte Carlo (MC) 3D dosimetry quality assurance (QA) tool employing patient geometry and actual delivery information.

METHODS

First, we generate fluence maps at all beam angles from the initial treatment plan. A GPU-based MC dose engine, gDPM, is employed for the secondary dose calculation (SDC) on patient CT. This SDC is used to verify the TPS plan dose (PD) accuracy. Before the 1st treatment fraction, we deliver the treatment plan on a Linac without any phantom setup to obtain machine log files. With the log files, we extract actually delivered fluence maps at all beam angles and perform delivered dose calculation (DDC) using gDPM. The difference between DDC and SDC indicates possible errors in data transferring and machine delivery. Lastly, the comparison between DDC and PD shows the accumulative errors from all the possible sources. Moreover, a web application for this QA tool is developed for clinical use. We have tested this QA tool on 6 patients, 4 VMAT and 2 IMRT patients. We reported mean gamma values and passing rates inside the 20% isodose line; DVH plot and dose difference matrix are also documented.

RESULTS

For all six patients, the gamma passing rates within the 20% isodose line for SDC, DDC and PD comparisons are all higher than 95%. In the DVH plot, the three dose distributions were found to be very close. A typical IMRT or VMAT case takes less than one minute to run the whole QA tool.

CONCLUSIONS

We have developed a GPU-based MC QA tool which can be used for efficient and easy IMRT and VMAT QA.

SU-E-T-154: Online Dose Verification with Gafchromic Film for Fixed-Gantry and Rotational Intensity Modulated Radiation Therapy: A Phantom Study

PURPOSE

The patient specific quality assurance (QA) measurements for fixed-gantry and rotational intensity modulated radiation therapy (IMRT and VMAT/RapidArc) are usually performed on a homogeneous phantom prior to the treatment. The purpose of this study is to develop an online method to verify the delivered dose to the patient on the treatment day.

METHODS

An anthropomorphic (Rando) head phantom was immobilized in treatment position with a thermoplastic mask to simulate a real patient. A sheet of gafchromic film (EBT2) was sandwiched between a 1-cm-thick solid water slab, which was fixed to the Type-S extension board, and the patient's head hold (a pillow used here). The CT images of the Rando phantom were acquired and exported to the treatment planning systems. One step-and-shot fixed-gantry IMRT plan and one RapidArc plan were generated and the dose distributions on the film plane were calculated. The two plans were delivered to the patient (Rando phantom in this study) in the treatment position on a Varian Trilogy linear accelerator with two new films. The films were scanned, and the measurements were compared with the planned doses.

RESULTS

The composite dose distributions measured on the film plane were the actual delivered dose for the treatment. The comparison between the measurement and planned dose profiles shows an agreement within 3% because of the good reproducibility of phantom positioning. Gamma pass rates (using 3mm and 3% criteria) for the IMRT and RapidArc plan were found to be 95% and 94%, respectively.

CONCLUSIONS

The phantom study has demonstrated the feasibility of using gafchromic film for online dose verification. This simple method takes into account the patient heterogeneity and the treatment associated uncertainties such as setup error, intrafraction motions and machine related variations. It can be implemented as an online physics and/or clinical QA tool without taking additional machine time.

SU-E-T-88: Evaluating Gantry Sag on Linear Accelerators and Introducing an MLC-Based Compensation Strategy

PURPOSE

Gantry sag is one of the well-known sources of mechanical imperfections that compromise the spatial accuracy of radiation dose delivery. This study aims to quantify the gantry sag on multiple linacs and to investigate a multiple leaf collimator (MLC)-base strategy to compensate for gantry sag.

METHODS

We used the Winston-Lutz method to measure the gantry sag on three Varian linacs. A ball-bearing phantom was imaged with a square radiation field during gantry rotation. The images were analyzed to derive the radiation isocenter and subsequently the gantry sag, that is, the superior-inferior wobble of the radiation field center from the radiation isocenter as a function of gantry angle. Compensation for gantry sag was attempted by offsetting the MLC leaves at 90-degree collimator angle. The amount of offset was the opposite of measured gantry sag, which was gantry angle-specific.

RESULTS

Gantry sag was reproducible within a six-month period. On the three linacs, the maximum gantry sag was found to vary from 0.7 mm to 1.0 mm, depending on the linac and the collimator angle. The radiation field center moved inferiorly, or away from the gantry, when the gantry was rotated from 0 to 180 degrees. Comparison of gantry sag at 0- and 90-degree collimator angles showed that the uncertainty in MLC leaf positions did not increase the gantry sag. Instead, gantry sag was caused primarily by nonideal gantry rotation. After the MLC compensation was applied, the maximum gantry sag was reduced to less than 0.2 mm.

