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.

Sci-Fri PM: Delivery - 04: Quantitative air communication testing of ion chambers for megavoltage dosimetry

The valid application of the standard correction for air density (P_TP in the TG-51 protocol) requires that for a vented ion chamber (basically all reference-class ion chambers) the air cavity does indeed communicate directly with the external environment. However, this assumption is not tested by users, and not universally verified by calibration laboratories. A system has therefore been developed at the National Research Council to test air communication of cylindrical and parallel-plate ion chambers. The systems is based on measurements in a vacuum vessel with a Sr-90 check source; the procedure is simple and quick and can measure ionization currents over the pressure range 0 kPa (atmospheric) to -20kPa (0.8 atm) with an uncertainty better than 0.2%. Investigation of a wide range of chamber types shows that for a coarse check on chamber performance (i.e., that the chamber is vented to atmosphere) measurement at a single polarity is sufficient (total test time less than 15 minutes) but for accurate characterization of the chamber performance, data at both polarities must be acquired. The accuracy of the system means that it can potentially be used to investigate: i) the validity of the pressure correction, and ii) the source of the polarity correction in cylindrical and parallel-plate chambers. The air communication test will be implemented as part of the standard calibration services provided by NRC for external beam radiotherapy.

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-320: A New Verification Phantom for GYN Brachytherapy Applicators Using GafChromic - Films

PURPOSE

Brachytherapy plays an important role in radiation therapy a wide range of tumor sits such as vaginal, cervical and endometrial cancers. The purpose of this project was to design, fabricate and verify a new phantom for dosimetric verification at small distances from GYN applicators used with GZP6 cobalt-60 HDR system.

METHODS

A new phantom has been designed and fabricated from 90 slabs of 18×16×0.2 cm³ Perspex to accommodate one tandem and two ovoids. The thin layer of the slabs was chosen to place GafChromic films in between the slabs for dosimetry with GZP6 cobalt-60 HDR system. For verification of this device, an assembly composed of a large ovoid size (3cm diameter) and tandem #1 with the least curvature was selected in this study. With this assembly, GafChromic films were exposed using a plan with 500 cGy dose delivery to point "A". The irradiated films were scanned. The responses of the films were converted to dose by calibrating samples of these films using a cobalt-60 teletherapy system in the range of 25 to 800 cGy dose. The measured isodose curves with the films were compared to calculated isodose lines by the treatment planning software.

RESULTS

The Result of these investigations indicated differences of up to ± 23 % between the planning and measured dosimetry at different points in GYN implant with cobalt-60 HDR source of GZP6 system. Therefore, this phantom enabled us to confirm the accuracy of radiation delivery to the GYN patients with cobalt-60 HDR source of GZP6 system.

CONCLUSIONS

The new phantom design could be utilized for the QA procedure of the GZP6 cobalt-60 HDR system as well as the Ir-192 HDR system to confirmation the accuracy of dose distribution in GYN implants, especially in non-traditional implants. The Radiotherapy Department of Shahid Beheshti University at Shohada hospital sponsored the purchase of the phantom materials and films used in the investigations.

SU-E-I-100: Feasibility Study of Gamma-Ray Medical Radiography

PURPOSE

The purpose of this research is to study an alternative technique to conventional x-ray radiography that requires less patient radiation dose, less cost, portable, requires less maintenance, and less power consumption. This research explores the feasibility of using gamma-ray radiography in medical imaging. Gamma-ray medical radiography has the potential to provide alternative diagnostic medical information to X-ray radiography.

METHODS

Approximately one Ci of Am-241 radioactive source which emits monoenergetic 59.5 KeV gamma rays was used in this study. Several factors that influence this feasibility were studied. These were the radiation source uniformity, image uniformity, image quality parameters such as contrast, noise, and spatial resolution. In addition, visual assessment of several human phantom gamma-ray and x-ray images were conducted. The images were recorded on computed radiography image receptors and displayed on a standard monitor.

RESULTS

The radioactive source provided a relatively uniform radiation exposure and uniform images. Image noise was mainly dependent on the exposure time and the source size. Although the contrast depended on the window and level setting, it was also dependent on the exposure time and the source size. Spatial resolution was dependent on the source size and the magnification. The generated gamma-ray images were of lower quality than the X-ray images which was mainly due to the low radioactivity used. However, the gamma-ray images displayed most of the main structures contained in the humanoid phantoms.

