Dose/Volume Relationship of Late Rectal Bleeding After External Beam Radiotherapy for Localized Prostate Cancer

Abstract

To date, five patients have had grade 1 late rectal toxicity (one CRT case and four SCIM-RT cases), one patient had grade 2 late rectal toxicity (CRT), and three patients had grade 3 late rectal toxicity (all CRT cases). Because of the low number of events, the analysis was performed with all patients experiencing rectal bleeding grouped together. The actuarial rectal bleeding rates at 18 and 24 months were 6% and 8%, respectively. The actuarial rectal bleeding rates at 24 months were identical (8%) for both SCIM-RT and CRT. A multivariate analysis of the following parameters was performed to determine independent predictors of rectal bleeding: age (continuous variable), race (Caucasian vs African American), coverage of seminal vesicles (yes vs no), adjuvant androgen deprivation (yes vs no), technique (CRT vs SCIM-RT), Radiation Therapy Oncology Group acute rectal toxicity score (continuous variable), VrPr (continuous variable in cubic centimeters), and %VrPr (continuous variable). Only the VrPr (cubic centimeter) was an independent predictor of rectal bleeding; %VrPr was not. With different cut-off levels being used, a VrPr of 15 cm3 was significant on univariate analysis; the actuarial rectal bleeding rates at 24 months for patients with a VrPr < or = 15 cm3 versus a VrPr > 15 cm3 were 5% versus 22%, respectively. CONCLUSION> In our study sample, which included both conformal and intensity-modulated radiotherapy patients, the volume of rectum receiving the prescribed radiation dose (the equivalent of 78 Gy) was an independent predictor of late rectal bleeding. The percent of rectal volume receivingthe full dose was not. Using actual volume rather than percent volume also avoids the dependence on the extent of rectal volume contours. We recommend 15 cm3 as the cut-off of the rectal volume not to exceed the prescription dose. The rectal bleeding rate at 2 years for cases with < 15 cm3 receiving the full dose was only 5%.