CONCLUSIONS

The results indicated that gantry sag on a linac can be quantitatively measured with sub-millimeter precision, using a simple ball-bearing phantom and the electronic portal imaging device. Reduction of gantry sag is feasible by applying a gantry angle-specific correction to MLC leaf positions at 90 degree collimator angle.

SU-E-T-208: The Secondary Malignancy Risk Estimation Due to the Neutron Contamination in 3D-CRT and IMRT Treatment Techniques by Using Bubble Detectors

PURPOSE

In this study, the neutron measurements were performed in free in air and RW3 solid water phantom to estimate the secondary malignancy risk for three dimensional conformal radiotherapy (3D-CRT) and intensity modulated radiotherapy (IMRT) techniques in prostate cancer treatment.

METHODS

Neutron dose were measured in 18 MV Elekta Synergy Platform and Varian Clinac linear accelerators by using bubble detector for personal neutron dosimetry (BD-PND). To determine the neutron equivalent dose in different depths and different distance from the edge of treatment field RW3 solid water phantom was used and organs location was defined in Alderson Rando phantom with respect to target (prostate) position in the treatment field. By using these data, we determined the neutron equivalent dose and effective dose for the standard prostate cancer patient treated with 3D-CRT and IMRT with 18 MV photon energy. The total dose was 70 Gy in 3D-CRT and 76 Gy in IMRT treatment in the current study. For both of these treatment techniques, we estimated the risk of secondary malignancies due to the neutron contamination by using the International Commission on Radiological Protection (ICRP) report 103.

RESULTS

The equivalent dose and effective dose due the neutron contamination were considerably high in 18 MV IMRT technique. The secondary malignancy risk estimation for 3D-CRT and IMRT were found to be 0.44% and 1.15% for Elekta Synergy Platform linear accelerator, 0.92% and 2.38% for the Varian Clinac DHX High Performance linear accelerator, respectively.

CONCLUSIONS

Therefore, one should take care of the secondary malignancy risk in case of using 18 MV in IMRT applications.

SU-E-T-190: Design, Development, and Evaluation of a Modified, Anthropomorphic, Head, Quality Assurance Phantom for Use in Stereotactic Radiosurgery

PURPOSE

To develop and evaluate a modified anthropomorphic head phantom for evaluation of stereotactic radiosurgery (SRS) dose planning and delivery.

METHODS

A phantom was constructed from a water equivalent, plastic, head-shaped shell. The original phantom design, with only a spherical target, was modified to include a nonspherical target (pituitary) and an adjacent organ at risk (OAR) (optic chiasm), within 2 mm, simulating the anatomy encountered when treating acromegaly. The target and OAR spatial proximity provided a more realistic treatment planning and dose delivery exercise. A separate dosimetry insert contained two TLD for absolute dosimetry and radiochromic film, in the sagittal and coronal planes, for relative dosimetry. The prescription was 25Gy to 90% of the GTV with >= 10% of the OAR volume receiving >= 8Gy. The modified phantom was used to test the rigor of the treatment planning process, dosimeter reproducibility, and measured dose delivery agreement with calculated doses using a Gamma Knife, CyberKnife, and linear accelerator based radiosurgery systems.

RESULTS

TLD results from multiple irradiations using either a CyberKnife or Gamma Knife agreed with the calculated target dose to within 4.7% with a maximum coefficient of variation of+/-2.0%. Gamma analysis in the coronal and sagittal film planes showed an average passing rate of 99.3% and 99.5% using +/-5%/3mm criteria, respectively. A treatment plan for linac delivery was developed meeting the prescription guidelines. Dosimeter reproducibility and dose delivery agreement for the linac is expected to have results similar to the results observed with the CyberKnife and Gamma Knife.

CONCLUSIONS

A modified anatomically realistic SRS phantom was developed that provided a realistic clinical planning and delivery challenge that can be used to credential institutions wanting to participate in NCI funded clinical trials. Work supported by PHS CA010953, CA081647, CA21661 awarded by NCI. DHHS.

WE-G-BRA-06: Calibrating an Ionisation Chamber: Gaining Experience Using a Dosimetry 'flight Simulator'

PURPOSE

Recently there has been great interest in the use of simulation training, with the view to enhance safety within radiotherapy practice. We have developed a Virtual Environment for Radiotherapy Training (VERT) which facilitates this, including the simulation of a number of 'Physics practices'. One such process is the calibration of an ionisation chamber for use in Linac photon beams.

METHODS

The VERT system was used to provide a life sized 3D virtual environment within which we were able to simulate the calibration of a departmental chamber for 6MV and 15 MV beams following the UK 1990 Code of Practice. The characteristics of the beams are fixed parameters in the simulation, whereas default (Absorbed dose to water) correction factors of the chambers are configurable thereby dictating their response in the virtual x-ray beam. When the simulation is started, a random, realistic temperature and pressure is assigned to the bunker. Measurement and chamber positional errors are assigned to the chambers. A virtual water phantom was placed on the Linac couch and irradiated through the side using a 10 × 10 field. With a chamber at the appropriate depths and irradiated iso-centrically, the Quality Indices (QI) of the beams were obtained. The two chambers were 'inter-compared', allowing the departmental chamber calibration factor to be calculated from that of the reference chamber.

