Comparison of rectal dose-volume constraints for IMRT prostate treatment planning

The purpose of this paper is to compare different sets of rectal dose-volume constraints and to develop input criteria for the intensity-modulated radiation therapy (IMRT) of prostate cancer. The IMRT treatment plans were created using Varian planning system (CadPlan with Helios module) for ten patients with localized prostate cancer (isocenter dose 78 Gy). The posterior portion of rectum was contoured as an extra volume (help volume). Three sets of input parameters for rectum with gradually increasing priorities (25-50-75-90) were designed for the inverse treatment planning: 1. dose-volume constraints allowing no more than 50, 40, 20, and 10% of the rectum volume be irradiated to 50, 60, 70, and 75 Gy, respectively-volume-based plans, priorities 25-90 (V25-90 plans); 2. maximum dose constraint of 74.1 Gy for rectum - plans with limited maximum rectal dose, priorities 25-90 (M25-90 plans); 3. maximum dose constraint of 50 Gy for the help volume - plans with limited maximum dose for the help volume, priorities 25-90 (H25-90 plans). Dose homogeneity in the planning target volume (PTV) and the rectal volumes (RV) irradiated to 50, 60, 70, and 75 Gy (RV 50-75 Gy) were recorded. Rectum sparing improved for all the plans with increasing priority. While the M plans had the lowest RV 75 Gy values, the H plans gave the minimum RV 50-70 Gy values. Plans were considered acceptable if at least 98% of the PTV was treated to 95% of the prescribed dose. In particular plan groups, the V75, M75, and H50 plans, together with the V50 + M75 and H50 + M75 combined plans, satisfied this condition and yielded the lowest rectal doses. The H50 + M75 combined plan allowed optimal sparing of rectum (RV 50 Gy 51.6%, RV 60 Gy 38.5%, RV 70 Gy 26.0%, and RV 75 Gy 9.1%, respectively). An optimal set of dose-based rectal constraints (maximum rectal dose 74.1 Gy, maximum help volume dose 50 Gy) has been developed for Varian planning software. These parameters will constitute a starting point for the prostate IMRT plan optimization.