Influence of rectum delineation (Rectal volume vs. rectal wall) on IMRT treatment planning of the prostate

Purpose: To evaluate the delineation of either the rectal volume (RV) or the rectal wall (RW) in intensity-modutated radiotherapy (IMRT) for prostate cancer: influence on dose distribution to the targets and organs at risk (OARs) was investigated. Material and Methods: For ten patients with localized prostate cancer IMRT treatment plans were generated with the RV, wall including the filling, and the RW without the Lumen as OAR (plan-RV and plan-RW), respectively. Two different IMRT treatment-planning systems (TPS) were utilized. The influence on target coverage and sparing of OARs was investigated. Results: No influence was seen on target coverage and sparing of the bladder and femoral heads. Doses to the RV were significantly reduced in plan-RV for all evaluated dose levels: maximum 26% and 17%, respectively, in both TPS. The dose distribution to the RW was not significantly different between plan-RV and plan-RW. Conclusion: The different delineation of the OAR rectum significantly affected the inverse IMRT treatment-planning process. The use of the RV as OAR resulted in improved dose distributions to the RV. Therefore, it is suggested using the RV as OAR in IMRT treatment planning of the prostate.