RESULTS

For the virtual 6/15 MV beams, the QI were found to be 0.668/ 0.761 and the inter-comparison ratios 0.4408/ 0.4402 respectively. The departmental chamber calibration factors were calculated; applying these and appropriate environmental corrections allowed the output of the Linac to be confirmed.

CONCLUSIONS

We have shown how a virtual training environment can be used to demonstrate practical processes and reinforce learning. The UK CoP was used here, however any relevant protocol could be demonstrated. Two of the authors (Beavis and Ward) are Founders of Vertual Ltd, a spin-out company created to commercialise the research presented in this abstract.

SU-E-T-499: Validation of the Varian Generic Phase Space Files for Monte Carlo Calculations of Dose Distributions for the TrueBeam Linac Head

PURPOSE

To validate the generic phase space files for Varian TrueBeam linac head simulations.

METHODS

The generic phase space files include the simulation results of 6MV, 10MV, 6MV FFF, and 10MV FFF (flattening-filter free) operating modes of TrueBeam for patient-independent linac head components. Using the generic phase space files as the radiation sources, the BEAMnrc Monte Carlo codes are used to simulate the patient-dependent parts of the TrueBeam linac and the resulting phase space files are generated at a plane just before entering a water phantom for 4 different field sizes (5×5, 10×10, 20×20, and 40×40 cm² ). Dose distributions are calculated by DOSXYZnrc in the water phantom of size 50×50×40 cm³ . The percentage-depth-dose (PDD) curves and lateral dose profiles at three different depths (dmax, 10cm, 20cm) are obtained. Comprehensive comparisons have been made for a total of 64 dose profiles (including PDDs) between the Monte Carlo calculations and the measured data. The gamma index analysis is performed for all the comparisons.

RESULTS

The matching of the calculated dose distributions to the measured ones is analyzed by the gamma index method with a criterion of 2% dose tolerance and 2 mm distance-to-agreement. Of the 64 comparisons, the minimum gamma index passing rate is at least 92%, after taking into account the statistical nature of the Monte Carlo calculated dose values. Despite the existence of latent variance of phase space files, the phantom dose calculation uncertainty can be less than 1% for field sizes as small as 5×5 cm² . The computing time saved by using phase space files could be a factor of 5-10.

CONCLUSIONS

The Varian generic phase space files are accurate and efficient radiation sources for Monte Carlo calculations of radiation dose distributions for TrueBeam linac head. This work was supported in part by Varian Medical Systems and the NIH (1R01 CA104205 and 1R21 CA153587).

SU-E-J-59: Dual Imaging Guided Localization System for Spine Radiosurgery

PURPOSE

To compare localization accuracies between an ExacTrac and cone beam computed tomography (CBCT) systems for single fraction spine adiosurgery. The work also aimed to evaluate the inherent systematic deviation of both ExacTrac and CBCT systems to achieve highly accurate localization in the spine radiosurgery.

METHODS

ExacTrac and CBCT imaging systems were evaluated using the linac isocenter as the mutual reference point. First, a BB was placed in an anthropomorphic pelvic phantom. The phantom was localized with both imaging systems and the procedure was repeated 12 times. These results were used to devise a localization protocol using both imaging systems in spine radiosurgery, and employed for 51 patients (81 isocenters) prescribed for single fraction treatment. The displacement discrepancy between the isocenter and two systems were quantified in four dimensions (three translations, one rotation). A Student's two-tailed t-test was used to test for significant differences between the two imaging systems.

RESULTS

The phantom study showed 1.4±0.5, 0.6±0.5, and 0.1±0.5 mm differences between the two imaging systems in the anterior/posterior (A/P), superior/inferior (S/I) and left/right (L/R) directions, respectively. The angular difference was minimal along all three axes. The patient study revealed similar isocenter discrepancies between ExacTrac and CBCT of 1.1 ± 0.7 mm, 1.0±0.9 mm, and 0.2±0.9 mm in the A/P, S/I, and L/R directions, respectively, with the A/P and S/I directions showing statistical significance ((t(80) = 13.5 and 7.6 respectively, p = 0.000). The couch yaw discrepancy was 0 ± 0.3°. Overall, 1 mm systematic differences were observed in the A/P and S/I directions between ExacTrac and CBCT localization systems, both in phantom and patient. A procedure was developed to mitigate this systematic discrepancy.

CONCLUSIONS

These findings have justified our patient localization tolerance levels of 2 mm translation and 1 degree rotation for spine SRS treatment.

