N. Al Hammadi

SU‐E‐J‐177: Comparison Between VMAT CT Planning and Segmented MRI Images with Assigned Bulk Density: A Dosimetric Study for Intact Prostate Patients

Purpose: Variability in CT delineation and visualization of target volume has long been a problem in radiotherapy. USe of MR based radiotherapy(MR‐RT) is currently being extensively investigated due to the superior soft tissue contrast which can be enhanced by using appropriate pulse sequences. Current MR Systems ensure that there is no loss of geometric accuracy and distortion however lack of electron density information remains a problem for MR‐RT. This is a retrospective study using VMAT planning on segmented MR images with bulk density assigned for intact prostate patients. Methods: MR images for 10 patients were acquired on GE Optima MRI scanner and segmented, the bulk density was assigned as per ICRU 46 to the contours. VMAT plans were created and optimized on Varian Eclipse TPS using the AAA algorithm on the Pseudo CT/ MR study data. The resulting dose distributions were assessed for PTV coverage and OAR constraints to obtain clinically valid plans.The resulting VMAT plan dose distributions were assessed by re calculating the MR optimized plans on the original CT data sets keeping the plan parameters the same. Using plan analysis features available in Eclipse the resulting differences in the dose distributions were analyzed using DVH data as well as slice by slice dose distribution. The TCP and NTCP were calculated for both sets of plans. The optimized results for TCP and NTCP data will be presented for PTVs and OARs together with DVH comparisons. Quantitative analysis of the differences in dose distribution using Gamma Index Analysis will be performed using SunNuclear ArccheckTM and all results will be presented. Results: Detailed data analysis of the 10 Patients results will be presented. Conclusion: Initial assessment of the data indicates VMAT planning on MR only images with appropriately assigned bulk density information is clinically acceptable for intact prostate cases.

SU‐E‐T‐546: On the Evaluation of GATE Monte Carlo Toolkit Performance for the Dosimetry of Ir‐192 and I‐125 Brachytherapy Sources

PURPOSE GATE is a Monte Carlo (MC) simulation toolkit based on the Geant4 package widely used for many medical physics applications. The latest version of GATE extends its applications to radiotherapy. Aim of the current study is to evaluate the validity of the code for accurate brachytherapy dosimetry, since this is a prerequisite for its integration within the armamentarium of validated and useful MC codes for this purpose. METHODS Towards this aim a number of most commercially used HDR and PDR Ir-192 as well as LDR I-125 brachytherapy sources have been simulated using the GATE MC toolkit. Their specific dosimetric parameters following the TG43 formalism have been derived and subsequently benchmarked versus the clinically used dosimetric data which are incorporated in several Brachytherapy treatment planning systems, and versus several MC codes - such as MCNP, EGSnrc, BrachyDose, GEANT, PENELOPE and MCPT - that have been used extensively for brachytherapy dosimetry. RESULTS The comparison of the GATE derived dosimetric results with the aforementioned established MC codes for the dose rate constant, the radial dose function, the anisotropy function as well as the along and away dose rate tables, exhibited an agreement within 3% in all cases. The use of different cross section libraries, material data tables as well as simulation geometries could justify these differences that are, however, comparable to the discrepancies observed among other MC codes for the same source designs. CONCLUSIONS The GATE Monte Carlo toolkit has been benchmarked with well established MC codes in brachytherapy dosimetry and can be safely added within the armamentarium of MC codes used for this purpose. Its versatility and dynamic developing structure promises its use not only in the dosimetric characterization of brachytherapy sources but also in the simulation of real voxel based patient anatomy with the patient related implemented brachytherapy treatment plan.

SU-E-T-97: A Methodology for Using Gafchromic EBT2-Films for Accurate Relative 2D-Dosimetry Without the Need of An Accurate Calibration Curve

PURPOSE Calibration curve (CC) for EBT2 film dosimeters is mainly dependend on film batch, film scanning and analysis conditions. CC errors are translated to 2D dosimetry errors. Aiim of study is to present a methodology for using EBT2-films for accurate 2D relative dose measurements without need of an accurate CC. METHODS A batch of EBT2 films has been used for calibration irradiations using doses up to 2500 cGy. An arbitrary parameter (AP) (inverse pixel intensity using ImageJ software) was used as the dependent variable in a film dose response relationship. Linear dose-response region was evaluated. Within this dose region, the relative changes of dose (D%) equals the relative changes of the APnet = [(AP-APbackground)%]. A film from same batch was irradiated using a Stereotactic Radiosurgery treatment plan (Maximum dose lied within the linear film dose range). The film derived relative dose map was compared against corresponding TPS calculations using Gamma Index (2%, 2mm)Results: Linear Dose AP response was observed for doses up to ∼400 cGy. Therefore, within this dose range, (D%) = (APnet%). 2D map of measured SRS irradiated EBT2 film (Apnet%) values have been measured and compared to corresponding TPS calculation (D%) values. Satisfying agreement between the two data sets was observed (gamma <1 for ∼96% of the pixels). CONCLUSION EBT2 films can be used for accurate 2D relative dose measurements as long as they are irradiated within the dose range of 0 to 400 cGy and their batch linear dose response is ensured. There is no need to ensure a certain slope of the linear response, but just the linearity of response itself. Since films are mainly useful for relative rather than absolute dose measurements, the proposed methodology could offer a simple solution for 2D dosimetry in a large number of radiotherapy QA and/or plan verification purposes.