Long-Term Durability of Active Surveillance of Small, Low-Risk Papillary Thyroid Cancer.

[IMPORTANCE] In managing early-stage cancers, active surveillance (AS) may be preferentially favored by older individuals. In counseling patients, it is important to understand the durability of AS in the context of age.

[OBJECTIVE] To evaluate the durability of AS in patients with small, low-risk papillary thyroid cancer (PTC) according to age at the time of choosing AS.

[DESIGN, SETTING, AND PARTICIPANTS] This single-center, prospective, long-term follow-up cohort study was conducted at a tertiary care hospital in Toronto, Ontario, Canada. Adult patients with small, localized, low-risk PTC less than 2 cm in maximal diameter were enrolled between May 2016 and February 2021. The clinical outcome data were analyzed up to the time point of May 25, 2025, and final data analysis was performed in June 2025.

[EXPOSURE] All patients were offered the choice of AS or thyroid surgery.

[MAIN OUTCOMES AND MEASURES] The primary outcome was the overall rate of AS crossover to definitive treatment (treatment completed or recommended by an investigator) and the indications. Cumulative crossover incidence function curves were examined according to age, with death from other causes as the competing risk.

[RESULTS] A total of 200 patients (155 patients under AS and 45 who had immediate surgery) were followed up for a median (IQR) duration of 71 (59-84) months. Overall mean (SD) age was 52.0 (14.9) years, and 153 patients (76.5%) were female. There were no observed thyroid cancer-related deaths or any distant metastatic disease. The overall crossover rate from AS was 23.9% (37/155; 32 completed treatment, 3 declined surgery for disease progression, and 2 awaiting treatment). Crossover reasons included disease progression (56.8% [21/37]), patient preference (40.5% [15/37]), and ultrasound imaging limitations precluding accurate tumor measurement under active surveillance (tumor border not clearly distinguishable from heterogeneous echotexture of the thyroid parenchyma in a patient with Hashimoto thyroiditis; 2.6% [1/37]). The 5-year age-stratified cumulative overall crossover incidence rates were 41.5% (95% CI, 25.6%-56.8%) in patients younger than 45 years, 20.9% (95% CI, 12.3%-31.1%) in those aged 45 to 64 years, and 5.1% (95% CI, 0.9%-15.2%) in those aged 65 years and older (P < .001).

[CONCLUSION AND RELEVANCE] This single-center Canadian cohort study found that AS is a durable long-term management strategy for small, low -risk PTC, particularly in older individuals. Older individuals may be less likely to cross over to surgery after choosing AS.