SU-E-T-168: Development of a Liquid Scintillation Detector for External Beam Dosimetry

PURPOSE

The goal of this research was to design a liquid scintillation dosimeter that could be used forrelative dosimetry of linear accelerator fields. The project emphasized minimization of cost and ease of use.

METHODS

The scintillator that was used in this research was BETAMAX- ES scintillation cocktail from MPBiomedical. This particular scintillator was selected due to its relatively high scintillation yield and lowcost. The entirety of the scintillator used the measurements was supplied free of cost. The housing for the liquid was constructed from PVC and is cylindrical with one tapered end. One fiber of the dual optical fibers transmits the generated photons to the CCD while the other fiber is used for Cerenkovsubtraction.The detector used comes from a Philips SPC880NC webcam. The plastic casing of the webcamwas removed so that only the printed circuit board, USB cable and lens eyepiece holder remained. Thesensor employed is the Sony ICX098QB CCD, which is 3.2mm by 2.4mm and each pixel is 5.6mm by 5.6mm. A small cylindrical insert was manufactured that was inserted into the lens eyepiece holder to get adequate mechanical coupling of the fibers to the CCD face. Images were acquired with a freeware image acquisition tool, SharpCap, and analyzed with theMatlab commercial math package from Mathworks.

RESULTS

Measurements have been performed that show that the detector is able to accurately measuretissue maximum ratio and the relative dose factor. The detector was able to accurately measurephysical wedge factors and made good predictions of the modulation factor for a patient's 7-field IMRT plan.

CONCLUSIONS

This work has shown that relative dosimetry can be performed using an inexpensive liquidscintillation detector. This could be expanded to include an array of liquid scintillator cells formeasurement of beam profiles and other more complex problems.

SU-E-T-21: Modeling a MLC Scatter Source for In-Air Output Factors

PURPOSE

Scattered radiation from multi-leaf collimators (MLCs) is no longer negligible for calculating in-air output ratio, Sc for small and irregular fields often used in intensity-modulated radiation therapy (IMRT). An extra-focal source model for scattered radiation from MLCs, namely MLC scatter source, has been developed to improve the accuracy of the Sc calculation.

METHODS

A conventional dual-source model was made by using Sc data that were measured for collimator-defined fields of Varian Clinac IX linear accelerator. Then, an MLC scatter source at the center of the MLC position of the linear accelerator was assumed in the model. The MLC scatter source model consisted of two Gaussian functions of which parameters were iteratively optimized against the Sc data measured for different MLC fields with fixed collimator sizes. To evaluate the effectiveness of the developed source model, measurements were made for various MLC-defined irregular or square fields. The calculated Sc data by using (1) the developed source model and (2) the conventional dual source model were compared with the measured data.

RESULTS

The mean discrepancy between the measured Sc and calculated Sc from the developed source model was 0.08+-0.28%, while one from the conventional source model was 0.44+-0.39%.

CONCLUSIONS

The developed MLC scatter source model in conjunction with the dual source model could improve the accuracy of the Sc calculation in IMRT fields.

SU-E-T-386: Gamma Analysis of Normalized and Un-Normalized Dose Distributions

PURPOSE

The gamma index method, as currently implemented in all commercial QA software, calls for selection of a normalization point to evaluate agreement between two dose distributions. The implication of this is that there is an infinite number of possible solutions! Which one to pick? A unique and more relevant solution is obtained only if no normalization point is used.

METHODS AND MATERIALS

The set of test cases suggested by the AAPM TG1 19 were planned using Pinnacle 8.0m and delivered on a Varian 21EX linac for 6 and 18 MV photons. The recommended point and planar dose measurements were obtained using a Pinpoint ion chamber, EDR2 film and MatriXX. The gamma index method using typical 3%, 3 mm criteria with and without a normalization point was used to assess the agreement between calculated and delivered planar dose distributions. The analysis was extended to a set of data for clinically treated patients.

RESULTS

The comparison with the TG119 benchmark data showed that all point dose and planar measurements for 6 MV were within the published range. Similar results, although without published data to compare with, were obtained for 18 MV as well. For all complex tests, the percentage of points passing the gamma criteria of 3%, 3 mm was (95.8±1.6)% and (95.6±1.0)% for 6 MV and 18 MV, respectively. Without a normalization point, however, the same gamma analysis fell to (20.7±6.7)% and (13.9±4.0)% for 6 MV and 18 MV, respectively. The clinical data set showed the same trend, with the gamma passing rate declining from (98.9±0.7)% to (33.4±13.1)%.

CONCLUSION

The gamma index method provides a unique answer for gamma passing rate only without normalizing dose distributions to any particular point. The common gamma criteria of 3%, 3 mm, however, is a very poor metric in that case.