Multiple primary breast and thyroid cancers: role of age at diagnosis and cancer treatments (United States).

BACKGROUND: Breast and thyroid cancer have been observed to occur more frequently than expected as multiple primary tumors in women. The study presented herein focuses on the effects of age at diagnosis and treatment for the first cancer on the development of the second cancer.
METHODS: This retrospective cohort study used a study population consisting of 38,632 women diagnosed with primary invasive breast cancer and 2189 women diagnosed with primary invasive thyroid cancer between 1974 and 1994. Cases were identified from records of the Cancer Surveillance System of western Washington and followed for subsequent cancer development through 1995.
RESULTS: Seventy-one women were diagnosed during their lives with both breast and thyroid cancers. Including cancers diagnosed during the same month as or after the initial cancer, the relative risk (RR) of breast cancer among women with thyroid cancer was 1.5 (95% confidence interval [CI] 1.1-2.0), and the RR of thyroid cancer among women with breast cancer was 1.5 (95% CI 1.1-2.2). Among women with thyroid cancer, risk of breast cancer was greatest when the latter cancer was diagnosed under 45 years of age (RR = 2.3, 95% CI 1.1-4.4). First course of treatment, including radiation or hormonal therapy to treat thyroid cancer, and radiation, chemotherapy, or hormonal therapy to treat breast cancer, did not alter a woman's risk of developing the second cancer.
CONCLUSIONS: The data suggest that the incidence of breast and thyroid cancer may be related, and that in particular women with thyroid cancer may be at a moderately increased risk of developing breast cancer before age 45.