THYROID NODULES
INCIDENTALLY IDENTIFIEND ON PET SCANS PERFORMED FOR NON-THYROID MALIGNANCIES
Positron-emission
tomography (PET) is increasingly used in patients with malignant disease for
staging and/or during post-treatment follow-up. 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. This can explain why the
rate of malignancy in patients with thyroid nodules incidentally identified on
PET scans is markedly higher than that in patients with nodules identified on
CT scans or MRI. In a large review the detection rate on PET scan (performed for non-thyroid malignancies) was only 1 % (compared to 10-12 %
for CT), but 33 % of these nodules found on PET scans were malignant (most
often papillary thyroid cancer, 82 %). Of note, many of these patients with
non-thyroid malignancies have been exposed to radiation therapy. The
pattern of standardized uptake value (SUV) on PET is diagnostically important,
since diffuse tracer uptake within the thyroid gland generally represents
benign disease, whereas focal uptake is associated with malignant nodule in 30 –
50 % of cases. The maximum SUV on PET appears to be associated with an increased
risk of malignancy, but a specific SUV cutoff to determine benign versus
malignant nodule has not been determined. Clinical characteristics, such as
age, sex, and tumor size, have not been associated with the risk of malignancy
in nodules incidentally identified on PET.
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