SU-E-T-227: Re-Evaluation of Dose Distribution and Margins for IMRT Prostate Plans

To evaluate the dose distribution on prostate and proximal seminal vesicles (SVs), rectum, and bladder when a certain geometry uncertainty is occurred during planning or delivery, and further determine the optimistic margin relation to different directions.

Thirty prostate cancer patients with prostate specific antigen (PSA) less than 10 and Gleason Score between 2 and 6 have been selected and planned in EclipseTM 10.0 treatment planning system. PTV is expanded from GTV (prostate plus proximal SVs) with uniform 6 mm margin in all directions. All patients are head-first-supine and planned with seven beam IMRT technique. At least 95% of PTV is covered by full prescription. Rectum V65 is less than 17%, rectum V40 is less than 35%, bladder V65 is less than 25%, and bladder V40 is less than 50%. To simulate the deviation from planning error or setup uncertainty, the original isocenter of each plan has been shifted every 2mm from 0mm to 10mm in superior (S), inferior (I), right (R), left (L), anterior (A), and posterior (P) directions. The dose is then re-calculated with fixed jaw technique. The new plan parameters such as PTV coverage, both bladder and rectum V65 and V40 are analyzed.

PTV full dose coverage is linearly decreasing with increasing isocenter shift and the absolute slope mean values are 2.2, 2.2, 3.0, 2.7, 2.0, and 2.0 corresponding to S-I-P-A-L-R directions. As for rectum and bladder, the further away from the shifted isocenter, the smaller values of V65 and V40.

This study shows that 2 to 3 percentage of decrements on PTV full dose coverage occurs while prostate target shifts every millimeter. To uniformly cover the dosimetric impact uncertainty, it shows that the optimistic margin ratio would be 1.1(S):1.1(I):1.5(P):1.35(A):1(L):1(R) for the six directions.