MULTIPLE PRIMARY BREAST AND THYROID-CANCER

The occurrence of breast and thyroid multiple primary cancers was evaluated using data from the Connecticut Tumor Registry. The study population consisted of 1618 women with primary thyroid cancer and 39.194 women with primary breast cancer diagnosed between 1935-1978. Thyroid cancer patients (34) subsequently developed breast cancer and 24 breast cancer patients later had thyroid cancer. A significantly elevated risk of thyroid cancer following breast cancer (SIR [standardized incidence ratio] = 1.68) and breast cancer following thyroid cancer (SIR = 1.89) was demonstrated. The finding was even more notable when compared with the risks obtained for other sites. The elevated risk was particularly evident in women under 40 yr of age at time of diagnosis of the 1st cancer. Analysis by histologic type revealed that the highest risk of 2nd primary breast cancer was found among patients with follicular or mixed papillary-follicular thyroid cancer. Women under age 40 with follicular carcinoma had a 10-fold risk of developing breast cancer (4 observed, 0.4 expected). An enhanced risk of the 2nd primary tumors was evident for the entire period after treatment of the 1st primary, although it was highest within 1 yr after diagnosis of the 1st primary. This may be due to the close medical surveillance of cancer patients which would increase early diagnosis of the 2nd tumors. Breast and thyroid cancer may share common etiologic features.