Poster - Thur Eve - 53: Analysis of the distribution of dose delivery during respiratory-gated step-and-shoot IMRT for lung cancer radiotherapy

Respiratory motion is a large source of dosimetric error when treating lung cancer with Intensity Modulated Radiation Therapy (IMRT). The asynchronicity of the tumour motion and the multileaf collimator (MLC) used to modulate the radiation beam intensity, leads to the interplay effect. One method to account for this effect is respiratory gating. Treatment planning optimization for gated IMRT is performed on a subset average 4D-CT which includes the phases surrounding end exhalation. However, this assumes that the beam delivery will be evenly distributed amongst those phases. This study investigates the distribution of beam delivery during gated step-and-shoot IMRT (SS-IMRT) for both early and late stage non-small cell lung cancer (NSCLC). Four Stage I NSCLC patients, prescribed a dose of 54 Gy in 3 fractions, and five Stage III NSCLC patients, prescribed a dose of 60 Gy in 30 fractions, were retrospectively planned with high and low modulation beams-IMRT, and delivered using the QUASAR™ Programmable Respiratory Motion Platform with 15 mm and 20 mm peak-to-peak sinusoidal motion and real patient breathing motion. The percent monitor units delivered at each phase were compared. For Stage I patients, the monitor units delivered were evenly distributed over the gating window due to a high number of monitor units delivered per control point. For Stage III patients, as the complexity of SS-IMRT increases, there were more monitor units delivered in the initial gating phase. This dose discrepancy could potentially lead to geographic miss of the tumour and should be taken into account during treatment planning.

Poster - Thur Eve - 29: Detecting changes in IMRT QA using statistical process control

Statistical process control (SPC) methods were used to analyze 239 measurement based individual IMRT QA events. The selected IMRT QA events were all head and neck (H&N) cases with 70Gy in 35 fractions, and all prostate cases with 76Gy in 38 fractions planned between March 2009 and 2012. The results were used to determine if the tolerance limits currently being used for IMRT QA were able to indicate if the process was under control. The SPC calculations were repeated for IMRT QA of the same type of cases that were planned after the treatment planning system was upgraded from Eclipse version 8.1.18 to version 10.0.39. The initial tolerance limits were found to be acceptable for two of the three metrics tested prior to the upgrade. After the upgrade to the treatment planning system the SPC analysis found that the a priori limits were no longer capable of indicating control for 2 of the 3 metrics analyzed. The changes in the IMRT QA results were clearly identified using SPC, indicating that it is a useful tool for finding changes in the IMRT QA process. Routine application of SPC to IMRT QA results would help to distinguish unintentional trends and changes from the random variation in the IMRT QA results for individual plans.

Poster - Thur Eve - 25: In vivo dosimetric verification of intensity-modulated radiation therapy

Dosimetric verification of patient treatment plans has become increasingly important due to the widespread use of complicated delivery techniques. IMRT and VMAT treatments are typically verified prior to start of the patient's course of treatment, using a point dose and/or a film measurement. Pre-treatment verification will not detect patient or machine-related errors; therefore, in vivo dosimetric verification is the only way to determine if the patient's treatment was delivered correctly. Portal images were acquired throughout the course of five prostate and six head-and-neck patient IMRT treatments. The corresponding predicted images were calculated using a previously developed portal dose image prediction algorithm, which combines a versatile fluence model with a patient scatter and EPID dose prediction model. The prostate patient image agreement was found to vary day-to-day due to rectal gas pockets and the effect of adjustable support rails on the patient couch. The head-and-neck patient images were observed to be more consistent daily, but an increased measured dose was evident at the periphery of the patient, likely due to patient weight loss. The majority of the fields agreed within 3% and 3 mm for greater than 90% of the pixels, as established by the χ-comparison. This work demonstrates the changes in patient anatomy that are detectable with the portal dose image prediction model. Prior to clinical implementation, the effect of the couch must be incorporated into the model, the image acquisition must be automatically scheduled and routine EPID QA must be undertaken to ensure the collection of high-quality EPID images.

Poster - Thur Eve - 57: Craniospinal irradiation with jagged-junction IMRT approach without beam edge matching for field junctions

PURPOSE

Craniospinal irradiation were traditionally treated the central nervous system using two or three adjacent field sets. A intensity-modulated radiotherapy (IMRT) plan (Jagged-Junction IMRT) which overcomes problems associated with field junctions and beam edge matching, improves planning and treatment setup efficiencies with homogenous target dose distribution was developed.

METHODS AND MATERIALS

Jagged-Junction IMRT was retrospectively planned on three patients with prescription of 36 Gy in 20 fractions and compared to conventional treatment plans. Planning target volume (PTV) included the whole brain and spinal canal to the S3 vertebral level. The plan employed three field sets, each with a unique isocentre. One field set with seven fields treated the cranium. Two field sets treated the spine, each set using three fields. Fields from adjacent sets were overlapped and the optimization process smoothly integrated the dose inside the overlapped junction.

