Limited Utility of Routine Surveillance Ultrasound in Differentiated Thyroid Cancer Patients With Undetectable Postoperative Thyroglobulin Levels

BACKGROUND: Neck ultrasound (US) and serum thyroglobulin (Tg) measurements are mainstays of long-term differentiated thyroid cancer (DTC) surveillance. Given the high sensitivity of serum Tg, we aimed to assess the utility of neck US in DTC patients who underwent total thyroidectomy and have undetectable serum Tg. METHODS: We performed a retrospective cohort analysis of DTC patients who underwent a total thyroidectomy at our institution (2010-2023) and received US-guided fine needle aspiration (FNA) during their surveillance. Patients were categorised into three lab categories based on serum Tg and Tg antibody (Tg Ab) status before the biopsy: (1) 'Negative Tg' if undetectable Tg ( < 0.2 ng/dL) and Tg Ab, (2) 'Positive Tg' if detectable Tg and undetectable Tg Ab, and (3) 'Positive Tg Ab' if detectable Tg Ab. To calculate the positive predictive value (PPV) of neck US, we defined the 'true positive' of US as findings that prompted an FNA biopsy resulting with DTC, and 'false positive' findings prompting an FNA biopsy that did not result as DTC. RESULTS: A total of 118 patients were included, encompassing 146 FNA biopsies: 33 (23%) had Negative Tg, 84 (57%) had Positive Tg, and 29 (20%) had Positive Tg Ab lab results before their biopsies. The PPV of neck US in the setting of Negative Tg was 3% (one true positive, 32 false positives), while the PPV was 50% (42 true positives, 42 false positives) for Positive Tg, and 52% (15 true positives, 14 false positives) for Positive Tg Ab cohorts. Sub-analysis of the Positive Tg cohort using different serum Tg level cutoffs revealed a PPV of 29% at just detectable serum Tg of 0.2 ng/dL, and PPV of 38% for Tg < 1.0 ng/dL. The PPV stabilised at 58% for Tg levels ≥ 1 ng/dL. CONCLUSION: With the low PPV of neck US, high cost of surveillance, and the advent of ultra-sensitive serum Tg measurements, future guidelines should consider reducing routine neck US surveillance in patients with undetectable serum Tg and only performing it when there is a rise in serum Tg levels.