Variation in the risk of radiation-related contralateral breast cancer by histology and estrogen receptor expression in SEER

Radiation exposure, particularly at a young age, is an established cause of breast cancer. It is not known whether radiation-related breast cancer risk varies by molecular subtype. We characterized the relative risk (RR) of contralateral breast cancer (CBC) related to radiotherapy by histology and estrogen receptor (ER) status of the CBC in five-year survivors in the Surveillance, Epidemiology, and End Results database using Poisson regression models adjusted for attained age and calendar year, age at and year of treatment, ER status of the first breast cancer, and disease stage. 205,316 female breast cancer survivors were followed for an average of 10 years from 1973 until 2007, during which time 6924 women developed a subsequent primary invasive breast cancer in the contralateral breast. The overall RR (and 95% confidence interval (CI)) of radiotherapy-related CBC was 1.11 (1.05–1.16). There was no heterogeneity in risk according to histology of the CBC (P > 0.50) for all ages or young age at exposure, but case numbers were small for subtypes other than ductal and lobular carcinomas. Information on ER status was available from 1990 onwards for 3546 CBC cases, of which 2597 (73%) were ER+ and 949 (27%) were ER−. The RRs were 1.10 (1.02–1.19) for ER+ CBC and 1.19 (1.04–1.35) for ER− CBC (P _difference = 0.33). Among women treated age <35 years, radiation-related risk of CBC was non-significantly elevated for ER− (RR = 1.38, 95% CI: 0.96–1.97) but not for ER+ tumors (RR = 0.80, 95% CI: 0.47–1.35) (P _difference = 0.09). We did not find clear evidence that radiation-related risk varies by histology or ER status, but our findings, which were the first to examine this question, were suggestive of possible differences by ER status that may merit further investigation.