RESULTS

For the Jagged-Junction IMRT plans vs conventional technique, average homogeneity index equaled 0.08±0.01 vs 0.12±0.02, and conformity number equaled 0.79±0.01 vs 0.47±0.12. The 95% isodose surface covered (99.5±0.3)% of the PTV vs (98.1±2.0)%. Both Jagged-Junction IMRT plans and the conventional plans had good sparing of the organs at risk.

CONCLUSIONS

Jagged-Junction IMRT planning provided good dose homogeneity and conformity to the target while maintaining a low dose to the organs at risk. Jagged-Junction IMRT optimization smoothly distributed dose in the junction between field sets. Since there was no beam matching, this treatment technique is less likely to produce hot or cold spots at the junction in contrast to conventional techniques.

WE-G-218-01: Management of RT Patients with Implanted Cardiac Devices: Updated Recommendations?

It has been long time since AAPM published TG-34 on cardiac pacemakers of older technology, which has been the standard document for clinical use, even today, for pacemakers (ICPs) and for defibrillators (ICDs), alike. Management of RT patients with recent technology cardiac devices has been widely published in literature without the provision of a new comprehensive and concise set of recommendations. The various effects of interaction of non-ionizing and ionizing radiations with those devices are crucial to be studied and accounted for during RT treatment deliveries. Thus, AAPM has formed the new TG-203 to work on this issue and provide recommendations to the clinical user for management of patients with cardiac devices when receiving RT. It has been numerous postings that we see in medical physics list server groups inquiring advices on dealing with these devices during patient imaging and radiation treatments. As treatment delivery technologies (IMRT, SBRT, dose escalations, proton beams, etc) and ICP/ICD technology advance, the need to address the management of patients with such devices receiving radiation treatment becomes increasingly important. ICDs offer the same functionality as ICPs, but they are also able to deliver a high-voltage shock to the heart, if needed. Finally, major discrepancies exist among manufacturer recommendations and wide variations exist among radiation therapy facilities regarding patient management precautions.

LEARNING OBJECTIVES

1. Provide a review on sources of potential malfunctions of modern ICPs and ICDs, including malfunction mechanisms from high-LET radiations and transient effects attributed to medical imaging procedures for radiotherapy. 2. Provide a review on management of radiotherapy patients with cardiac implanted devices. 3. Utilize recently available data and computation methods of out-of-field/peripheral dose by scattered photons and secondary neutrons in order to assess cumulative doses on the ICPs and ICDs, during current treatment deliveries (IMRT, SBRT, proton beam therapy, etc). Risk of failure associated with these doses will be discussed. 4. Provide recommendations for management of radiotherapy patients with implanted cardiac devices including the initial patient evaluation stage, dosimetric evaluation to the ICP/ICD during treatment simulation, treatment planning and treatment delivery. Recommendations for the final evaluation of the integrity and functionality of the device after treatment completion will be assessed.

SU-E-T-595: Comparison of Volumetric Modulated Arc Therapy (VMAT) and Static Intensity Modulated Radiotherapy (IMRT) for Malignant Pleural Mesothelioma in Patients with Intact Lungs/Post Pleurectomy

PURPOSE

This planning study compares VMAT and static gantry, sliding window IMRT for malignant pleural mesothelioma for post pleurectomy.

METHODS

We compared plans for a left sided (L) and a right sided case (R). Plans used clinically approved planning target volumes (PTVs) and critical organ contours. IMRT plans employed 7-8 6 MV photon beam directions over a 215° range centered on the ipsilateral lung. VMAT plans used 4 partial arcs within the same range and energy. Prescription dose per fraction was 1.8 Gy; case L went to 50.4 Gy, case R to 46.8 Gy. Planning objectives were: Lyman model NTCP for both lungs < 25%; contralateral lung, mean dose < 8 Gy; heart, V30 Gy < 50%, mean < 30 Gy; Each Kidney, V18 Gy < 33%; liver_not_GTV, mean < 30 Gy, V30 Gy < 50%; stomach not PTV, mean < 30 Gy; cord maximum < 45 Gy; bowel maximum < 55 Gy, D05 < 45 Gy; PTV D95 = 94%, V95 = 94%, D05 = 115%. Dose calculation was done with the AAA algorithm.

RESULTS

VMAT and IMRT both met the dosimetric constraints. The VMAT MU were 887 (L)_and 896 (R) and for IMRT were 1691 (L) and 2409 (R). IMRT required 14-16 fields (wide-field splitting). The delivery times were 8 minutes (VMAT) and 20 minutes (IMRT). For coverage and plan homogeneity parameters within 1.5% - 2%, VMAT better spared organs at risk.

