OBJECTIVE
The aim of this study was to determine the effectiveness of retinoic acid redifferentiation therapy (RAT)-induced radioiodine therapy (RIT) for radioiodine-refractory thyroid cancer based on the 4 response-to-therapy categories.
METHODS
A total of 20 patients (16 papillary, 4 follicular) with radioiodine-refractory thyroid cancers underwent 31 high-dose RITs after receiving 1.0-1.5 mg/kg retinoic acid for 6 weeks. Serum thyroglobulin levels were measured before and at the end of RAT, and 9 months after RIT. CT (chest or head/neck), whole body radioiodine scintigraphy, F-18 FDG PET/CT or spine MRI was performed to evaluate structural lesions.
RESULTS
Fourteen patients (70%) had N1b lymph node metastasis, while 60% patients had distant metastases (10 lungs, 3 bones). Mean thyroglobulin level before RAT was 677 (13-4794) ng/mL. Mean cumulative radioiodine dose before RAT was 779 (100-2300) mCi, while mean radioiodine dose with RAT was 252 (150-400) mCi. Using 30% and 50% decrease as cutoffs for thyroglobulin level, 41.9% and 25.8% patients showed good response after RAT-induced RIT, respectively. Before RAT, there were 21 structural incomplete and 10 biochemical incomplete responses by 4 response-to-therapy categories, which changed to 18 structural incomplete, 9 biochemical incomplete, 1 indeterminate and 3 excellent responses. All follicular thyroid cancer patients showed poor response. At the end of RAT, thyroglobulin level decreased in 57% (16/28) cases, of these, 63% (10/16) showed good response (> 30%) after RIT. Meanwhile, those with increased thyroglobulin level, 66% (8/12) showed poor response (p < 0.001). Post-therapeutic whole body scintigraphy did not predict the response of RAT-induced RIT.
CONCLUSION
RAT-induced RIT changed response categories in some patients with radioiodine-refractory thyroid cancer. The decrease in thyroglobulin level at the end of RAT could predict the response of RAT-induced RIT.