Thomas B. Repas, DO, FACP, FACE, CDE, is an endocrinologist, lipidologist and physician nutrition specialist in clinical practice at the Regional Medical Clinic Endocrinology and Diabetes Education Center in Rapid City, SD. Dr. Repas is the former chairman of the professional diabetes advisory committees of the Wyoming and the Wisconsin Diabetes Prevention and Control Programs. He is board certified in the areas of endocrinology, diabetes and metabolism, clinical lipidology, internal medicine and nutrition, and is also a certified diabetes educator.

Incidental finding of an 18FDG positive thyroid nodule on PET CT imaging

A 70-year-old woman had a PET/CT scan to evaluate her prior history of endometrial adenocarcinoma. No evidence of metastatic adenocarcinoma was seen. However there was an incidental finding of a focal area of increased uptake seen in her left thyroid (see image below).

The patient was sent to see me in consultation. I performed a thyroid ultrasound and found a left-sided nodule (see image below). It had relatively benign appearing characteristics. It was isoechoic, with only peripheral flow, and was well-defined and without calcifications. There were no other thyroid nodules or suspicious cervical lymphadenopathy. Had I not known of the positive uptake on PET scan, this is a nodule I might have chosen to follow by observation instead of biopsy. Nevertheless, given the PET findings, I performed an ultrasound guided fine needle aspiration (FNA). The results were consistent with a benign thyroid nodule.

Thyroid incidentalomas are common and frequently identified on carotid ultrasound, CT, MRI and other imaging studies. The prevalence of thyroid incidentaloma on 18FDG PET /CT is about 2% to 4%. The risk for cancer in a thyroid nodule found on PET imaging ranges from 14% to almost 40%. Thyroid cancer is more likely in nodules with standard uptake values (SUV) above 5.

Some authors suggest that PET imaging may be useful in determining which thyroid nodules are at increased risk for malignancy in situations where the diagnosis is unclear, such as nodules with a suspicious or indeterminate FNA. A thyroid nodule that is positive on PET is suspicious and certainly should be evaluated further. However, not all thyroid cancers are FDG-avid. It is also possible for very small thyroid cancers to be missed. Furthermore, many insurers refuse to reimburse PET imaging for the evaluation of thyroid disorders, including in patients who have already confirmed radioiodine-negative residual thyroid cancer.

I will be following this nodule by ultrasound. If there is any change in size or other characteristics in the future, I would reconsider our decision to follow by observation. Depending on what that change is, I may even recommend surgical resection. However, with both reassuring ultrasound characteristics and benign FNA results, right now, my level of concern is low.

For more information:

Kang WK. J Clin Endocrinol Metab.2003;88: 4100-4104.

Choi JY. J Nucl Med. 2006;47:609-615.

Mitchell JC. Surgery. 2005;138:1166-75.