SU‐E‐T‐797: Effects of the Number of Intensity Levels in Step&shoot IMRT for Prostate Treatments

Purpose: Step&shoot IMRTTreatment planning requires the conversion of an optimized fluence into deliverable sequences of MLC segments. The objective of this paper is to study the effects of varying the intensity levels (IL's) number in prostate step&shoot IMRT treatments. Methods: Patient data with prostate cancer and IMRT treatment indication was selected. Treatment planning was done using a 6MV 7‐field arrangement (Primus linac, Optifocus MLC, 10mm leaf‐width) and Konrad v2.2 (Siemens) TPS. The treatment plan includes the simultaneous irradiation of the prostate to 82Gy and the seminal vesicles to 64Gy in 41 fractions. Four plans were generated using 5, 7, 10 and 15 IL's and normalized to deliver the same minimum dose to the prostate. Comparisons between plans were done in terms of total segment number, MU, treatment time and DVH for PTVs and OARs (rectum, bladder and femoral heads). Results: The total segment numbers were 44, 60, 84 and 116 from 5 to 15 IL's. The MU's total numbers decreased with the increase of the IL's (527, 522, 518 and 508). Increasing the number of IL's was accompanied by the treatment times raise (11%, 23% and 37% in relation to 5 IL's, 387seg) and resulted in increases of Prostate PTV dose homogeneity and conformity index (CI). The maximum dose (CI) was 87.7Gy (0.957) and 89.7Gy (0.836) for 15 and 5 IL respectively. No difference in minimum dose and CI was found for the seminal vesicle PTV. All plans showed insignificant OAR DVH differences. Conclusions: The increment in the IL's number resulted in an improvement of the Prostate PTV homogeneity and treatment time without any extra sparing to OAR. The benefit of a better uniform dose in the Prostate PTV should be analyzed for each treatment in order to set planning guidelines.