PET shows thyroid cancer metastasis

Thyroid cancer is different from most adult cancers in that it affects mainly young people (two out of every three cases occur in patients 20 to 55 years old). Initial diagnosis usually is done by fine-needle aspiration biopsy, and long-term follow-up often consists of some form of radioactive iodine whole-body scanning technique. However, radioactive iodine usually reveals cancer only inside the thyroid bed — but not outside it, where the cancer also can spread — and some thyroid cancers will not take up radioactive iodine at all.

Dr. Ali S. Alzahrani from King Faisal Specialist Hospital and Research Centre in Riyadh, Saudi Arabia, and his colleagues compared the use of F-18-fluorodeoxyglucose PET with diagnostic radioiodine whole-body scanning and postablation radioiodine whole-body scanning after thyroid surgery to see whether PET might reveal more locoregional and distant cancerous tissue.

As reported in the May 2008 issue of the European Journal of Endocrinology, the researchers tested 26 patients with newly diagnosed differentiated thyroid cancer. All underwent diagnostic radioiodine whole-body scanning and PET scanning. Twenty-four patients also received postablation scanning. The researchers compared the results of the three scans and also correlated the PET results with the stage of the disease and the long-term prognosis.

The PET technique revealed 18 positive cases with 40 specific cancer focal points and eight negative cases. The diagnostic whole-body scans were all positive and showed 47 focal points, and the postablation scans showed similar results. However, whereas the diagnostic scans displayed 45 of the focal points inside the thyroid bed, the PET technique showed uptake of 26 focal points outside the bed. This suggested that the focal points seen on the diagnostic scans were more likely to be residual thyroid tissue, whereas those seen on PET scans were more likely to be metastatic cancerous tissue. In addition, the positive PET scans correlated strongly with later stages of the disease, and the negative scans correlated well with cancer that was more likely to go into remission.

The researchers believe that their results show that F-18-fluorodeoxyglucose PET could play a role after thyroid surgery in evaluating patients with differentiated thyroid cancer.