Occurrence of second primary neoplasms in patients with differentiated thyroid cancer treated with radioiodine

Differentiated thyroid cancer (DTC) is a relatively indolent malignancy and with the current therapeutic strategy of surgery, followed by I-131 ablation and suppressive thyroxine therapy, the 5 year survival exceeds 90%. However, second primary neoplasms (SPN) have been seen to occur in some of these patients during follow up. In the previous decade, few studies proposed an association between 131-I therapy and occurrence of SPNs while many others refuted this. Aim: We aimed to assess the prevalence of second primary neoplasms in patients treated for DTC with radioiodine and their site of occurrence. 4712 patients, who received radioiodine therapy at the All India Institute of Medical Sciences between 1985 and 2012 for DTC, were included in the study. The data was analyzed retrospectively for occurrence of SPNs in these patients. All patients underwent thyroidectomy prior to radioiodine therapy. Low dose I-131 whole body scan was performed to determine site of disease. At our center, we follow a policy of empiric dosing following thyroid hormone withdrawal. Age, gender, histopathology, ablation dose, and subsequent I-131 treatments were recorded. Details of occurrence of the SPN including type, site, date of occurrence, treatment and time interval between the two neoplasms were reviewed. A total of 44 cases (22 male, 22 female) of DTC with SPN were observed in our study. Mean age of these patients was 44.5 years (10-78 years). The most common site of SPN in these 40 patients was breast (10/40; 22.7%), followed by brain (including meningiomas)) 8/44; 18.2%), nerve sheath tumor (3/40; 6.8%), ovary (3/40; 6.8%), Head and neck squamous cell (3/40; 6.8%) and Leukemia (3/40; 6.8%). Lymphoma, adenocarcinoma prostate and renal cell carcinoma were found in 2 patients each (4.5%) while hepatocellular carcinoma; chondrosarcoma, rectal cancer, cervical cancer, angiomyolipoma, giant cell tumor, eosinophilic granuloma, Wilm's tumor were present in one patient each (2.5%). Mean interval between diagnosis of the two malignancies was 62.9 months (1-360 months). Mean I-131 dose given to these patients was 132.93mCi. Of these 44 patients, 22 patients (50%) had SPNs after thyroid cancer while 22 (50%) had SPNs prior to the detection of thyroid cancer. Occurrence of second primary neoplasms is a well known occurrence in patients with differentiated thyroid cancer. Breast cancer was the most frequently associated malignancy followed by CNS tumors in our data. There appears to be no causal relationship between radioiodine therapy and the occurrence of these SPNs, however a meticulous follow up for a longer period may yield newer observations. (author)