Incidental Thyroid Nodules at Non-FDG PET Nuclear Medicine Imaging: Evaluation of Prevalence and Malignancy Rate

The purpose of this study was to estimate the prevalence of thyroid nodules detected incidentally on non-FDG PET nuclear medicine imaging studies, the malignancy rate, and predictors of malignancy.

A retrospective review of more than 10 years of patient records at an academic medical center identified the cases of 31 patients with incidental focal radiotracer-avid thyroid findings on non-FDG PET nuclear medicine studies who underwent biopsy or surgical excision. Statistical analysis of patient and imaging features was performed to identify features predictive of malignancy. Society of Radiologists in Ultrasound and American Thyroid Association biopsy criteria were applied to patients who had ultrasound images for review.

Thirty-one patients had incidental thyroid findings on (99m)Tc-sestamibi parathyroid scans (80.6%), (111)In-pentetreotide scans (16.1%), and (99m)Tc-tetrofosmin cardiac scans (3.2%). These three types of scans accounted for 21,402 total examinations in the study period. Thus, the prevalence of incidental thyroid findings on non-PET nuclear medicine studies that were evaluated by pathologic examination was 0.14%. The malignancy rate was 16.1% (5/31). No clinical or imaging features were identified as predictive of malignancy. Society of Radiologists in Ultrasound and American Thyroid Association criteria were applied to 23 thyroid nodules with available ultrasound images. According to both sets of criteria, biopsy was recommended for 19 of 23 (82.6%) nodules, and one of three (33.3%) cases of thyroid cancer was missed.

Most thyroid nodules incidentally detected on non-FDG PET nuclear medicine studies are detected on (99m)Tc-sestamibi parathyroid scans and (111)In-pentetreotide scans. Because these nodules are extremely rare and the malignancy rate is high, further evaluation of incidental focal radiotracer-avid thyroid findings with ultrasound is an appropriate recommendation.