SMARCB1/INI1 Deficient Sino-Nasal Carcinoma: Extending the Histomorphological Features

SMARCB1/INI1 deficient sinonasal carcinoma is a variant of sinonasal undifferentiated carcinoma (SNUC). There is a paucity of literature describing the histomorphological features of this relatively new entity. Herein we describe the histomorphological features of three such cases and review the literature. We retrospectively reviewed the cases of SNUC diagnosed in our institute in the last 6 years. Immunohistochemistry for INI1, NUT & p16 were performed on these cases. Three cases showed loss of INI1. The histomorphology and clinicopathological features of these cases were studied and compared with non INI1 deficient SNUC. A total of 9 cases of SNUC were identified. Three of these cases showed loss of INI1. These three cases had presented with large sinonasal mass and with intracranial extension. Histopathology of these cases showed a diffuse infiltrative pattern, nest, and islands of predominantly basaloid cells with focal rhabdoid morphology. Additional features like small cell carcinoma like pattern, pseudoalveolar and pseudoglandular patterns, clear vacuoles and pseudopapillary appearance were also noted. We conclude that in presence of a mixed pattern of cells with a predominance of basaloid morphology, the possibility of SMARCB1/INI1 deficient sinonasal carcinoma must be strongly suspected and immunohistochemistry for INI1 must be performed.Electronic supplementary materialThe online version of this article (10.1007/s12105-020-01246-9) contains supplementary material, which is available to authorized users.