For patients with thyroid incidentalomas detected on positron emission tomography/computed tomography (PET/CT), emerging research suggests that small nodule size, irregular CT margin and high maximum standardized uptake value (SUVmax) are independently indicative of higher malignancy risk.
For the retrospective study, recently published in European Radiology, researchers reviewed 18F-fluorodeoxyglucose PET/CT data as well as cytologic and histopathological results for 233 patients (with a total of 234 nodules) who had PET/CT detection of thyroid incidentalomas (TIs) and subsequent ultrasound imaging for focal thyroid uptake.
Multivariable analysis revealed that thyroid nodule size < 1.5 cm was associated with a 2.65-fold higher risk of malignancy and an irregular CT margin was linked to over a threefold higher risk of thyroid cancer. The study authors also determined that an SUVmax > 6.25 was associated with a 2.55-fold higher risk for malignancy.
“These findings support an imaging-driven triage strategy for PET/CT-detected TIs: margin irregularity primarily enhances sensitivity, whereas applying a metabolic threshold preferentially improves specificity. In practice, small FDG-avid nodules with irregular margins may indicate more aggressive biology and therefore warrant targeted ultrasound evaluation,” wrote Myoung Kyoung Kim, M.D., who is affiliated with the Department of Radiology and the Thyroid Center at Samsung Medical Center and the Sungkyunkwan University School of Medicine in Seoul, Korea, and colleagues.
The researchers noted other studies that showed conflicting results with respect to the impact of SUVmax and thyroid nodule size in differentiating between malignant and benign nodules. While a 2006 study showed no significant difference based on mean nodule size, the researchers maintained that the current findings demonstrated an association between malignancy and small nodule size.
“This result is likely due to the high proportion of (papillary thyroid carcinoma) cases among malignant nodules in our study (104/118),” posited Kim and colleagues.
Three Key Takeaways
• Small, FDG-avid nodules are not low risk. Thyroid incidentalomas < 1.5 cm detected on PET/CT were associated with a 2.65-fold higher malignancy risk, challenging previous research showing no impact with nodule size in differentiating between malignant and benign nodules.
• CT margin irregularity is a strong morphologic predictor. An irregular CT margin conferred more than a threefold increased risk of thyroid cancer, underscoring the importance of careful CT feature assessment alongside metabolic data.
• SUVmax thresholds can refine triage but should not stand alone. An SUVmax > 6.25 was linked to a 2.55-fold higher malignancy risk and improved specificity when combined with size and margin features, supporting a multiparametric PET/CT-driven approach to prioritize targeted ultrasound evaluation.
The study authors also noted that the combination of young age (< 35), nodule size (< 1.5 cm), irregular CT margin and SUVmax (> 6.25) yielded the highest sensitivity (80.5 percent) and the lowest specificity (52.6 percent) for detecting malignant TIs. Alternately, the combination of nodule size (< 1.5 cm), irregular CT margin and SUVmax (> 6.25) offered a 75 percent specificity regardless of patient age. While the researchers noted comparable sensitivity with this combination for patients < 35 years of age (62.7 percent) and the overall cohort (61 percent), they noted that only five patients in the cohort were younger than 35.
“ … The relatively low sensitivity and specificity of the individual cutoff values for age, size, and SUVmax highlight the necessity for a comprehensive evaluation. Therefore, nodules with a high SUVmax, small size, and margin irregularities on PET/CT images should be recommended for further ultrasound evaluation,” emphasized Kim and colleagues.
(Editor’s note: For related content, see “Study: AI Boosts Ultrasound AUC for Predicting Thyroid Malignancy Risk by 34 Percent Over TI-RADS,” “Study Shows PET/CT is Superior to SPECT/CT in Managing Patients with Primary Hyperparathyroidism” and “Ultrasound-Guided Thermal Ablation Shows Low Recurrence of Thyroid Carcinoma at Five Years.”)
In regard to study limitations, the authors acknowledged possible bias with patient selection, a relatively small cohort size, the potential for slight variability with SUV measurements due to differences in PET/CT scanning protocols and the lack of considering patient comorbidities and age in the assessment of patient outcomes.
