This multi-institutional study investigated non-invasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP) frequency and its diagnostic significance in Japan. We reviewed 4008 thyroid nodules resected in six institutions before NIFTP was proposed. Overall, 26 cases diagnosed as non-invasive encapsulated follicular variant of papillary thyroid carcinoma (PTC) and 145 cases of follicular thyroid adenoma (FTA) were included. Of these nodules, 80.8% and 31.0%, respectively, were NIFTPs. In five institutions, NIFTPs were more commonly found in FTA than in PTC nodules. When NIFTP was included with PTC, the overall prevalence was 2.3%, with rates in five institutions below 5.0% (0.8%–4.4%). One NIFTP case with nuclear score 3 revealed nodal metastasis 2.5 years post-resection, and the carcinoma cells were immunohistochemically positive for BRAF. FTAs or NIFTPs with nuclear score 2 did not metastasize. NIFTP was more common among FTA than among PTC nodules, possibly due to underdiagnosis of PTC on nuclear findings. Considering the clinical findings, molecular pathogenesis, and therapeutic strategy in Japan, NIFTP with nuclear score 2 is not different from FTA, and use of this entity terminology is not meaningful. In contrast, NIFTP with nuclear score 3 has potential for metastasis and BRAF^V600E mutation. Therefore, in NIFTP cases, nuclear scores 2 and 3 should be separately reported.

中文翻译：

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日本具有乳头状核特征的非侵袭性滤泡性甲状腺肿瘤的患病率和诊断意义——一项多机构研究

这项多机构研究调查了日本具有乳头样核特征的非侵袭性滤泡性甲状腺肿瘤 (NIFTP) 的频率及其诊断意义。在 NIFTP 提出之前，我们回顾了 6 个机构切除的 4008 个甲状腺结节。总体而言，纳入了 26 例诊断为非侵袭性包膜滤泡型甲状腺乳头状癌 (PTC) 病例和 145 例滤泡性甲状腺腺瘤 (FTA) 病例。在这些结节中，分别有 80.8% 和 31.0% 是 NIFTP。在五个机构中，NIFTP 在 FTA 中比在 PTC 结节中更常见。当 NIFTP 纳入 PTC 时，总体患病率为 2.3%，五个机构的患病率低于 5.0% (0.8%–4.4%)。1 例核评分为 3 的 NIFTP 病例在切除后 2.5 年发现淋巴结转移，且癌细胞 BRAF 免疫组化阳性。核评分为 2 的 FTA 或 NIFTP 未发生转移。NIFTP 在 FTA 中比在 PTC 结节中更常见，可能是由于核发现对 PTC 诊断不足。考虑到日本的临床发现、分子发病机制和治疗策略，核评分为2的NIFTP与FTA没有什么不同，使用这个实体术语没有意义。相反，核评分为3的NIFTP具有转移和BRAF ^V600E突变的潜力。因此，在 NIFTP 病例中，应分别报告核评分 2 和 3。