CONCLUSIONS

Both VMAT and IMRT are feasible techniques for the treatment of malignant pleural mesothelioma with intact lungs, with less MU and a shorter delivery time for VMAT. Additional cases must be planned to test generality of our preliminary results.

SU-E-T-463: Biological-Based Optimization and VMAT is Unnecessary for Stereotactic Body Radiation Therapy

PURPOSE

This study shows that there is no clear dosimetric benefit of biological-based optimization for either fixed-beam IMRT or VMAT. Other than shorter delivery times, even VMAT does not offer additional advantage to fixed-beam IMRT.

METHODS

A small number of patients for lung, pancreas, spine and brain CA were planned with fixed-beam IMRT, optimized with (gEUD) and without (DV) biological objectives and, also planned for VMAT with and without gEUD, for comparison. For the lung and brain cases, a non-coplanar 7-11 beam arrangement was used for fixed- beam IMRT and a coplanar 'hybrid' arc simulated VMAT with beams set every 5° spacing. For the other treatment sites, all beams were coplanar. For each case, the fixed-beam IMRT and VMAT plans were optimized with the same objectives. It is important to note that, only 2 segments/beam were allowed for each plan, in order to create small fluence modulation, appropriate for small target volumes during SBRT.

RESULTS

For all plans we noticed that there were minor or no dosimetric differences between fixed- beam IMRT and VMAT, whether DV or gEUD objectives were used or whether fixed-beam IMRT or VMAT is used. Keeping the level of beam modulation as-low-as possible, for small SBRT targets, one can show that VMAT with or without gEUD optimization does not offer any dosimetric advantage against fixed-beam IMRT with multiple non-coplanar beams. This is against the expectation that gEUD-optimization can Result superior plans than DV-optimization. The difference is that, for small target volumes like those encountered in SBRT, the complexity of the fluence is not as high as in large field intensity modulated cases.

CONCLUSIONS

The fact that VMAT with or without gEUD can produce as good plans as fixed-IMRT does not make VMAT a preferred treatment modality, other than the fact that requires reduced treatment time.

TU-F-BRCD-01: Tolerance Levels and Methodologies for IMRT Verification QA

Intensity modulated radiation therapy (IMRT) is a technology intensive treatment modality involving the delivery of highly conformal dose distributions to patients. IMRT is becoming a standard of care for many disease sites and approximately 30%-60% of cancer patients in the United States receive IMRT treatments. Given the complexity of the IMRT treatment planning and delivery processes, a number of AAPM reports and guidance documents addressed the technical aspects of IMRT, including the need for comprehensive acceptance testing, commissioning, and QA programs for IMRT planning and delivery equipment. The implementation of these verification programs is essential to ensure the accuracy of IMRT delivery. Despite the critical role of patient-specific IMRT verification QA to ensure the safe delivery of IMRT treatments to patients as planned, there is little systematic guidance on the type of methodologies, tools, and acceptable tolerance levels that are needed in clinical practice. Furthermore, there are limited discussion on the pros and cons of the different delivery methods for QA measurements, and no recommendations on how to assess the clinical relevance of failed IMRT plans.

LEARNING OBJECTIVES

1. To discuss commonly employed IMRT measurement methods and discuss the pros and cons of each method. 2. To review methodologies for absolute dose verification (single small-volume, 1D, 2D methods), and review dose-difference, DTA, and Gamma analysis techniques including the variability of vendors implementation 3. To review IMRT QA passing rates for given tolerances and action levels, and discuss the clinical relevance of failed IMRT QA.

WE-E-BRCD-01: SBRT Treatment Planning: Practical Considerations

Stereotactic Body Radiation Therapy (SBRT) is being increasingly used in the radiation oncology community to treat tumors in extra-cranial sites such as spine, lung, liver, pancreas and prostate. There are well established RTOG protocols (e.g. RTOG 0631 for spine, RTOG 0813 and 0915 for lung and RTOG 0438 for liver) that specify detailed requirements for treatment planning of SBRT plans. SBRT plans have to meet strict criteria on maximum PTV dose, prescription isodose, prescription isodose surface coverage, high dose spillage and intermediate dose spillage in addition of the constraints of dose limiting organs at risk. This lecture aims to educate members on treatment planning issues for SBRT cases with concentration on strategies to create highly inhomogeneous dose distributions inside the PTV and steep dose gradients in the surrounding normal tissues.

