A significant decrease in rectal volume and diameter during prostate IMRT

A significant decrease in RV and RD occurs during prostate IMRT delivery. More than half of patients had decreased RV and over a third had decreased RD. This observation is pertinent to prostate localization, planning margins, and implies that dose-volume histogram (DVH) analysis of rectal irradiation based on pre-treatment CT scanning may inaccurately estimate the risk of rectal toxicity when the initial RV is larger than 70 cm3.