Abstract 4827: Does beam radiation treatment of prostate cancer increase rectal cancer risk.
Authors: Morgan John W. , Jabo Brice , Ghamsary Mark , Bush David 
Published: August 2014

Background: Prostate cancer (PC) is the most common invasive cancer among US men. The majority of PCs are organ-confined at diagnosis and are candidates for treatment using external beam radiation alone (RAD), prostatectomy alone (SURG), or other protocols. This research was conducted at Loma Linda University using California Cancer Registry (CCR) population-data for 1988-2010. The CCR is part of the Surveillance Epidemiology and End Results (SEER) program. Since 1988, there has been mandatory state-wide reporting of information for all invasive cancers to the CCR including cancer diagnosis and stage, treatment, and demographic characteristics, with greater than 99% case reporting. From 1988-2010, nearly half a million California men were diagnosed with prostate cancer (PC). The majority were organ-confined at diagnosis.

Problem: We sought to assess whether RAD treatment of prostate cancer, that exposes peri-rectal tissue to ionizing radiation, was followed by increased risk of rectal cancer, relative to SURG.

Methods: We conducted record linkage for all new prostate and rectal (rectum and rectosigmoid junction) cancers in California 1988-2010, identifying men diagnosed with rectal cancer 5+ years following RAD or SURG treatment of organ-confined prostate. Among the men treated with RAD vs SURG, the Cox proportional hazards ratio (HR) for rectal cancer was assessed. Demographic covariates included: age (&lt;50, 50-74, & 75+ years), race/ethnicity as Asian/Other (A-O), non-Hispanic black (NHB), Hispanic (Hisp), and non-Hispanic white (NHW), and socioeconomic status quintiles (1-5 Highest) .

Results: There were 194 new rectal cancers among 54,130 PC cases that had been treated with RAD and 254 cases among 69,105 SURG patients. Adjusting for demographic covariates and year of PC diagnosis, the rectal cancer HR with 95% CI for RAD vs. SURG was: HRRAD/SURG=1.58, 95% CI=1.28-1.94. Following are HRs for age (continuous variable) (HRAge=1.02, 95% CI=1.00-1.34) and categories of race/ethnicity (HRA-O/NHW=0.99, 95% CI=0.66-1.49; HRNHB/NHW=1.09, 95% CI=0.75-1.57; HRHisp/NHW=1.07, 95% CI=0.78-1.47); SES (HRSES1/SES5=0.92, 95% CI=0.64-1.34; HRSES2/SES5=1.17, 95% CI=0.87-1.57; HRSES3/SES5=1.20, 95% CI=0.92-1.57; HRSES4/SES5=1.19, 95% CI=0.93-1.54); and PC diagnostic year (continuous variable) (HRYear=0.94, 95%CI=0.92-0.97).

Discussion/Conclusions: Findings reveal increased rectal cancer hazards among organ-confined prostate cancer patients treated with RAD, relative to SURG, that is substantially independent of demographic covariates. Treatment of rectal cancer among PC patients treated with RAD is further complicated because they may have already received maximum pelvic RAD dose. Further analyses that seek to distinguish roles of different dose and delivery methods for RAD are ongoing.

Citation Format: John W. Morgan, Brice Jabo, Mark Ghamsary, David Bush. Does beam radiation treatment of prostate cancer increase rectal cancer risk. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4827. doi:10.1158/1538-7445.AM2013-4827