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

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.