A Comparison of Detection Rates Between Ultrasound and Post‐Therapeutic I‐131 SPECT / CT for Cervical Lymph Node Metastasis in Papillary Thyroid Carcinoma

OBJECTIVE: This study compares the diagnostic performance of Ultrasound and post-therapeutic I-131 SPECT/CT for detecting cervical lymph node metastases in postoperative patients. We also characterize the sonographic features of metastatic nodes and map their distribution across cervical levels. METHODS: A retrospective, reference-standard-led cohort study was conducted. The cohort comprised 172 post-thyroidectomy PTC patients, from which 266 individual lymph nodes with pathologically proven metastasis were analyzed. The detection rates of US and I-131 SPECT/CT were compared using McNemar's test. Multivariate logistic regression was used to identify independent sonographic predictors of a node being classified as suspicious on ultrasound. RESULTS: US demonstrated a significantly higher detection rate for proven metastatic nodes than SPECT/CT (75.0% versus 55.8%, P < .001). Microcalcifications [adjusted odds ratio (aOR) = 1.64, P = .03] and hypoechogenicity (aOR = 1.57, P = .02) were the strongest independent predictors of a node being classified as suspicious on ultrasound. Level VI was the most frequently involved nodal station (55%) and the only independent predictor of metastatic involvement (OR = 2.64, P = .028). CONCLUSION: Ultrasound demonstrated superior detection of metastatic lymph nodes, identifying a substantial subset (32.7%) of RAI-refractory disease missed by SPECT/CT. SPECT/CT remains valuable for confirming RAI avidity and detecting occult functional disease. These findings support a combined imaging strategy according to the metastatic characteristics. The presence of microcalcifications and hypoechogenicity was an important sonographic predictor of malignancy, with Level VI being the most common and predictable site of metastatic involvement.