THYROID
NODULES INCIDENTALLY DIAGNOSED ON PET SCAN
Occasionally,
incidental thyroid nodules (ITN) may be detected in patients undergoing PET
scan for other indications (typically with previous malignant disease, during
the follow-up). Among these group of patients ITN will be detected in 1 – 3 %
of them. It should be noted that the rate of malignancy in patients with ITN
detected on PET scans is markedly higher than that in patients with ITN
detected on CT scans or MRI. Although the resolution for nodules on PET scan is
lower than that of CT and/or MRI, the radiotracer used for PET is specific for
metabolically active tissue, leading to a higher rate of malignancy within
detected abnormalities. This may be due – at least in part – to the
characteristics of this patient
population since many of them have a current or prior non-thyroid malignancy
and are more likely –compared to the general population – to have been exposed
to radiation therapy. It is important to note that the pattern of standardized
uptake value (SUV) on PET has clinical / prognostic significance: a diffuse
tracer uptake within the thyroid gland generally represents benign disease,
whereas focal uptake is associated with thyroid malignancy in a significant
percentage of patients (30 – 50 %). Moreover, the maximum SUV on PET is
associated with an increased risk of malignancy; however, a specific SUV cutoff
to determine benign versus malignant thyroid nodule has not been defined.
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