Paediatric follicular thyroid carcinoma – indolent cancer with low prevalence of RAS mutations and absence of PAX8–PPARG fusion in a Japanese population Huy Gia Vuong, 1 Tetsuo Kondo, 1 Naoki Oishi, 1 Tadao Nakazawa, 1 Kunio Mochizuki, 1 Akira Miyauchi, 2 Mitsuyoshi Hirokawa 3 & Ryohei Katoh 1 1 Department of Pathology, University of Yamanashi, Yamanashi, 2 Department of Surgery, Kuma Hospital, Kobe, and 3 Department of Pathology, Kuma Hospital, Kobe, Japan Date of submission 31 March 2017 Accepted for publication 12 June 2017 Published online Article Accepted 16 June 2017 Vuong H G, Kondo T, Oishi N, Nakazawa T, Mochizuki K, Miyauchi A, Hirokawa M & Katoh R (2017) Histopathology 71, 760–768. https://doi.org/10.1111/his.13285 Paediatric follicular thyroid carcinoma – indolent cancer with low prevalence of RAS mutations and absence of PAX8–PPARG fusion in a Japanese population Aims: Paediatric follicular thyroid carcinomas are uncommon, and their clinicopathological features and molecular profiles are still unknown. In the pre- sent study, we aimed to investigate the clinicopatho- logical aspects of a large series of follicular thyroid carcinomas (FTCs) in paediatric patients and to anal- yse the point mutations in codons 12, 13 and 61 of NRAS, HRAS and KRAS genes and the rearrange- ments of PAX8–PPARG. Methods and results: A total of 41 paediatric FTCs less than 21 years of age were enrolled into the pre- sent study. We used direct sequencing and reverse transcription–polymerase chain reaction (RT–PCR) to detect RAS mutations and PAX8–PPARG fusions, respectively. The paediatric FTCs were 6:1 in a female to male ratio, with a mean tumour size of 52.7 mm. Distant metastasis was found in one case at the time of presentation. During a median follow-up time of 69 months, two cases had lung metastasis and all patients were alive. Histologically, all cases were min- imally invasive FTCs and varied in growth patterns: microfollicular (39%), follicular (14.6%), solid/trabec- ular (6%), oncocytic (4.9%) and mixed patterns (26.8%). The mean Ki67 index was 5.7% and it was not statistically different among the growth patterns. NRAS mutations were found in five cases (12.2%) and associated significantly with small tumour size (P = 0.014). PAX8–PPARG fusion was not detected in our series. Conclusion: Paediatric FTCs are indolent in clinical course in spite of their large tumour size and have a distinct genetic background. RAS mutations and PAX8–PPARG fusions may not play major roles in the tumorigenesis of paediatric FTCs. Keywords: adolescent, cancer, childhood, follicular thyroid carcinoma, paediatric, RAS mutation Introduction Thyroid cancer in paediatric patients is uncommon, accounting for only 5% of all thyroid cancers. 1 Most studies on paediatric thyroid cancer have focused mainly on its most common histological type, papillary thyroid carcinoma (PTC). In paediatric thy- roid cancer, PTCs account for 80–85% and follicular thyroid carcinomas (FTCs) comprise only of 10–15% of cases. 2,3 Interestingly, paediatric PTCs were reported to manifest different clinicopathological fea- tures compared with adult PTCs, which tend to have solid growth in histology, more aggressive clinical behaviour but a favourable outcome and different genetic profiles. 1,4,5 In contrast, little is known about the clinicopathological features of FTC in paediatric Address for correspondence: R Katoh, Department of Pathology, University of Yamanashi, 1110 Shimokato, Chuo-shi, Yamanashi 409-3898, Japan. e-mail: rkatoh@yamanashi.ac.jp © 2017 John Wiley & Sons Ltd. Histopathology 2017, 71, 760–768. DOI: 10.1111/his.13285