Thymic uptake of radioiodine in children and young adults with thyroid cancer

Objectives Radioiodine is used as a diagnostic and therapeutic agent in children and adults with thyroid cancer. Thymic radioiodine uptake not representing cancer has been reported occasionally in adults, but limited information has been published on the incidence of thymic radioiodine uptake in children. The aim of this study is to evaluate the incidence of thymic uptake on whole-body radioiodine scans in children and young adults with differentiated thyroid cancer.

Methods 55 subjects (mean age 15±4y; range 8-29y; 12M, 43F) were treated for differentiated thyroid cancer (51 papillary, 4 follicular and 1 hurtle cell) with I-131 (dose range 28-377 mCi) and had one or more whole body scans between 1992 and 2009 at our institution. Eight subjects received two radioiodine treatments. 63 whole-body scans were performed 4-9 days after radioiodine therapy. 112 diagnostic whole-body scans were performed using either I-131 (37) or I-123 (75), with 55 diagnostic scans performed before and 57 diagnostic scans performed at some time after initial radioiodine therapy. The presence or absence of thymic uptake was assessed independently by two nuclear medicine physicians.

Results Radioiodine uptake was identified in the location of the thymus on 19 of 63 post-treatment scans and 9 of 112 diagnostic scans. Patient sex or age did not predict thymic uptake. On post-therapy scans, thymic uptake did not correlate with I-131 therapy dose. On diagnostic scans, thymic uptake did not correlate with either iodine isotope (I-131 vs I-123) or absence of prior radioiodine therapy.

Conclusions In this group of children and young adults with thyroid cancer, thymic radioiodine uptake was identified in 30% of post-therapy whole body scans and 8% of diagnostic whole body scans. Thus, thymic uptake appears much more frequently in children and young adults than has been reported in the general adult population. Recognizing thymic radioiodine uptake as a potential false-positive finding on radioiodine whole-body scans will help avoid unnecessary evaluation or therapy in patients with thyroid cancer