Active Surveillance of Low-Risk Papillary Thyroid Microcarcinoma: A Multi-Center Cohort Study in Korea.

Background: Active surveillance has been introduced as a management option for low-risk papillary thyroid microcarcinoma (PTMC) due to its mostly indolent course.

Methods: This was a multicenter study of 370 PTMC patients who underwent active surveillance more than one year. The changes in volume and maximum diameter between initial and last ultrasonography were evaluated to identify the natural course of PTMC during active surveillance.

Results: Patients' age at diagnosis was 51 ± 12 years, and 110 (30%) patients were <45 years of age. The initial maximum diameter and volume of PTMCs were 5.9 ± 1.7 mm and 81.0 ± 77.7 mm, respectively. During the median 32.5 months of follow-up, 86 (23.2%) patients were found to have an increase in tumor volume, and 13 (3.5%) patients showed an increase in the maximal diameter of the tumor. The cumulative incidence of volume increase gradually rose with time (6.9%, 17.3%, 28.2%, and 36.2% after two, three, four, and five years, respectively). The risk of volume increase in patients <45 years of age was twice as high as in older patients (p = 0.002). There was no significant difference in tumor size change according to sex, levothyroxine treatment, or presence of Hashimoto's thyroiditis. During the period, 58 (15.7%) patients underwent delayed thyroid surgery due to anxiety (37.9%), tumor size increase (32.8%), or appearance of cervical lymph node metastasis (8.6%). Lymph node metastasis was found in 29.3% of patients on pathological examination.

Conclusions: A significant number of PTMCs grow during active surveillance, and tumor volume change is a more sensitive means of evaluating tumor growth. Active surveillance can be carefully applied for selected patients. Although it is not contraindicated, it should be applied more cautiously for younger patients.