FDG-PET scanning in patients with differentiated thyroid cancers. Institutional experience

Full text: Patient with detectable serum thyroglobulin (Tg) and negative radioiodine whole body (RAI-WB) scan should have FDG-PET scan to locate tumor tissue. Sometimes scan fail to detect any pathologic accumulation of FDG. Some well-differentiated thyroid cancers do not accumulate FDG. In addition, tumor burden could be so small that imaging becomes impossible. To explore this possibility, several studies explored relationship between serum Tg level and result of FDG-PET scans. Results were inconclusive and it is not clear if there is some low level of serum Tg below which PET scanning is not cost effective. We examined all cases of thyroid cancer patients who underwent FDG-PET scanning in our institution (N-33) for relationship between Tg level and results of the scan. There were 16 females, 17 males. Age was 19-81 years. Follicular cancer was present in 2 patients while 31 had papillary cancer. 14 patients had PET scan while withdrawn from T4 therapy, and 19 patients were scanned while TSH was suppressed. Overall, 20 scans were considered positive, while 12 were negative. One patient was considered to have positive scan but after scan was repeated interpretation was changed into - positive uptake due to inflammation. In patients whose PET scan was positive, Tg level was in a range 6.6-7,108 ng/ml, while those with negative scan had Tg level in 1.7-36.5 ng/ml range. In group withdrawn from T4 therapy and positive scan Tg level ranged from 13.8-7,108 ng/ml, and with negative scan 2.8-36.5 ng/ml. In PET positive patients in euthyroid group Tg ranged 6.6-432 ng/ml and in negative 1.7-13.3 ng/ml. Conclusion: These results suggest that patients who have negative RAI scan and low Tg level may not benefit from PET scanning. In our study Tg level less than 6.6 ng/ml for the whole group, and below 13.8 ng/ml for patient withdrawn from T4 therapy did not result in positive FDG-PET scan result. Our group is small and more data need to be collected to clarify these observations. In addition, relationships between Tg levels after rh TSH injections and results of PET scans should be studied to clarify the best strategy for evaluation of these patients. (author)