The development of breast carcinoma in women with thyroid carcinoma.

BACKGROUND: Breast carcinoma and thyroid carcinoma are two malignancies that occur most commonly in women. An association between the incidence rates of thyroid and breast carcinoma in women after a diagnosis of the other malignancy has been suggested in a retrospective analysis of a single institution's tumor registry. In that study, an increased incidence of breast carcinoma in premenopausal women previously treated for thyroid carcinoma was observed.
METHODS: The purpose of this study was to investigate further this relation utilizing a large database, the National Cancer Institute's Surveillance, Epidemiology and End Results (SEER) database. The SEER database is maintained by the National Cancer Institute, and it represents 11 population-based cancer registries covering approximately 14% of the United States population. The study was a population-based retrospective cohort analysis using external comparisons. From 1973 to 1994, 365 women in the SEER database were identified as having both thyroid and breast carcinomas. The SEER database from 1973 to 1994 was utilized to calculate age specific and calendar year specific incidence rates for each year for thyroid and breast carcinomas. The expected number of second cancers for each age group, calendar year, and follow-up period were determined by multiplying these incidence rates by the age specific and calendar year specific number of person-years at risk. The risk ratio (RR) was calculated by dividing the observed by the expected number of second cancers. Statistical significance was determined by the Poisson test.
RESULTS: A total of 1,333,115 person-years were available for analysis. One hundred thirteen thyroid carcinoma cases were diagnosed after breast carcinoma cases (RR, 0.99; P = 0.576). Two hundred fifty-two breast carcinoma cases were diagnosed after thyroid carcinoma cases (RR, 1.18; P = 0.007). Premenopausal women (age 20-49 years) with an index thyroid carcinoma have a significantly increased risk of developing subsequent breast carcinoma (RR, 1.42; P = 0.001). Black premenopausal women with an index thyroid carcinoma do not have an increased risk of developing breast carcinoma, but the statistical power is lower due to low numbers. No women with index breast carcinoma have an increased risk of developing thyroid carcinoma.
CONCLUSIONS: Women with a history of thyroid carcinoma have a greater than expected risk of developing breast carcinoma. This risk is most pronounced in premenopausal white women. The implications of this observation with respect to breast carcinoma screening guidelines and thyroid carcinoma treatment guidelines deserve further investigation. Copyright 2001 American Cancer Society.