Risk of thyroid cancer in relatives of patients with medullary thyroid carcinoma by age at diagnosis.

The familial risk of medullary thyroid carcinoma (MTC alone or as part of multiple endocrine neoplasms, MEN2A/MEN2B) is high, so we aimed to answer open questions about the lifetime cumulative risk of thyroid cancer (LCRTC at 0-79 years) among relatives of MTC patients by age and sex. For this nationwide study, a cohort of 3217 first-/second-degree relatives (FDRs/SDRs) of 389 MTC patients diagnosed in 1958-2010 in the Swedish Family-Cancer Database was followed for the incidence of thyroid cancer. The LCRTC in female relatives of patients with early-onset MEN2B (diagnosis age <25 years) was 44-57%, representing 140-520 times increase over the risk in their peers without a family history of endocrine tumors (men: LCRTC=22-52%, 320-750 times) depending on the number of affected FDRs/SDRs. The LCRTC in female relatives of patients with late-onset MEN2B (diagnosis age ≥25 years) was about 15-43% (men=24%). The LCRTC among relatives of early-onset MTC-alone patients was 3-20%. The LCRTC among relatives of late-onset MTC-alone patients was 5-26%. The LCRTC in female relatives of MEN2A patients was 16-63% (men=52%). The relatives of patients with early-onset MTC exhibited a high tendency to develop early-onset thyroid cancer. Simply available data on the number of FDRs and even SDRs affected with MTC and their age at diagnosis were quite informative for the estimation of the risk of thyroid cancer in probands. In settings where genetic testing is not available or affordable for all, evidence-based cumulative risks reported in this nationwide study may help physicians to identify very high-risk individuals.