Hypermetabolism on ¹⁸F-Fluorodeoxyglucose Positron Emission Tomography Scan Does Not Influence the Interpretation of Thyroid Cytopathology, and Nodules with a SUVmax <2.5 Are Not at Increased Risk for Malignancy

Background: Hypermetabolism of thyroid nodules on ¹⁸F-fluorodeoxyglucose positron emission tomography (PET) is associated with a higher prevalence of malignancy. However, the definition of hypermetabolism and its impact on cytological interpretation are unclear. Methods: Medical records of all patients with thyroid nodules who had undergone cytological evaluation at the Moffitt Cancer Center between October 2008 and May 2014 were retrospectively reviewed. Those with a PET scan performed within one year of the cytology composed the study group, and the rest were used as controls. The distribution of the cytological categories, percentage of resection, and prevalence of malignancy among each Bethesda category was compared between both groups. Results: Fifteen percent (436) of all thyroid nodules with cytological evaluation were in the study group. Maximum standardized uptake values (SUVmax) were directly associated with the probability of having a malignant or a follicular neoplasm cytological diagnosis; and inversely associated with the probability of having a benign cytological diagnosis. However, the prevalence of cancer within each Bethesda category was not associated with SUVmax values. It was found that the prevalence of malignant cytology increased to >5% with SUVmax values ≥2.5. SUVmax values were significantly higher in malignant than in benign nodules on histology (mean values 10.8 vs. 5) but with significant overlap between both groups for either the whole cohort or nodules with indeterminate cytology only limiting its use for differential diagnosis. Conclusions: The prevalence of malignancy in thyroid nodules with a SUVmax <2.5 is similar to the general population, and management should not be modified in those patients. The increased prevalence of malignancy among hypermetabolic thyroid nodules (SUVmax ≥2.5) is well characterized by cytology and does not impact the interpretation of cytological categories. Therefore, SUVmax value does not add relevant information once cytology is available.