Spermatocytic tumours containing an intratubular component: a multi‐institutional study emphasizing diagnostic pitfall with germ cell neoplasia in situ

INTRODUCTION: Spermatocytic tumours (ST) are uncommon germ cell tumours generally considered unrelated to germ cell neoplasia in situ (GCNIS). Morphologically, ST can occasionally mimic seminoma, while CD117 can be positive in both. Intratubular spread of ST can also mimic intratubular seminoma/GCNIS, further raising diagnostic consideration for seminoma. METHODS: Herein, we performed a morphological and immunohistochemical investigation of ST focusing on the intratubular component to raise awareness of this potential diagnostic pitfall. RESULTS: Twenty cases of ST demonstrating intratubular spread were collected, re-reviewed and stained (OCT4, CD117, SSX C-terminus and DMRT1). Intratubular ST involved a variable number of tubules (range 3 to >50) typically along the periphery of the invasive ST (90%) and rarely diffusely throughout the testes (10%). ST cells usually filled the entire tubule (95%); rarely, ST cells were present at the base, seemingly in the spermatogonial niche (5%). Intratubular ST typically consisted of homogeneous intermediate-sized cells (80%); only a subset showed the characteristic tripartite morphology (20%). Unlike GCNIS, intratubular ST lacked thickened basement membranes and often demonstrated concomitant spermatogenesis (60%). All cases were SSX C-terminus positive and OCT4 negative. CD117 was variably positive in 80% of cases (1 case only positive in the invasive tumour, 3 cases only positive in intratubular components). DMRT1 was variably positive in all cases, with differing extents/intensities between the invasive tumour and intratubular components. CONCLUSION: Intratubular spread of ST is not an uncommon finding. Awareness of the morphological features as well as judicious use of immunohistochemistry can help avoid mistaking this entity for GCNIS/seminoma.