Clinical conditions related to positive thymic uptake on I‐131 post‐therapeutic scans in thyroid cancer patients

Abstract

Objective Thymic uptake is a well‐known cause of false‐positives on I‐131 post‐therapeutic scans. This study identified the clinical conditions associated with thymic uptake on I‐131 post‐therapeutic scans in thyroid cancer patients. Design This was a retrospective study that investigated the clinical conditions associated with thymic uptake on I‐131 post‐therapeutic scans of patients obtained between January 2010 and December 2010. Patients: Six hundred and eighty‐five patients were included follwing a therapeutic dose of I‐131 (3.7–9.25 GBq). Methods: We reviewed the patients' clinical characteristics, including age, sex, histology, serum thyrotropin (TSH) stimulation regimen, prior history of RAI therapy, and labaratory parameters such as the serumTSH, thyroglobulin, and anti‐thyroglobulin antibody. At follow‐up, patients were assessed in terms of disease‐free status, structural persistence, and biochemical disease. Results In total, 107 I‐131 post‐therapeutic scans (15.6%) evidenced thymic uptake. The mean age of the positive thymic uptake group was significantly lower than that of the negative group (p < .001). Significant indicators for thymic uptake were thyroid hormone withdrawal and a history of repeated radioactive iodine (RAI) therapy (p < .05). Logistic regression analysis showed that young age and a history of repeated RAI therapy correlated with thymic uptake (p < .001). At the end of follow‐up, 487 patients (86.5%) were disease‐free, 44 (7.8%) still had biochemical disease, and 32 (5.7%) showed structural persistence. Ten patients (11.5%) in the positive thymic uptake group and 22 (4.6%) in the negative thymic uptake group showed structural persistence. Five patients (5.7%) in the positive thymic uptake group and 39 (8.2%) in the negative thymic uptake group had biochemical disease. The final follow‐up results of the two groups were statistically different. Conclusions Thymic uptake tended to be more prominent in young patients with a history of repeated RAI therapy. Structural recurrence during follow‐up was much more common in the positive thymic uptake group, while the incidence of biochemical recurrence during follow‐up was higher in the negative thymic uptake group.