Comparison of recombinant TSH-stimulated serum thyroglobulin (Tg) test, I-131 SPECT/CT, and FDG PET/CT in anti-Tg antibodies (TgAb) positive thyroid cancer

Objectives To compare diagnostic value of serum Tg tests, I-131 SPECT/CT, and FDG-PET inl differentiated cancer with positive TgAb.

Methods Serum Tg assay, SPECT/CT with diagnostic dose I-131 , and FDG PET/CT were performed under rhTSH stimulation in 13 consecutive patients with differentiated thyroid cancer who had undergone total thyroidectomy. All patients had positive TgAb titer (8.1-239.8IU/ml, average 53.7) that baseline Tg was undetectable in all but one. Low iodine diet were applied from 8days before to the day of I-131 ingestion. Patients received i.m.injection of rhTSH for 2 consecutive days. On the next day of 2nd rhTSH, 5-10mCi of I-131 was administrated. On 3 days after 2nd rhTSH, I-131 SPECT/CT along with whole body scan and FDG PET/CT were imaged. Also, blood tests of FT4, TSH, Tg and TgAb were done.

Results On day 3 post rhTSH stimulation, despite TSH values of 28.25±4.8(μU /ml) However, Tg was undetectable in 12 out of 13pts. I-131 SPECT/CT was positive in 9 pts ( thyroid bed in 5, neck/mediastinum in addition to thyroid bed in 3, and lung in 1). PET/CT was positive in 9 and did depict I-131 negative lesions in 6 . However, 4 pts showed false positive uptake (inflammatory nodes in 2, primary lung cancer and colon cancer in 1, and granuloma in 1). True positive uptake associated with false positive uptake was found in one. Also, co-existence of metastatic lung tumor from thyroid cancer and primary lung cancer was confirmed in another patient.

Conclusions In TgAb positive thyroid cancer, Tg test under rhTSH stimulation is unreliable. I-131 accumulates in thyroid bed or functioning tumors although negative predictive value of the modality remains uncertain. PET/CT detects additional lesions that were missed by I-131. However, false positive uptake of FDG was found in about 30 % of the patients. Thus, pathologic correlation should be performed as much as possible before considering aggressive therapies on basis of FDG PET/CT findings alone