SU-FF-T-72: Absolute Rectal Volumetric Dose as a Meaningful Predictor to Its Late Side Effect in Prostate IMRT

Abstract

Purpose: To investigate a meaningful correlation between rectal volumetric doses and the rectal late side effect in prostate IMRT.Method and Materials: Six cases with complaints of prolonged rectal bleeding after one year of prostate IMRT course were analyzed. The DVH's were compared with the control group comprised of 14 similar IMRT cases without late rectal side effects. The follow‐up time was 24–45 months. In each case, a total of 75.6 Gy radiation dose was delivered in 32 fractions using 7‐field 18 MV photon beams. All CTV's were delineated on fused MR/CT images. The posterior margins of PTV's were 7 mm for initial 45 Gy and 4 mm for the 30.6 Gy reduction dose. Nonexclusively, PTV protruded into the anterior portion of rectal wall. During patient daily setup, an ultrasound targeting system was employed to ensure the accuracy. Results: There were no significant deviations seen in relative volumetric DVH's between the two groups for rectal D5 through D50 (p> 0.05). However, DVH's for absolute rectal volume indicated significant higher values (p< 0.01) in the group with late rectal side effect, as seen in table1. Conclusions: No statistical meaningful correlation was found in our data between relative volumetric DVH and the occurrence of late rectal side effect. Rectal DVH using absolute volume is suggested by this study to be a more sensitive indicator to predict late rectal side effect in prostate IMRT. This study is valuable for future follow‐ups and investigations.