Purpose: The primary objective of the study was to assess the risk for second primary malignancy after radioiodine treatment for differentiated thyroid carcinoma (DTC). Other objectives were to study the different variables associated with the occurrence of synchronous or metachronous dual malignancies in association with DTC.
Materials and methods: The patient population studied comprised patients with histopathologically proven DTC referred for radioactive iodine treatment after thyroidectomy followed up at a single centre during the period from January 1963 to March 2011. The data collected were analysed with respect to different variables associated with two primary tumours in the setting of DTC.
Results: Out of the total 8614 patients studied from 1963 to March 2011, 44 dual malignancies in association with DTC were detected, showing a prevalence rate of 0.5%. The most common site of second primary malignancy was the head and neck (H&N) in men and the breast in women. Of the 44 dual malignancies, 18 were synchronous in nature and 26 were metachronous. In the metachronous group, thyroid carcinoma was the first primary malignancy in only 5/26 patients, whereas the remaining 21 patients had thyroid carcinoma as the second primary malignancy. In 5/26 patients in whom carcinoma of the thyroid was the first primary malignancy, the second primary malignancy was breast carcinoma in two cases, renal cell carcinoma in one case, colorectal carcinoma in one case and metastatic NET of unknown primary in one case. All these five patients received radioiodine treatment with different activities. In 21/26 patients in whom thyroid carcinoma was the second primary malignancy, the most common first primary malignancy comprised H&N tumours. In this group, 12 patients had a history of external beam radiotherapy (EBRT). Among synchronous malignancies (18/44 patients), H&N tumours were the most commonly associated.
Conclusion: There was no significant risk for second primary malignancy after radioactive iodine treatment in patients with DTC. There is increased risk for thyroid malignancy after EBRT delivered for other primary tumours, especially in children. Hence, any thyroid abnormality on long-term follow-up of patients after EBRT for other tumours should be considered carefully. H&N malignancies are the most common synchronous, metachronous and overall associated second malignancy with thyroid carcinoma.