Second primary malignancies in thyroid cancer patients

The late health effects associated with radioiodine (¹³¹I) given as treatment for thyroid cancer are difficult to assess since the number of thyroid cancer patients treated at each centre is limited. The risk of second primary malignancies (SPMs) was evaluated in a European cohort of thyroid cancer patients. A common database was obtained by pooling the 2-year survivors of the three major Swedish, Italian, and French cohorts of papillary and follicular thyroid cancer patients. A time-dependent analysis using external comparison was performed. The study concerned 6841 thyroid cancer patients, diagnosed during the period 1934-1995, at a mean age of 44 years. In all, 17% were treated with external radiotherapy and 62% received ¹³¹I. In total, 576 patients were diagnosed with a SPM. Compared to the general population of each of the three countries, an overall significantly increased risk of SPM of 27% (95% CI: 15-40) was seen in the European cohort. An increased risk of both solid tumours and leukaemias was found with increasing cumulative activity of ¹³¹I administered, with an excess absolute risk of 14.4 solid cancers and of 0.8 leukaemias per GBq of ¹³¹I and 10⁵ person-years of follow-up. A relationship was found between ¹³¹I administration and occurrence of bone and soft tissue, colorectal, and salivary gland cancers. These results strongly highlight the necessity to delineate the indications of ¹³¹I treatment in thyroid cancer patients in order to restrict its use to patients in whom clinical benefits are expected.