The aim of this study was to investigate risk factors for multifocality of papillary thyroid microcarcinoma (PTMC).

A retrospective study of 301 patients operated for benign thyroid diseases in a five year period. These patients had a PTMC incidentally discovered on definite histopathological findings following total or near-total thyroidectomy. Since distinguishing between intrathyroid extension of PTMC as the result of intrathyroid dissemination or as the result of multicentricity is challenging we observed them together as multifocality. In statistical analysis we used standard descriptive statistics and logistic regression analysis to determine independent risk factors.

In our study there were 85.4% females, and 14.6% males with a median age of 54 years. Most patients had a follicular variant of PTMC (52.8%) that was 5 mm or smaller. A multifocal PTMC was present in 26.6% of cases. An independent protective factor for multifocality of PTMC was a large thyroid gland (OR 0.55, 95% CI 0.31-0.97, p=0.039). Size of PTMC greater than 5 mm was an independent risk factor for a multifocal PTMC (OR 3.26, 95% CI 1.85-5.75, p=0.000). Finally, the mixed follicular-papillary variant of PTMC represents an independent risk factor for a multifocal PTMC (OR 2.42, 95% CI 1.09-5.36, p=0.030).

Intrathyroid extension is present in more than a quarter of PTMCs found in patients operated for benign thyroid disease. Independent risk factors for intrathyroid extension are size of PTMC greater than 5 mm and the mixed follicular-papillary variant of PTMC, while a large thyroid gland is an independent protective factor.