Analysis of OAR Doses in DMPO-Based Prostate IMRT

Abstract

Radiotherapy plays an eminent role in the treatment of the prostate cancers with rotational radiotherapy taking the leading edge in the recent years. The study aims to find the relevance of the Direct machine parameter optimization (DMPO) based static gantry IMRT treatment planning in the step and shoot environment for ca prostate case with involved nodes in the era of rotational radiotherapy. Also we aim to determine the relation between the femur dose and the rectum dose, and mean rectum dose and rest rectum volume if any. 32 ca prostate patients were selected retrospectively for this study with prescribed dose of 68Gy (EQD2 72.08Gy α/β 10) and 50Gy in 25 fractions to the prostate and nodes respectively treated with the simultaneous integrated boost technique. The plans were done on a treatment planning system with DMPO algorithm and delivered using step and shoot method in LINAC. For PTV, 95% volume covering 100% prescribed dose was achieved with posterior rectum wall receiving less than 34Gy and organs at risk (OARs) within the tolerance limit. Along with the percentage volume of the rest rectum (rectum minus PTV with 0.5cm margin), the mean doses of rectum, rest rectum, rest bladder (bladder minus PTV with 0.5cm margin) and femur were noted. The relation between rectum mean dose and femur mean dose were studied. Also comparison was done between mean rectum dose and percentage (%)rest rectum volume. The doses received by 5% of the femoral heads were also studied. Plot of mean rectum dose versus mean femur dose received by each patients shows the inverse relation with correlation coefficient of -0.401 (p<0.05). Also the mean rectum dose and %rest rectum volume are inversely related with correlation coefficient of -0.81 (p<0.001). The mean of the dose received by 5% volume of right and left femur is 40.39±4.7Gy and 40.2±3.8Gy respectively with p<0.001. The mean dose of rectum was 36.5±5.5Gy with median dose of 37.6Gy. For rest rectum the mean dose was 28.7±5.6Gy with median dose of 27.8Gy. The mean dose of the left and right femoral heads and rest bladder were 24.2±4.5Gy, 23.7±4.5Gy and 31.6±6.2Gy respectively with p value <0.001. For an optimal plan for ca prostate case achieved on a TPS with DMPO algorithm, we could predict the mean dose of rectum to be 36.5±5.5Gy with 95% Confidence interval for mean femoral head dose of 23.9±4.2Gy. We could also infer that the femur dose and mean rectum dose, and the %rest rectum volume and mean rectum dose were inversely correlated with correlation coefficient of -0.401 and -0.810 respectively. Ca Prostate being the fourth most commonly occurring cancers is widely treated using radiotherapy as an important mode of treatment with different treatment techniques all over the world. Although the rotational radiotherapy delivers the treatment in comparably lower beam on time, the DMPO based static gantry IMRT plans still holds the dosimetric relevance with comparable OAR doses.