Higher Breast Cancer Risk in Thyroid Cancer Patients, and Vice Versa

The overall risk of developing a second primary thyroid or breast tumor is higher in patients with prior breast or thyroid cancer, according to a retrospective, case-controlled study. This could be due to early detection of the second cancer, but expression of hormone receptors suggests a link in molecular pathogenesis of the two cancers.

“Thyroid cancer (TC) and breast cancer (BC) are the two most common malignancies in females,” wrote study authors led by Young Joo Park, MD, PhD, of Seoul National University College of Medicine in South Korea. “Several lines of evidence suggest that TC and BC occur together in the same individual more frequently than would be expected by chance.”

To test that idea, the researchers conducted a retrospective study including 4,234 women with primary TC, and 6,822 women with primary BC. The results were published online ahead of print in Thyroid.

Of the TC patients, 55 women (4.3%) developed subsequent BC during a 5-year median follow-up period. This yielded a standardized incident ratio (SIR), which compared the incidence to what would be expected in the general Korean population, of 2.45. Similarly, among the BC patients, 81 women (2.6%) developed a subsequent thyroid tumor during a 6.2-year follow-up period. This yielded an SIR of 2.81.

The analysis also found that the expression of both estrogen and progesterone receptors were significantly higher in first primary BC patients who developed TC than in a BC group who did not develop TC. Estrogen receptor expression was 67.9% compared with 55.2%, and progesterone receptor expression was 59.3% vs 46.1% (P < .05 for both).

The authors wrote that those second primary tumors detected within 5 years of the initial primary diagnosis show “favorable histopathological findings and prognosis,” especially in TC patients subsequently diagnosed with BC, and thus close monitoring for BC may be beneficial.

“Although the increased incidence appears to be principally due to increased detection rates, the greater expression of estrogen receptor and progesterone receptor in BC in patients with co-existing TC suggests that a specific molecular pathogenesis might underlie this association,” they concluded.