LEARNING OBJECTIVES

1. What characteristics SBRT plans have and how they are different from conventional fractionated radiotherapy plans. 2. For IMRT/VMAT SBRT plans (such as spine SBRT plans that concave distributions around the cord are required), we will discuss strategies that can be employed to overcome many commercial treatment planning system's IMRT algorithms' intrinsic penalty for inhomogeneous plans. Additional issues related to VMAT will be also discussed. 3. For non-IMRT SBRT plans (such as many lung or liver cases where target motion is a concern), we will discuss techniques to control dose inhomogeneities and dose fall off. 4. We will also discuss policy and procedures on SBRT patient setup and verification issues to ensure safe delivery of SBRT treatments.

SU-E-J-102: The Impact of the Number of Subjects for Atlas-Based Automatic Segmentation

PURPOSE

To determine the impact of atlas size on the performance of atlas-based automatic segmentation (ABAS) in delineation of organs at risk for adaptive radiation therapy.

METHODS

A total of 25 patients who had undergone intensity modulated radiation therapy for various head and neck cancers were retrospectively selected for inclusion in a library to be used for ABAS with the MIM VISTA software package (MIM Software, Cleveland OH). Treatment planning computed tomography (CT) scans and subsequent organ at risk (OAR) contours generated as part of the treatment planning process for these patients were added to the library. This library of 25 patients was then successively pruned to generate 5 atlases with 25, 20, 15, 10, and 5 patient subjects respectively. Atlas based segmentation was performed on 10 retrospectively selected treatment planning CT scans to automatically generate right and left parotid glands and brainstem contours. These planning CT scans belonged to a unique set of 10 patient subjects different from the ones used for generating the atlases. One physician (JW), who was blinded to the ABAS results, manually delineated gold-standard contours for the right and left parotid glands and brainstem. Dice similarity coefficients were calculated and analyzed as a function of atlas subject size.

RESULTS

For the sites selected in this study, the performance of ABAS was relatively insensitive to atlas size. Furthermore, some patient subjects were repeatedly selected implying that the adoption of a single standard patient for ABAS may be of benefit.

CONCLUSIONS

Our preliminary results indicate that the performance of the atlas based segmentation module in MIM VISTA Version 5.2 for the organs studied here may be relatively insensitive to the atlas size.

MO-A-BRB-04: Treatment Plan Data Collection, Beam Modeling and Clinical Validation for Unfiattened Beams

The use of an unfiattened photon beam for radiotherapy treatment is a new concept in the Radiation Oncology field. In the past, the non-optimal radiation area coverage obtained with unfiattened photon beams was of concern, and the flattening filter was introduced to overcome this pitfall. With the advance of technology, Intensity Modulated Radiotherapy (IMRT) emerged and non-optimal coverage was no longer an issue since the beam intensity could be modulated to obtain the desired target coverage. Unfiattened photon beams offer several advantages over flattened beams, namely: (1) they provide 2 to 4 times the dose rate, which significantly shortens the treatment time, especially for the high dose irradiation techniques, (2) they provide a purer beam spectrum, which is easier to model in a treatment planning system, (3) the head scatter is dramatically reduced, giving a lower dose outside of the field and (4) they provide a sharper penumbra, which makes planning easier. This lecture will provide an overview of the data collection, the treatment planning system (TPS) parameter modeling and the TPS validation for clinical implementation of unfiattened photon beams.

LEARNING OBJECTIVES

1. Understand the physics data collection and related issues. 2. Understand the TPS parameter modeling, planning validation and related issues. 3. Understand the optimal usage of unfiattened beams with clinical examples.

SU-E-T-596: Choroidal Metastases Irradiation with Volumetric Modulated Arc Radiotherapy

PURPOSE

Intraocular metastases are present in up to 10% of patients with systemic malignancies. With tumor cells spreading hematogenously, the posterior choroid is frequently involved because of its rich vasculature. We treated a woman who developed bilateral choroidal metastases from breast cancer. For the patient, we designed three radiotherapy techniques. The study was to find the dosimetric differences between them.

METHODS

For the patient with bilateral choroidal metastases, three-dimensional conformal radiotherapy (3D-CRT), inverse intensity-modulated radiotherapy (IMRT) and volumetric modulated arc radiotherapy (V-MAT) plans were designed. The prescription dose was 60 Gy in 30 fractions. The dose distribution in the target, the dose to the organs at risk, total MU and delivery time were compared.

RESULTS

In 3D-CRT, IMRT and V-MAT plans, the volume of target in 100% prescription dose were 92.5%, 93.3% and 92.0% respectively. V-MAT plan showed the best conformity and homogeneity in target. IMRT plan showed a better homogeneity than 3D-CRT plan. In 3D- CRT, IMRT and V-MAT plans, the max doses on left lens were 975.3 cGy, 739.9cGy, 870.2 cGy respectively. The max doses on right lens were 929.9 cGy, 802.9 cGy and 889.0 cGy respectively. V-MAT plan showed the lowest value for the max dose on left optic nerve and the lower value for the right one compared to 3D-CRT plan. The MUs were 386, 973, 709 and the delivery times were 10 min, 14min and 1.6 min respectively in 3D-CRT, IMRT and V-MAT. V-MAT could significantly reduce the delivery time.

