Should subcentimeter non-invasive encapsulated, follicular variant of papillary thyroid carcinoma be included in the noninvasive follicular thyroid neoplasm with papillary-like nuclear features category?

Objective: In 2016, non-invasive, well-circumscribed and encapsulated, follicular variant of papillary thyroid carcinoma (NI-EFV PTC) was reclassified as noninvasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP) in order to reduce overtreatment of this indolent tumor. However, the study cohort did not include subcentimeter tumors, i.e., papillary thyroid microcarcinoma (mPTC) with NI-EFV morphology, and such lesions are still regarded and staged by most pathologists as microcarcinomas. It is therefore crucial to evaluate the clinical outcome of subcentimeter NI-EFVs. Methods: A total of 52 patients with unifocal mPTC, NI-EFV from five tertiary hospitals who had at least one year clinical follow-up (FU) without post-operative RAI administration were included in the study. A control group of 57 invasive mPTC follicular variant was also included. Results: The median tumor size was 0.44 cm (range 0.1–0.9 cm). There were no distant or lymph node metastases at diagnosis in all patients. Twenty-three patients (44%) underwent lobectomy alone, while the remaining received total thyroidectomy. No recurrence was observed in the entire cohort (n = 52) including all 38 patients with at least 2 years of FU (median FU: 6.3 years). Among 25 patients with ≥5 years of FU, none recurred with a median FU of 9.6 years (range 5.2–18.1 years). In contrast, in the control group with invasive mPTC follicular variant, there were 5 (9%) patients with nodal metastasis at presentation and 1 (2%) who displayed nodal recurrence. Conclusion: Papillary thyroid microcarcinoma, NI-EFV, when stringently selected for, lacks metastasis at presentation and follows an extremely indolent clinical course, even when treated conservatively without RAI therapy. Provided stringent inclusion criteria are met, classification of subcentimeter mPTC, NI-EFV as NIFTP should be considered in order to avoid overtreatment of these biologically indolent lesions.