Context: Papillary thyroid microcarcinoma (PTMC) usually has an excellent prognosis. Objective: To evaluate the three-dimensional structures of PTMCs, using serial neck ultrasonography (US) in patients under active surveillance. Design and Setting: A retrospective cohort study. Participants: In total, 192 patients diagnosed with PTMC under active surveillance for> 1 year were included in a median 30-month follow-up. Changes in tumor size were evaluated not only using the maximal tumor diameter but also the tumor