CONCLUSIONS

V-MAT plan showed the best conformity and homogeneity in target. IMRT showed the lowest value in the max doses to lenses though the delivery time was longest. Overall, V-MAT showed a similar or better dose distribution, and most importantly, improved treatment delivery efficiency. With the results, V-MAT technology could be expected to choroidal metastases.

SU-E-J-47: Evaluation of the Mismatches Between DRR's and Port Films for IMRT Fields

PURPOSE

To quantify the frequency and magnitude of mismatches between the MLC shape on the DRR created by the Pinnacle planning system and the corresponding port film MLC shape generated by the Mosaiq record-and-verify system for IMRT fields.

METHODS

A retrospective review was made of the most recent 60 patients to receive IMRT at our clinic. The MLC shape on the DRR created by the Philips Pinnacle planning system for each treatment field was reviewed (573 fields total) and compared with the MLC shape of the port film image, which was generated by the Mosaiq treatment planning system.

RESULTS

Of the 60 patients studied, 20 had at least one leaf mismatched between the MLC shape on the DRR and port film (142 of the 573 fields). Of the affected cases, on average 59% of the fields had a mismatch. The affected fields had an average of 6.7 leaves mismatched with a mean discrepancy of 27mm. The average maximum discrepancy for each affected patient was 69mm. Discrepancies were most common for head and neck cancers.

CONCLUSIONS

The MLC shape Mosaiq generates for the port film is the CIAO, or Complete Irradiated Area Outline, which is the area that is actually treated. The Pinnacle DRR displays the maximum leafmotion, which can be different for larger fields in which the MLC leaves abut within the collimator jaw opening. The discrepancy can create substantially different MLC shapes. The problem can be solved by not filming the MLC shape and only using the films for isocenter placement; however, displaying the area receiving treatment can be a useful safety check, possibly preventing a treatment error. The persons assessing the films must be aware of this issue and evaluate the films carefully.

SU-E-T-545: Dose Comparison between Intravenous Contrast-Enhanced CT and Non Contrast CT in Treatment Planning

PURPOSE

With increasing concern for patient dose from CT scan, we are trying to reduce CT scan and use intravenous contrast-enhanced CT (contrast CT) in treatment planning. This study is to investigate dose calculation accuracy using contrast CT in treatment planning for lung, esophagus and pancreas cancer.

METHODS

We analyzed treatment plans for 8 patients for whom CT simulation was performed both with and without intravenous contrast agent (CA) (non-contrast CT). IMRT/3D plans were generated with inhomogeneity correction on the non-contrast CT scan. Contrast CTs were fused to the non-contrast studies and all contours and plans were copied to the contrast CT scans. For each patient, we analyzed dose-volume histograms (DVH) for planning volumes (PTV) and the organs-at-risk (OAR), comparing the doses generated on non-contrast CT scans with those generated on contrast CT scans.

RESULTS

Maximum doses ratio Dmax(contrast)/Dmax (non-contrast) in PTVs was 1.0009±0.0013. The ratio of D05 (contrast)/D05 (non-contrast) was 0.996±0.005. The ratio of mean PTV dose Dmean(contrast)/Dmean(non-contrast) was 0.990±0.005%. The ratio of minimum dose Dmin(contrast)/Dmin(non-contrast) and D95(contrast)/D95(non-contrast) were 0.970±0.030 and 0.984±0.009, respectively. Contrast CT raised cord dose slightly. The ratio of cord Dmax was 1.005±0.026. However there were two cases the ratio of cord Dmax were 1.035.

CONCLUSIONS

The PTV D95 is usually normalized to prescription dose and the D95 differences between contrast and regular CT were within 2%. In most cases, the contrast CT could be used to treatment planning clinically. However more attention should be paid to maximum cord dose if it is already close to criteria limit.

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.

SU-E-T-583: Feasibility of Constraining Dose to the Nausea Center (area Postrema and Dorsal Vagal Complex) in IMRT Treatment Planning of the Head and Neck

PURPOSE

Nausea and vomiting have been known to occur in patients undergoing external beam radiation treatments for head&neck cancers. We sought to determine the feasibility of limiting the dose delivered to the nausea center, area postrema (AP) and dorsal vagal complex (DVC), for these patients without compromising target coverage and critical organ doses.

