Prevalence and diagnostic significance of noninvasive follicular thyroid neoplasm with papillary-like nuclear features among tumors previously diagnosed as follicular adenoma: a single-institutional study in Japan.

ABSTRACT

The incidence of noninvasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP) in papillary thyroid carcinoma (PTC) is significantly lower in Asian countries than Western countries; however, the difference remains unexplained. This study aimed to evaluate the incidence of NIFTP in tumors diagnosed as follicular adenoma (FA) in a Japanese institution and discuss the significance of NIFTP. In this study, 44 tumors were investigated, which were histologically diagnosed as FA at the Kuma Hospital in 2008. Of the 44 tumors, 13 (29.5%) were revised as NIFTP. In the remaining 31 tumors, the FA diagnosis was reconfirmed. On aspiration cytology, most of the NIFTPs were categorized into follicular neoplasm or suspicious for a follicular neoplasm. On histological examination, 9 (29.0%) of 31 FA nodules showed a nuclear score of 1. Twelve (92.3%) of 13 NIFTP nodules showed a nuclear score of 2, and the remaining nodule had a nuclear score of 3. No metastasis of FA or NIFTP was detected. There were no evidences of distant metastasis during follow-up. This is the first study to describe that NIFTP is more frequently included in tumors diagnosed as FA rather than PTC in Japan. As clinical management of FA and NIFTP is the same, in Japan, there is no reason to distinguish between FA and NIFTP. Conclusively, the necessity of using the disease entity "NIFTP" is not found in Japan.