CONCLUSIONS

Thisresearch explored the feasibility of using gamma-ray radiography in medical imaging and showed that gamma-ray medical radiography has the potential to provide alternative diagnostic medical information to X-ray radiography. Finally, this research also paves the way for the usage and production of high radioactive Am-241 source that will show high quality medical gamma-ray radiography is feasible.

SU-E-T-570: Improvement to the Histogram Analysis in Radiation Therapy (HART): An Open Source Software System for the Multi-Dimensional Dose- Volume Histogram Analysis in Digital Image Communication in Medicine - Radiation Therapy (DICOM-RT) Treatment Plans

PURPOSE

Histogram Analysis in Radiation Therapy (HART) is an efficient and accurate dose-volume histogram (DVH) computational tool in radiotherapy research. Several applications of the program have been presented previously (J Appl Clin Med Phys 11(1): 3013, 2010; Med Phys 38(6), p.3678, 2011) for the Radiation Therapy Oncology Group (RTOG) users. The program has been further developed to incorporate various types of DVH analysis features to support the research using DICOM-RT plans. The main objective of this work was to present the improvement and compatibility of the program for the DICOM-RT plans.

METHODS AND MATERIALS

MATLAB based codes were primarily designed to read and write a simpler HART format from the standard DICOM-RT data objects exported from the Xio treatment planning system (CMS Inc., St. Louis, MO). This format employed an optimal polynomial fitting technique to interpolate the co-ordinates of the contours in the regions-of-interest. The format was efficient for the (a) precise extraction of the cumulative DVH (cDVH) and spatial DVH (sDVH; x-,y-, and z-DVHs respectively) data- statistics, (b) universal-plan indices evaluation, (c) biological modeling based outcome analyses (BMOA), (d) radiobiological dose-response modeling, and (e) physical parameterization modules. The fundamental DVH statistics were validated using the DVH statistics extracted from the Computational Environment for Radiotherapy Research program.

RESULTS

HART offers various types of DVH computational functionalities, several plan evaluation and radiobiological outcome analysis modules in a user- friendly software package for the RTOG and DICOM-RT planners. The cDVH and BMOA modules were found to be the most applicable features for the global researchers.

CONCLUSIONS

HART is a novel and universal multi-dimensional DVH analysis tool for the radiation therapy research. We further expect to develop HART for the space-time DVH analysis and proton therapy applications. The software is available online (http://www2.uic.edu/∼apyaku1) for the radiotherapy research. This work was partially supported by NIH-NIDCD grant.

SU-E-J-90: 2D/3D Registration Using KV-MV Image Pairs for Higher Accuracy Image Guided Radiotherapy

PURPOSE

In this work, we investigate the impact of using paired portal mega-voltage (MV) and kilo-voltage (kV) images, on 2D/3D registration accuracy with the purpose of improving tumor motion tracking during radiotherapy. Tumor motion tracking is important as motion remains one of the biggest sources of uncertainty in dose application. 2D/3D registration is successfully used in online tumor motion tracking, nevertheless, one limitation of this technique is the inability to resolve movement along the imaging beam axis using only one projection image.

METHODS

Our evaluation consisted in comparing the accuracy of registration using different 2D image combinations: only one 2D image (1-kV), one kV and one MV image (1kV-1MV) and two kV images (2-kV). For each of the image combinations we evaluated the registration results using 250 starting points as initial displacements from the gold standard. We measured the final mean target registration error (mTRE) and the success rate for each registration. Each of the combinations was evaluated using four different merit functions.

RESULTS

When using the MI merit function (a popular choice for this application) the RMS mTRE drops from 6.4 mm when using only one image to 2.1 mm when using image pairs. The success rate increases from 62% to 99.6%. A similar trend was observed for all four merit functions. Typically, the results are slightly better with 2-kV images than with 1kV-1MV.

CONCLUSIONS

We evaluated the impact of using different image combinations on accuracy of 2D/3D registration for tumor motion monitoring. Our results show that using a kV-MV image pair, leads to improved results as motion can be accurately resolved in six degrees of freedom. Given the possibility to acquire these two images simultaneously, this is not only very workflow efficient but is also shown to be a good approach to improve registration accuracy.

SU-E-T-547: Rotating Shield Brachytherapy (RSBT) for Cervical Cancer

PURPOSE

To assess rotating shield brachytherapy (RSBT) delivered with the electronic brachytherapy (eBT) source comparing to intracavitary (IC) and intracavitary plus supplemental interstitial brachytherapy (IC+IS BT) delivered with conventional isotope radiation source.