METHODS

In a retrospective study 23 oropharyngeal cancer patients were identified as being treated with definitive or adjuvant radiotherapy at Memorial Sloan Kettering Cancer Center. Patients were treated solely with external beam radiation using intensity modulated radiation therapy (IMRT). The nausea center was carefully contoured in the treatment CT with the assistance of a board certified neuroradiologist. The doses delivered to the nausea center were calculated for each plan delivered. Cases were replanned offline to determine the lowest achievable nausea center dose that does not compromise the overall PTV coverage or critical structures doses, these being brainstem, spinal cord, cochleas, and temporal lobes.

RESULTS

Patients reporting higher nausea grade had median AP and DVC doses of 38.7Gy and 40.4Gy, respectively. Patients reporting higher vomiting grade had median AP and DVC doses of 39.5Gy and 44.7Gy, respectively. Replanning resulted in reduced dose to AP by an average of 18% and to the DVC by an average of 16% while maintaining adequate target coverage and doses to the critical organs the same or decreased by 1-4% . We aim to achieve a max dose of 36Gy to AP and 38Gy to DVC for these cases.

CONCLUSIONS

It is feasible to limit the doses to the nausea center without compromising target coverage or critical organ limits for oropharyngeal cancer patients undergoing IMRT treatment. Clinical results indicating an association between radiation dose to the nausea center and development of nausea and/or vomiting can potentially be addressed by implementing this technique.

SU-E-T-86: Development and Implementation of the Use of Optically Stimulated Luminescent Detectors in the Radiological Physics Center Anthropomorphic Quality Assurance Phantoms

PURPOSE

To study the angular dependence of optically stimulated luminescence dosimeters (OSLD) in the Radiological Physics Center anthropomorphic quality assurance pelvic phantom to provide accurate dosimetric measurements as a replacement for TLD.

METHODS

A spherical phantom was constructed to investigate the angular response of the OSLD as oriented in the RPC pelvic phantom. Three OSLD per irradiation angle, placed at the center of the spherical phantom, were irradiated with 100 cGy from six different angles. The angular response at each angle was determined relative to the OSLD response when the beam was incident normally on the OSLD surface. A pelvic phantom dosimetry insert was modified to include both TLD and OSLD. Three treatment plans were developed in Pinnacle v9.0 and one in Accuray's Multiplan, each with increasing angular beam delivery (4 field, IMRT, SmartArc, CyberKnife) for the pelvic phantom using a common dose prescription and constraints. Each plan was delivered to the phantom three times, containing two TLD and two OSLD, oriented in the transverse plane, at the center of the PTV. The dose delivered to the TLD and OSLD was calculated for each treatment and then compared.

RESULTS

The angular dependence correction factor for the spherical phantom was found to be uniformly 1.041 ± 0.003 from single beam edge-on irradiations. The angular dependence correction in the pelvic phantom from multiple beam orientation irradiations was 1.024 ± 0.002, such that the OSLD dose agreed with the TLD dose. Applying the OSLD pelvic phantom correction factor, the RPC measured dose to planning system calculated dose ratio was 0.995 ± 0.009. The established RPC phantom TLD dose to calculated dose ratio was 0.995 ± 0.010.

CONCLUSIONS

An anthropomorphic phantom OSLD angular dependence correction factor was established such that the final OSLD dose measurements agreed with RPC's TLD dose measurements to within 1%. Work supported by grant CA 10953, awarded by NCI, DHHS.

SU-E-T-387: Validation of a New System for Patient Specific IMRT QA and Comparison with Other Commerical Systems

PURPOSE

The focus of this project is to compare the Octavius 4D with current commercial available dose validation systems: MatriXX MultiCube and Delta4.

METHODS AND MATERIALS

Many challenges are faced with properly measuring Intensity Modulated Radiotherapy (IMRT). It has become common practice for clinics to use film, arrays, or multiple detectors to validate dose measurements pretreatment for static and dynamic treatments. IMRT QAs for various treatment sites were measured for patients using three different dose validation systems. All measurements were taken on a Varian CLinac 2100 C/D, SN-757, 80 MLC with 6MV. The treatment plans evaluated were Step-N-Shoot. Data analysis was performed using the software provided with each dose validation system. Detailed information was gathered from each system with their perspective advantages. The latest system, Octavius 4D, allows one to calculate the Gamma Index for Coronal, Sagittal, and Transversal views for every slice included in the measurement along with the traditional data analysis provided; histograms, horizontal and vertical profiles, DTA.

RESULTS AND DISCUSSION

The Gamma Index values were observed using the MatriXX Multicube, Delta4, and Octavius 4D. The treatment plan included five fields at various gantry angles. Also the gamma index and profiles were calculated for various treatment sites. Delta 4 and the Octavius 4D appears to be quite comparable. Each device has the ability to allow one to verify segmented and composite fields, measure dose profiles and analysis using the Gamma Method. ConclusionsSimilar IMRT QA measurements will be made for more Step-N-Shoot cases with the addition of SmartArcs. The limitations of each system will be determined for each system using the Gamma Index as a reference while varying the Region of Interest, Threshold, and Gamma Method (local, normalization, and maximum dose), as well as the 2D- profiles for these cases.

