The association between breast cancer and thyroid cancer: a meta-analysis.

Previous studies have suggested an association between breast cancer and thyroid cancer; however, there has not been a formal meta-analysis which collates the existing evidence supporting the hypothesis that breast cancer or thyroid cancer predisposes an individual to developing the other. A systematic search was carried out using PubMed and Medline. We searched for articles containing epidemiological evidence of breast cancer following thyroid cancer and vice versa. Additionally, we searched for articles that included epidemiological data involving the incidence of all second primary malignancies (SPMs) following both breast cancer and thyroid cancer, and compared the datasets. The meta-analysis performed in a total of 18 studies showed that there is a significantly increased risk of developing thyroid cancer as a second primary malignancy of breast cancer (SIR = 1.59, 95 % confidence interval (CI) 1.28-1.99). Additionally, there was marginally increased risk of developing breast cancer as a second primary malignancy of thyroid cancer (SIR = 1.24, 95 % CI 1.16-1.33), compared to the general risk of developing a second primary malignancy following thyroid cancer. The findings suggest that the risk of developing thyroid cancer as a second primary malignancy of breast cancer and vice versa is increased compared to the background risk of developing other SPMs. The risk of developing thyroid cancer after a primary breast cancer was higher than the risk of developing breast cancer as a second primary malignancy of thyroid cancer. This suggests that the effects of treatment-related factors and specific pathological processes of each cancer may contribute to the increased risk rather than common risk factors including genetic factors. Elucidation of the common mechanisms between breast cancer and thyroid cancer will have important implications in both diagnostic and therapeutic management of these cancers. Benefit of thyroid ultrasound screening after breast cancer surgery needs to be assessed.