The incidence of second primary tumors in thyroid cancer patients is increased, but not related to treatment of thyroid cancer.

Objective: The aim of the present study is to assess the prevalence of second primary tumors in patients treated for thyroid cancer. Furthermore, we wanted to assess the standardized risk rates for all second primary tumors, but especially for breast cancer, as data in the literature indicate an excessive risk in differentiated thyroid cancer (DTC) patients for this tumor.

Materials And Methods: We included consecutive patients, who received ablation treatment with I-131 at the Leiden University Medical Center between January 1985 and December 1999 (n = 282). The mean period of follow-up was 10.6 +/- 4.1 years.

Results: Thirty-five of the 282 patients (12.4%) had a second primary tumor (SPT), either preceding or following the diagnosis of thyroid cancer. Five other patients had three primary tumors, including DTC. As a result, 40 additional tumors were found in this group, revealing an overall prevalence of 14.2%. Twenty tumors (7.1%) preceded the thyroid cancer with a mean interval of 5.7 years (range: 0.5-22.0 years), whereas 20 tumors (7.1%) occurred after this tumor with a mean interval of 6.7 years (range: 1.0-15.0 years). In 13 female patients, breast cancer was found as SPT. The standardized incidence rate (SIR) for all cancers after the diagnosis of DTC in this study population was not increased (1.13; confidence interval (CI): 0.68-1.69). However, we found an increased SIR of 2.26 (CI: 1.60-3.03) for all cancers either following or preceding DTC, which is mainly caused by a SIR of 3.95 (CI: 2.06-6.45) for breast cancer.

Conclusion: Patients with DTC have an overall increased standardized incidence rate for second primary tumors, but not for second primary tumors following I-131 therapy. These findings suggest a common etiologic and/or genetic mechanism instead of a causal relation.