Author: Min Su 👨🔬
Affiliation: Mount Sinai School of Medicine 🌍
Purpose: To evaluate dosimetric impact of patient setup yaw error to target coverage for pelvis/prostate radiation external beam IMRT-VMAT treatment.
Methods: 9 Pelvis/prostate IMRT-VMAT treatment plans from 6 patients were analyzed in this study. Plans included pelvis, pelvis-SIB, prostate and prostate boost. All IMRT-VMAT plans were created with 2-3 full arc fields in Varian Eclipse V16.1 based on a 0.25 cm slice interval 3D image set. 54 plans were simulated to the patient yaw position error by rotating the couch at ±0.5°, ±1°, and ±2° from their planned treatment position. 14 target coverages, V100% in percentage, were calculated. Total 84 V100% were compared with their treatment plan for the dosimetic impact evaluation. Varian Aria was used as a record/verify system. 207 initial daily fraction setup couch rotation data were collected to observe a patient inter-fraction rotation setup error.
Results: Results showed that the maximum errors of pitch, roll and yaw from an initial daily patient setup were 3°, 2.9°, and 2.8° with an average of 1.247°±1.009°, 0.936°±0.700° and 0.889°±0.671°, respectively. 80 out of 84 comparisons showed V100% reduced. 4 out of 84 comparisons showed V100% improved, which 3 from 0.5° and 1 from 1° yaw error. The maximum improvement was 0.064% seen from a 0.5° yaw error case. In general, a coverage decreasing with the yaw error increasing was seen. The maximum V100% reduced from 0.5°, 1°, and 2° yaw error was 0.603%, 1.571%, and 3.911% with average (0.114±0.122) %, (0.357±0.300) % and (1.138±0.808) %, respectively.
Conclusion: In order to accurately deliver a planned dose, an on-board imager verification system and 6-degree-of-freedom couch are necessary. Several parameters are contributed to a dosimetric impact from patient yaw setup error: 1) A distance of target board from treatment planned isocenter; 2) OARs relative location to the target.