A.A. van 't Veld

55: TOF-PET scanner configurations for quality assurance in proton therapy: a patient case study

In order to determine the clinical benefit of positron emission tomography (PET) for dose delivery verification in proton therapy, we performed a patient case study comparing in-situ with in-room time-of-flight (TOF) PET. For the in-situ option, we consider both a (limited-angle) clinical scanner and a dual-head scanner placed close to the patient.

SU-E-T-527: Is CTV-Based Robust Optimized IMPT in Non-Small-Cell Lung Cancer Robust Against Respiratory Motion?

Purpose: To determine which proton planning technique on average-CT is more vulnerable to respiratory motion induced density changes and interplay effect among (a) IMPT of CTV-based minimax robust optimization with 5mm set-up error considered, (b, c) IMPT/SFUD of 5mm-expanded PTV optimization. Methods: Three planning techniques were optimized in Raystation with a prescription of 60/25 (Gy/fractions) and almost the same OAR constraints/objectives for each of 10 NSCLC patients. 4D dose without/with interplay effect was recalculated on eight 4D-CT phases and accumulated after deforming the dose of each phase to a reference (exhalation phase). The change of D98% of each CTV caused by density changes and interplay was determined. In addition, evaluation of the DVH information vector (D99%, D98%, D95%, Dave, D50%, D2%, D1%) which compares the whole DVH by η score = (cosine similarity × Pearson correlation coefficient − 0.9) × 1000 quantified the degree of DVH change: score below 100 indicates changed DVH. Results: Three 3D plans of each technique satisfied our clinical goals. D98% shift mean±SD (Gy) due to density changes was largest in (c): −0.78±1.1 while (a): −0.11±0.65 and (b): − 0.59±0.93. Also the shift due to interplay effect most was (c): −.54±0.70 whereas (a): −0.25±0.93 and (b): −0.12±0.13. Moreover lowest η score caused by density change was also (c): 69, while (a) and (b) kept around 90. η score also indicated less effect of interplay than density changes. Note that generally the changed DVH were still acceptable clinically. Paired T-tests showed a significantly smaller density change effect in (a) (p<0.05) than in (b) or (c) and no significant difference in interplay effect. Conclusion: CTV-based robust optimized IMPT was more robust against respiratory motion induced density changes than PTV-based IMPT and SFUD. The interplay effect was smaller than the effect of density changes and similar among the three techniques. The JSPS Core-to-Core Program (No. 23003), Japan Society for the Promotion of Science Grant-in-Aid for Scientific Research (No. 23390300), Grant-in-Aid for Young Scientists (B) (No. 21791194) and Grant-in-Aid for Cancer Research (H22-3rd Term Cancer Control-General-043)

SU‐E‐T‐522: Modeling the Agility MLC for Monte Carlo IMRT and VMAT Calculations

PURPOSE The Agility multileaf collimator (MLC) mounted on an Elekta Synergy linear accelerator for 6 MV was modeled for IMRT and VMAT calculations using the BEAMnrc Monte Carlo (MC) code and verified versus measurements. METHODS To describe the Agility MLC in BEAMnrc, the available Component Module code was modified to include its characteristics; 5 mm leaf width, flat leaf sides with a focus point shifted from the radiation source. The MLC model was verified by comparison of the calculated interleaf leakage and tongue-and-groove effect for a closed MLC field and an irregular field to measurements with EBT2 film in a solid water phantom and diode measurements in a water phantom, respectively. We have developed a time dependent phase space data (PSD), which include a parameter based on MU index. Because leaf, jaw, collimator and gantry positions of each segment are controlled by MU index, this PSD enabled to simulate dynamic motions by interpolating positions between each segment. IMRT and VMAT calculations were compared with film measurements in a solid water phantom to validate the accuracy of the overall MLC model. MC statistical uncertainty was below 2% for all simulations. RESULTS We found a good agreement with our measurements on interleaf leakage. Agreement between mean calculated and measured leaf transmissions with fully opened jaws normalized to the center of a 10×10 cm2 field at the same depth was within 0.1%. Discrepancy between MC calculation and measurement for the irregular field was below 2%/2 mm. The gamma analysis of the comparison of MC and EBT2 film measurements in IMRT and VMAT fields showed 99.1%, 99.5% pass rates with 3%/3 mm criteria, respectively. CONCLUSION The Agility MLC produced by Elekta could be accurately MC modeled with an adaptation in BEAMnrc. The MC model proved to be applicable for IMRT and VMAT calculations. Japan Society for the Promotion of Science (JSPS) Core-to-Core Program.

429 poster AN EFFICIENT AND INDEPENDENT 3D DOSE CALCULATION TOOL FOR QUALITY ASSURANCE OF COMPLEX RADIOTHERAPY TREATMENT PLANS

To validate the model-based 3D dose calculation performed by the COMPASS system, and to evaluate the use of the combination of model-based and measurement-based 3D dose determination for clinical QA purposes.