Histology, 12p status, and IMP3 expression separate subtypes in testicular teratomas

INTRODUCTION: Two types of testicular teratomas are distinguished by the current WHO classification. Prepubertal-type teratomas are benign, while postpubertal-type teratomas are considered malignant with metastatic potential, and are associated with germ cell neoplasia in situ. Prepubertal-type cases have been reported in the adult testis potentially causing confusion and overtreatment. Demonstration of the absence of 12p abnormalities with fluorescence in situ hybridization may facilitate diagnosis. Recently, IMP3 has emerged as a potential marker of malignancy in this context. AIMS: The aim of this study was to assess histological characteristics, IMP3 expression and the presence of 12p abnormalities of pure testicular teratomas. RESULTS: Thirty-seven cases were studied, 7 patients were children and 30 were adults. Six out of 7 pediatric cases showed no 12p abnormality and were IMP3 positive. Seventy-four percent and 79% of adult cases showed 12p abnormalities and IMP3 expression, respectively. Negative cases were not associated with in situ neoplasia or metastasis, they were smaller (mean, 14 vs 39 mm), showed less histological diversity (2.4 vs 4.0 types of tissues on average) compared to positive cases. CONCLUSION: Our study provides further evidence that prepubertal-type (type I) teratomas may appear in adult testes, thus teratomas in adults may be either benign (type I) or malignant (type II). IMP3 expression may aid the distinction between type I and type II teratomas of the postpubertal testis even when GCNIS and 12p status cannot be assessed. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1007/s00428-020-02771-2) contains supplementary material, which is available to authorized users.