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Objective:
To evaluate the clinical utility of surveillance ultrasound in the detection of disease recurrence in pediatric patients with thyroid cancer beginning 1 year after initial therapy.

Materials and Methods:
We studied pediatric patients (<19 years) diagnosed with differentiated thyroid cancer between 2010-2020 whose disease status 1 year after initial therapy (thyroidectomy - radioactive iodine) was no evidence of disease (NED) or indeterminate. We evaluated the association between sonographic and biochemical findings at 1 year with the subsequent cytological or histological diagnosis of residual/recurrent structural disease in the neck (SDN) and with final disease status at last follow-up.

Results:
Seventy-two patients with 1-year disease status NED (n = 35, 49%) or indeterminate (n = 37, 51%) were included. Median length of subsequent follow-up was 5.4 (range 1.6 - 10.4) years. There were 409 surveillance ultrasounds were performed in the cohort, a median of 5 (range 1-15) ultrasounds per patient. One year after initial therapy, 16 patients had an indeterminate or abnormal ultrasound finding. One (6.7%) was diagnosed with SDN after one additional year of follow-up. Of 56 patients with a negative ultrasound at 1 year, 21 patients had elevated thyroglobulin (Tg) or Tg antibody levels, and 35 patients had undetectable Tg or Tg antibody levels. Among the 21 patients with negative ultrasound but detectable Tg or Tg antibody at 1 year, SDN was detected in one patient (4.8%) after one additional year of follow-up. In this case, SDN was detected by an elevated thyroglobulin (Tg) level and I-123 scan, not by ultrasound. Final disease status was NED in 9/21 (42.9%), indeterminate in 10/21 (47.6%, six due to elevated Tg/Tg antibody levels and four due to indeterminate ultrasound findings), and 2/21 (9.5%) were diagnosed with pulmonary metastases. Among the 35 patients with negative ultrasound and undetectable Tg and Tg antibodies at 1 year, none were diagnosed with SDN over a median 4.8 (range 1.6 - 10.1) years of subsequent follow-up. Final disease status was NED in 33/35 (94.3%) and indeterminate in two patients (5.7%) due to elevated Tg levels with negative ultrasound.

Conclusion:
In pediatric patients with thyroid cancer and negative ultrasound 1 year after initial therapy, the utility of further routine ultrasound surveillance is low. However, patients with indeterminate or abnormal ultrasound findings or abnormal Tg or Tg antibody levels may benefit from continued ultrasound surveillance, in addition to biochemical monitoring to detect structural disease recurrence in the neck.