Objectives. Papillary thyroid carcinomas frequently occur as two or more separate foci within the thyroid gland (18%-87%). However, those multifocal tumors are easy to be undetected by preoperative radiologic evaluations, which lead to remnant disease after initial surgery. We aimed to study the incidence of multifocal papillary thyroid micro-carcinomas (PTMCs), diagnostic accuracy of preoperative radiologic evaluation, predictive factors, and the chance of bilateral tumors. Methods. Two hundred and seventy-seven patients with PTMC were included in this study. All patients underwent total thyroidectomy as an initial treatment. Medical records, pathologic reports, and radiological reports were reviewed for analysis. Results. Multifocal PTMCs were detected in 100 of 277 patients (36.1%). The mean number of tumors in each patient was 1.6 +/- 1.1, ranging from 1 to 10. The additional tumor foci were significantly smaller (0.32 +/- 0.18 cm) than the primary tumors (0.63 +/- 0.22 cm) (P<0.001). There was no significant relationship between primary tumor size and the presence of contralateral tumors. With more tumors detected in one lobe, there was greater chance of contralateral tumors; 18.8% with single tumor focus, 30.2% with 2 tumor foci, and 46.2% with 3 or more tumor foci in one lobe. Sensitivity of preoperative sonography was 42.7% for multifocal tumors and 49.0% for bilateral tumors. With multivariate analysis, nodular hyperplasia was the only significant factor for multifocal tumors. Conclusion. In cases of PTMCs, the incidence of multifocal tumors is high. However, additional tumor foci are too small to be diagnosed preoperatively, especially under the recent guidelines on radiologic screening tests for papillary thyroid carcinoma. Multifocal PTMCs have high risk of bilateral tumors, necessitating more extensive surgery or more thorough follow-up.
