Association of Breast Cancer Risk After Childhood Cancer With Radiation Dose to the Breast and Anthracycline Use: A Report From the Childhood Cancer Survivor Study

IMPORTANCE: Chest irradiation for childhood cancer is associated with increases in breast cancer risk. Growing evidence suggests that anthracyclines increase this risk, but the outcome of combined anthracycline use and radiotherapy has not been studied. OBJECTIVES: To evaluate breast cancer risk in childhood cancer survivors following radiotherapy and chemotherapy and assess whether risks varied by estrogen receptor (ER) status. DESIGN, SETTING, AND PARTICIPANTS: In a North American hospital-based nested case-control study, a retrospective cohort of 14 358 five-year survivors of childhood cancer, diagnosed from 1970 to 1986 and followed up through December 31, 2016, was analyzed. Cases (n = 271) were defined as women with subsequent breast cancer. Controls (n = 1044) were matched 4:1 to cases by age at first cancer and duration of follow-up (± 2 years). Data analysis was conducted from September 2017 to July 2018. EXPOSURES: Radiation dose to breast tumor site and ovaries and cumulative chemotherapy doses, including anthracyclines and alkylating agents. MAIN OUTCOMES AND MEASURES: Odds ratios (ORs) for subsequent breast cancer by ER status. RESULTS: A total of 271 women served as breast cancer cases (median age at first cancer diagnosis, 15 years [range, 3-20]; median age at breast cancer diagnosis, 39 years [range, 20-57]): 201 invasive (113 ER positive [ER+], 41 ER negative [ER–], and 47 unknown) and 70 in situ breast cancers. The OR for breast cancer increased with increasing radiation dose to the breast (OR per 10 Gy, 3.9; 95% CI, 2.5-6.5) and was similar for ER+ (OR per 10 Gy, 5.5; 95% CI, 2.8-12.6) and ER– (OR per 10 Gy, 4.8; 95% CI, 1.7-22.3) cancers. For women who received ovarian doses less than 1 Gy, the OR per 10 Gy to the breast was higher (OR, 6.8; 95% CI, 3.9-12.5) than for women who received ovarian doses greater than or equal to 15 Gy (OR, 1.4; 95% CI, 1.0-6.4). The OR for breast cancer increased with cumulative anthracycline dose (OR per 100 mg/m(2), 1.23; 95% CI, 1.09-1.39; P