The role of 131I whole-body scanning (131I WBS) for surveillance of patients with differentiated thyroid carcinoma (DTC) has declined in the United States of America over the past decade. As discussed by Gwyther, these patients are more often followed with neck ultrasound (US) and serum thyroglobulin (Tg) levels than with 131I WBS. Recombinant human thyroid-stimulating hormone (rhTSH, Thyrogen™), approved by the United States Food and Drug Administration in 1998, as an adjunctive diagnostic tool for serum Tg testing with or without 131I WBS, is one factor that has contributed to this paradigm shift. The subsequent approval of rhTSH, in 2007, for use with 131I ablation has led to a further decrease in radioiodine scanning. In this setting, diagnostic 131I WBS is omitted and a single post-ablation scan is performed. Improved US techniques and standardized criteria for characterizing cervical lymph nodes, as Gwyther notes, as well as efforts to minimize radiation exposure, have also contributed to the decline in radioiodine imaging.
In conjunction with rhTSH stimulation or withdrawal of thyroid hormone, 131I continues to have an important role in patients with treated DTC who have a negative neck US and suspected tumour recurrence based on detectable/rising serum Tg levels. In addition to localizing disease, 131I WBS identifies patients who are likely to benefit from 131I therapy. If 131I WBS is negative, 18-fluoro-2-deoxyglucose (FDG) PET/CT may provide valuable information about the location and extent of disease. The so-called ‘flip-flop phenomenon’, the finding that DTC is iodine-avid/FDG negative, while dedifferentiated tumours are iodine-negative/FDG avid, provides the basis for this evaluation strategy.
As Gwyther indicates, FDG-PET/CT provides accurate localization of recurrent or metastatic disease in patients with 131I-negative thyroid cancer. It simultaneously provides functional and anatomical information, and has the advantage of surveying the entire body in a single examination. Recently, brain metastases from papillary thyroid carcinoma (PTC) were detected using FDG-PET/CT. Although rare, brain metastases may occur in up to 10–15 % of patients with metastatic PTC who develop new sites of disease. The median survival for patients with brain metastases from thyroid carcinoma (4.7 months) is similar to that reported for patients with brain metastases from other systemic malignancies (4 months). Finally, FDG-PET/CT provides prognostic information because patients with FDG-avid disease have a worse prognosis than those with iodine-avid disease.
Fortunately, most patients with DTC have an excellent prognosis and can be adequately followed with serial neck US and serum Tg levels. For those patients with recurrent or metastatic disease, 131I WBS is useful for determining the extent of disease and for identifying those patients likely to benefit from 131I therapy. For patients with iodine-negative disease, FDG-PET/CT is useful for localizing the disease and providing prognostic information.