Incidental hypermetabolic, FDG-avid thyroid nodule on a PET scan

Issue: August 2011

ByStephanie L. Lee, MD, PhD, ECNU

Stephanie L. Lee, MD, PhD
Stephanie L. Lee

A 52-year-old man was sent for consultation for a hypermetabolic thyroid nodule found on 18F-fluorodeoxyglucose-PET imaging. The patient had the surveillance scan because of history of a non-Hodgkin’s lymphoma diagnosed and treated with chemotherapy 12 years before. He had no history of head and neck radiation and no family history of thyroid disease, including thyroid cancer. The patient was unaware of the thyroid nodule and had no symptoms of dysphagia, change in voice or anterior neck pressure. On exam, he had a normal thyroid, except for a firm, 1.6-cm mass in the left thyroid lobe that was nontender and mobile. He had no palpable adenopathy in the neck.

Laboratory testing showed a normal thyroid-stimulating hormone of 1.96 mU/L. Review of the PET-CT scan demonstrated a 1.8 cm × 1.5 cm heterogeneous, hypodense nodule in the posterior left thyroid lobe with chunky peripheral calcifications (Figure 1A). No enlarged nodes in the neck were seen.

After injection of 18F-fluorodeoxyglucose (18F-FDG), the nodule was mildly hypermetabolic, shown by the accumulation of the isotope with a standardized uptake value (SUV) of 3.1 (Figure 1B). Sonographic examination of the thyroid showed a 1.9 cm × 1.7 cm × 1.7 cm nodule in the posterior left thyroid. Sonographic features of the nodule of concern for malignancy included: indistinct margins; hypoechogenicity; taller than wide on transverse view; microcalcifications; and discontinuous peripheral macrocalcifications (Figure 2). The nodule did not have increased peripheral or intranodular vascular blood flow by Doppler analysis. Sonographic node survey did not reveal any adenopathy.

Figure 1. CT and PET/CT scan of the neck. Axial image of (A) CT scan and (B) fused 18F-FDG PET and CT scan through the thyroid gland. A. Heterogeneous nodule is located in the posterior left thyroid lobe (yellow arrow). The nodule has peripheral calcifications (red arrow). B. This nodule is hypermetabolic as shown by excess accumulation of the 18F-FDG with a standardized uptake value of 3.1 (blue arrow).

Photos courtesy of: Stephanie L. Lee, MD, PhD

Figure 2. Transverse thyroid ultrasound. Transverse image of the thyroid obtained with a 14 MHz linear probe. The hypoechoic nodule is seen in the left lobe of the thyroid. Concerning sonographic characteristics of this nodule include hypoechoic echogenicity (darker than the surrounding normal thyroid gland), blurred margins (blue arrow), microcalcifications (yellow arrows), discontinuous peripheral microcalcification (red arrow) with posterior shadowing (black or no image posterior to the calcification) and taller than wide shape. A taller than wide nodule will have an anteroposterior (red dashed line): transverse (yellow dashed line) ratio >1.

An ultrasound-guided fine-needle aspiration (FNA) biopsy of the left nodule showed follicular cells with enlarged nuclei, nuclear grooves, fine vesicular chromatin, membrane-bound nucleoli and tridimensional cell groups consistent with papillary thyroid carcinoma.

Imaging of this nodule demonstrated several of the new imaging characteristics that are associated with an increase in the risk for thyroid malignancy: 18F-FDG uptake; taller-than-wide appearance; and macrocalcifications on ultrasound.

18F-FDG uptake in the thyroid can occur in a diffuse pattern throughout the thyroid or focally within a nodule. Diffuse 18F-FDG uptake is benign and has been reported to occur in 0.6% to 3.3% of the population. This pattern usually occurs in patients with a chronic thyroiditis, such as Hashimoto’s thyroiditis or Graves’ disease, but may occur as a normal variant. Focal uptake of 18F–FDG in a thyroid nodule is associated with a risk for malignancy that ranges between 25% and 50%. Other investigators have shown that the intensity (ie, SUV) of 18F-FDG is not correlated with an increased risk for malignancy. Thus, an 18F-FDG positive thyroid nodule has an increased risk for malignancy and should be further evaluated by FNA biopsy. Although 18F-FDG uptake in recurrent and metastatic differentiated thyroid cancer suggests a less differentiated tumor with a more aggressive course and poor prognosis, it is not yet clear that an 18F-FDG positive thyroid cancer detected in the thyroid indicates a more aggressive clinical course.

New sonographic characteristics of thyroid malignancy include macrocalcifications, especially when discontinuous in a peripheral location and a taller (anteroposterior measurement) than wide (transverse measurement) ratio of at least 1 or more on a transverse image. Taken together, Cappeli and colleagues have shown that the estimated risk for the combination of sonographic characteristics is high, with a sensitivity of 99% but a specificity of 57%. Thus, either the hypermetabolic activity on the 18F-FDG PET scan or the taller-than-wide plus two additional worrisome sonographic characteristics (hypoechoic, microcalcifications, blurred margins and increased intranodular vascular flow) should signal the need to biopsy a thyroid nodule regardless of the size.

Stephanie L. Lee, MD, PhD, is director of the Thyroid Health Clinic at Boston Medical Center and associate professor of medicine at Boston University School of Medicine. She is also an Endocrine Today Editorial Board member.

For more information:

Cappelli C. Eur J Endocrinol. 2006;155:27.

Eloy JA. Am J Neuroradiol. 2009;30:1431-1434.

Liu Y. Ann Nucl Med. 2009;23:17-23.

Disclosure: Dr. Lee reports no relevant financial disclosures.