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-T-476: GPU-Based Monte Carlo Radiotherapy Dose Calculation Using Phase- Space Sources

PURPOSE

To design an efficient method for utilizing phase-space source models in the GPU-based Monte Carlo (MC) dose calculation engine gDPM.

METHODS

In GPU-based MC algorithms, particles are transported in parallel on different threads. Particles of different types and energies can require significantly different execution times. This can cause "thread divergence" and lower efficiency when source particles are read sequentially from a phase-space file. We have developed a strategy for utilizing phase- space files in a GPU compatible manner whereby the particles are grouped into phase-space-lets (PSLs) by type, energy, and location in the phase- space plane. This allows for dose calculations using only particles inside the field opening defined by the secondary collimators. For validation, the gDPM PSL implementation is compared with DOSXYZnrc using a BEAMnrc phase-space source model as input.

RESULTS

Two phase-spaces were generated using a BEAMnrc head model of a 6MV Varian Clinac 21EX, one above the upper jaws used to generate PSLs for gDPM and the other below the lower jaws used for DOSXYZnrc dose calculation. Profiles and depth dose curves for a variety of field sizes were generated in a water phantom. The agreement between gDPM and DOSXYZnrc is within 2% for all field sizes. For the 10 cm × 10 cm field, the calculation times of 650 million histories were 147 CPU hours and 54 GPU seconds for DOSXYZnrc and gDPM, respectively. In addition, we have tested the gDPM PSL implementation for dose calculation in a realistic 7-field IMRT tongue treatment plan. The calculation times were 59 CPU-hours and 66 GPU- seconds for DOSXYZnrc and gDPM for 485 million histories, respectively. Gamma pass rate for the two dose distributions was 99.54% for 3 mm/3% criteria within the 10% isodose.

CONCLUSIONS

Methods for the efficient use of phase-space sources for GPU-based MC dose calculations have been developed.

SU-E-T-632: Metrics for Comparing Dose Volume Histograms

PURPOSE

The process of IMRT planning is an iterative inverse process, where a planner seeks to attain a desired dose distribution, specified for tumors and OARs, hence creating a plethora of competing constraints. However, the final product is rather the unpredictable outcome of a series of trial-and-error attempts at meeting these competing objectives. A key tool to inspect the quality of a plan is the DVH. We provide a set of metrics for unbiased DVH comparison.

METHODS

The treatment of one prostate case was planned by seven planners, while imposing the same clinical objectives for PTV and OARs. The resulting seven DVHs were compared based on their deviation of from the ideal coverage, namely 100% of PTV receiving 100% of the dose. Our deviation function L measures the area between the ideal and realized DVH. A weight function W is used for dose dependency. We employ a set of Ws: constant, piece-wise linear, quadratic, normal and beta-distributional.

RESULTS

We show that the deviation of some planners are consistently higher than others for all eight Ws. Similarly, we observe that one of the planners consistently exhibits the lowest deviation, while another one is low for linear and quadratic Ws and worsen for the distributional functions independent of their slope and range. Further, the normal and beta-density function weights, as they primarily penalize the range of 95% < Dose < 105%, hence discriminating only deviations in the respective region.

CONCLUSIONS

The outcome of treatment planning is strongly related to the planning personal. The proposed set of DVH metrics allow for unbiased comparison, beyond the visual inspections. An optimized superposition of these metrics may yield a practical tool for daily treatment planning. Furthermore, these results exhibit the need for methods whose outcomes are independent of the planning personal.

SU-E-T-622: A Rapid Hybrid VMAT-IMRT Planning Method Using an Abbreviated Beam Angle Optimization Search

PURPOSE

To generate hybrid VMAT-IMRT treatment plans by utilizing an IMRT beam-angle-optimality (BAO) search under a commercially available TPS without the aid of custom optimization software or high performance computing.

METHODS

The high dose modulation provided by IMRT beams can be exploited to improve the quality of VMAT treatments. To achieve this, a VMAT treatment was created where the gantry pauses at predetermined angles to deliver IMRT segments. To determine IMRT BAO, an IMRT plan ('poly-IMRT') was made with many beams (>30) equally spaced around the patient. For practical reasons and to reduce the total time to approximately 1 hour, BAO was approximated by removing one beam out of the set and noting the new objective score. Determining this'score penalty' for each of the beams serves as a proxy for true BAO. The hybrid plan was created by combining the VMAT arc with a user-determined number of top-ranked beams from the poly-IMRT set. The BAO from this approach was compared with a more rigorous method ('VMAT+1'), in which a VMAT plan was optimized with 1 IMRT beam at various angles, allowing a direct determination of objective score versus gantry angle. The overall hybrid planning process was demonstrated by creating separate plans for a SBRT lung patient, with dose normalized to the limiting maximum aorta dose.

