Papillary Thyroid Microcarcinoma: Reclassification to Non-Invasive Follicular Thyroid Neoplasm with Papillary-Like Nuclear Features (NIFTP): a Retrospective Clinicopathologic Study.

Papillary thyroid microcarcinoma (PTMC) accounts for nearly 50% of newly diagnosed PTC cases. There is considerable debate in literature about the clinicopathologic features and prognostic significance of PTMC and whether it should be treated as a separate entity. Due to lack of agreement and supportive data, the consensus study group that established the criteria for non-invasive thyroid neoplasm with papillary-like features (NIFTP) kept its size above 1 cm i.e., excluding PTMC from this new group. As a result, to date, some patients diagnosed with PTMC get aggressive treatments such as partial or total thyroidectomy and even radioactive iodine ablation. We retrospectively studied clinicopathologic features and long-term follow-up of 48 cases of papillary thyroid microcarcinoma. Of these, 7 cases (15%) had capsular invasion, 2 cases (4%) had extrathyroidal extension, 1 case (2%) had lymphovascular invasion, 5 cases (25%) had lymph node metastases, no case (0%) had any distant metastases, and 1 case had recurrence after long-term follow-up (mean 13.7 years, range 1-21 years). Upon slide review, 8 cases fulfilled the criteria for NIFTP and were sub-classified under this new category. These 8 cases had no recurrence after long-term follow-up (mean 12.1 years, range 7-19 years). In this study, we confirmed the previous published reports exhibiting indolent nature of PTMC and also suggested that PTMC cases that fulfill all the criteria for NIFTP can be sub-classified under this term in order to avoid unnecessary aggressive treatment.