Thymic uptake after high dose I¹³¹ treatment in patients with differentiated thyroid carcinoma: interpretation and therapeutical management

Full text of publication follows. Aim: I¹³¹ is a very important for initial staging, treatment and monitoring of differentiated thyroid carcinoma after a total thyroidectomy. Doubtful uptake foci may appear due to physiological tracer distribution or true concomitant pathology. Increased functional activity of the thymus may be physiological in children and adolescents. In adults is associated with thymic hyperplasia, primary neoplasms, metastases, inflammatory processes or even in normal parenchyma. Thymic uptake after high dose radioiodine treatment is frequent in young adults and deciding the need of additional therapy is difficult in some cases. Materials and methods: we present 5 cases of female patients with a mean age of 36.6 years(24-43), which were treated with a mean dose of 106 mCi of I¹³¹(100-150 mCi) that showed tracer uptake in the thymic area. A blind therapeutic dose of I¹³¹ was administered to 3 patients due to high Tg levels. In the remaining two, one corresponds to a ¹³¹I-WBS (Whole Body Scan) after remnants ablation treatment and the remaining one to a ¹³¹I-WBS after a treatment dose given for lymph node metastases. The ¹³¹I-WBS was performed 7 days after treatment. Conventional anterior and posterior planar images and SPECT / CT of head, neck and upper mediastinum were performed in all patients. Tg levels were measured with and without hormone replacement therapy in all cases. In every case thymic samples were sent to pathology for analysis. Results: in one patient an elective total thymectomy was performed due to the macroscopic appearance and the prominent tracer uptake. In a second one also a total thymectomy was decided due to the intense tracer uptake and because the patient was catalogued as high risk and had history of several bilateral lung metastases on a previous ¹³¹I-WBS which disappeared. In the remaining three, non additional invasive procedure was considered due to the normal previous pathology reports of the thymus parenchyma from the initial thyroidectomy that involved compartment VII dissection. In the first two cases a thymic hyperplasia was reported. Conclusion: with the characteristics of the cases mentioned above and what its described in literature we can include thymus as a region of normal physiological uptake in most young adults. We also propose to include I-131 therapy as a possible cause of true thymic or transitory hyperplasia in these type of patients. Reminding the importance to always take the precaution of studying each case separately to exclude all risk factors of potential thymic metastasis or primary tumors. (authors)