RESULTS

Large score penalties from poly-IMRT coincided with large score benefits from VMAT+1, indicating both methods identified the same optimal beams. The VMAT, IMRT, and hybrid plans delivered the prescription dose to 84.3%, 85.6% and 87.7% of the PTV and had homogeneity indices of 1.38, 1.41, and 1.32 respectively. Normal tissue doses were within 0.5%.

CONCLUSION

The presented method can create hybrid VMAT-IMRT plans which combine delivery efficiency with improved target coverage. The planning process takes about an hour using a standard TPS.

SU-E-J-22: Effect of MLC Leaf Width on MLC Leaf Shifting Algorithm for Concurrent Treatment of Prostate and Pelvic Lymph Nodes

PURPOSE

Our previous study showed that adjusting selected MLC leaf pairsto follow prostate movement is an effective strategy to account for daily prostate displacement during concurrent treatment with pelvic lymph nodes. MLC leaf width affects the quality of MLC shifting plans for longitudinal prostate motion compensation. This study is to investigate the effect of the MLC leaf width in compensation of the prostate movement.

METHODS

Fifty-one daily CT on-rail scans from three patients were available for this study. On these CTs, the prostate, bladder and rectum were manually contoured, and the lymph nodes contours were transferred from the planning CT after rigid bony registration. For each patient, three different IMRT plans were created based on a planning CT using leaf width of 2.5, 5, and 10 mm, respectively. For each CT, the prostate displacement was determined by dual imaging registration and compensated by shifting MLC resulting in a total of 153 MLC shifted plans.

RESULTS

Among 51 daily CTs, the average prostate movement along the superior/inferior direction was 1.1±3.7 mm (range: -6 to 6.5 mm). The differences in D99 of the prostate between the dose of the day and dose of the plan were 2.3±3.3%, 1.3±2.0%, and 4.4±5.1% for 2.5, 5, and 10 mm leaf width plans, respectively (p<<0.05). The corresponding differences in D99 of the lymph nodes were 0.7±0.9%, 0.6±0.9%, and 1.4±0.8%. The mean differences in D50 were 0.8%, 1.6%, and 2.7% for the bladder, and 10.0%, 3.9%, and 5.7% for the rectum, respectively.

CONCLUSIONS

Using the MLC Shifting method to compensate for prostate movement in the longitudinal direction depends on the MLC leaf width and the magnitude of the prostate motion. The use of leaf width of 5 mm can provide sufficient tumor coverage without significantly affecting doses to the critical structures.

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-525: Dosimetric Validation of the Algorithm Based on Linear Boltzmann Transport Equations for Photon 4MV Dose Calculation

PURPOSE

To evaluate a dosimetric accuracy of AcurosXB dose calculation algorithm for 4 MV photon beam.

METHODS

Four MV beam (Clinac-6EX) and AAA and AcurosXB algorithms (pre-release version 11.0.03.) were used in this study. The differences of the calculation with AAA (EAAA) and AcurosXB (EAXB) to the measurement were evaluated in the depth doses to 25 cm depth and dose profiles within the water and slab phantoms (water, lung and bone equivalent). In addition, the clinical cases, including three whole breast plans and three head and neck IMRT plans, were evaluated. First the AAA plans were calculated, then AcurosXB plans were recalculated with dose-to-medium with identical beam setup and monitor units as in the AAA plan.

RESULTS

In the water phantom study, the EAAA and EAXB were up to 2.2% and 1.5% in the depth doses for the open field (field size = 4 - 40cm square), respectively. Under the heterogeneity conditions, the EAAA and EAXB were less than 4.4% and 2.2% in lung region, and less than 12.5% and 6.3% in bone region, respectively. In the re-buildup region after passing through the lung phantom, the AAA overestimated the doses about 10%; however AcurosXB had good agreement with measurement within 3%. Dose profiles with AcurosXB were better agreement with measurement than AAA. In the clinical cases, the dose of the skin surface region with AcurosXB were higher than AAA by at least 10%, and the dose differences over 5% appeared in heterogeneous region. However, DVH shapes of each organ were similar between AAA and AcurosXB within 2%.

CONCLUSIONS

In phantom study, AcurosXB had better agreement to measurement than AAA, especially in heterogeneous region and re-buildup region. In the clinical cases, there were large differences between AcurosXB and AAA in the surface region. Evaluation Agreement of non-clinical versions of Acuros XB with　Varian Medical Systems.