METHOD AND MATERIALS

IC, IC+IS and RSBT plan was simulated for 5 patients with advanced cervical cancer (>40cc). One BT plan for each patient (fraction 1) guided by magnetic resonance imaging (MRI) was used in our treatment planning system (TPS). A bio- and MRI-compatible polycarbonate (Makrolon Rx3158) intrauterine applicator was simulated for IC and RSBT, and the vienna applicator was simulated for IC+IS BT. 192Ir was used as the radiation source of IC and IC+IS BT; Xoft AxxentTM eBT source was used for RSBT. A 0.5 mm thick tungsten shield was used for RS-BT with different azimuthal and zenith angles. The total dose for each plan was escalated as the external beam radiation therapy (EBRT) plus BT times fraction number (5 in our case).

RESULTS

RSBT and IC+IS BT had higher dose conformity in terms of D90 than IC BT for all the patients. The advantage of RSBT over IC+IS BT was dependent on the shield emission angle, tumor shape and tandem applicator location. The delivery time of RSBT was increased as finer emission angle was selected.

CONCLUSIONS

RSBT is a less-invasive potential alternative to conventional IC and IC+IS BT for treating bulky (>40cc) cervical cancer. RSBT can provide better treatment outcome with clinically acceptable increased delivery time if proper emission angle is selected based on the tumor shape and tandem applicator location. supported in part by NSF grants CCF-0830402 and CCF-0844765; and the NIH grant K25-CA123112, and American Cancer Society seed grant (IRG-77-004-31).

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-T-479: Skin Dose from Flattening Filter Free Beams: A Monte Carlo Investigation

PURPOSE

Flattening filter free (FFF) beams in radiotherapy have advantages such as shorter treatment delivery time and lower out-of-field dose compared with conventional flattened beams. This study investigates in detail the skin dose induced by FFF beams from a TrueBeam accelerator (Varian Medical Systems, Palo Alto, CA) using Monte Carlo method.

METHODS

Phase space files generated using real geometry of a TrueBeam accelerator above the jaws, were used as the input radiation source files in beam simulation for various field sizes using BEAMnrc. Phase space files for various field sizes were generated at the phantom surface. DOSXYZnrc was used for dose calculations in phantom and in patient using the generated phase space files as source input files.

RESULTS

The calculated percentage depth dose curves and profiles in water agreed with measurements within ± 2% for the high dose region and ±2 mm in the penumbra. The peak fluence of a 6 MV FFF beam with the same electron beam incident on the target is about 3 times that of a flattened beam . The mean energy of a 6 MV FFF beam is 0.92-0.95 MeV while it is 1.18-1.30 MeV for the flattened beam. Due to the mean energy difference, the dose in a 6 MV FFF beam is about 6% (of the maximum dose, or 12% of local dose) higher at depth of 1 mm compared with a flattened beam.

CONCLUSIONS

Due to the lower mean photon energy, in an FFF beam the surface (skin) dose is slightly higher compared to the conventional flattened beam of the same field size.

Poster - Thurs Eve-42: A revision of the γ-evaluation: Initial interpretation of dose disagreements on γ-vector fields

Gamma evaluations are a common clinical tool used as a quantitative comparison between dose-distributions, combining both dose difference and distance to agreement criteria. Because gamma evaluations permit rapid analysis of agreement between complex dose distributions, they are often a preferred comparison method for assessing delivery of conformal radiotherapy distributions. Although the comparison provides a useful measure of agreement between distributions when the index is less than one, the scalar gamma value provides little information into the clinical significance or source of disagreements of failing gamma values (i.e., when γ>1). Previously, Stock et al., have presented the gamma angle as an indicator of the relative influence of the distance to agreement versus the dose difference on gamma. We present a modification to the gamma evaluation such that the complete 3D gamma vector information is considered. The predictive nature of each vector component was investigated by simulating various dose disagreements in test distributions. Misalignment tests revealed that the mean gamma vector components indicate the offset direction and relative magnitude for all test distributions. The mean dose component of the gamma vector was prognostic of double Gaussian overdoses and underdoses in a virtual conformal delivery. The response of the vector field depends on properties distinctive to each distribution, such as the local dose gradient. Understanding how these unique properties affect the vector field may permit better diagnosis of dose disagreement sources. Other vector field properties, such as curl and divergence, may yet provide more information for interpreting the cause and significance of γ>1.