Incidental "hot" thyroid nodule

Case contributed by Matt A. Morgan


Non-smoker with a 1.6 cm lung nodule presents for FDG PET-CT evaluation

Patient Data

Age: 45 years
Gender: Female
Annotated image

On the initial CT study, a 6 mm hypoattenuating non-calcified thyroid nodule was noted (red arrow). It was not followed up with a thyroid ultrasound.

On the follow-up PET-CT, in addition to evaluating the SUV of the lung mass, the incidental thyroid nodule demonstrated FDG uptake.

Case Discussion

An incidental "hot" thyroid nodule on an FDG-PET scan should not be ignored or buried in the report. A meta-analysis of multiple studies reports a 39.4% rate of malignancy in these nodules.

If the patient has not had an ultrasound of the neck and thyroid (and if the patient has a normal life expectancy), then an ultrasound should be the first step (looking in particular for suspicious neck lymph nodes). However, even if the ultrasound reveals a relatively benign-appearing nodule, the FDG-avid nodule still carries an 11-13% rate of malignancy 2.

Since patients who receive FDG-PET scan very frequently have a history of malignancy, it might seem reasonable to include a metastasis in the differential for the FDG-avid thyroid lesion. The likelihood of this event depends somewhat on the primary malignancy and the clinical presentation, but it has been reported that metastases to the thyroid have been found clinically in 1.5-3% of patients who receive surgery for a thyroid malignancy 3 (higher % in autopsy studies). The most common primaries for clinically presenting metastases are renal cell (~48%), lung , colorectal, and breast carcinoma. As a side note, some authors report a high false negative rate for fine needle aspiration of some thyroid metastases 4, and if this is in the differential diagnosis, a core biopsy could be considered.

But whether primary or secondary, the point is that an FDG-avid thyroid nodule should not be buried in the report, and may need biopsied. This "hot" nodule eventually turned out to be papillary thyroid carcinoma.

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