INI1 (SMARCB1)-Deficient Sinonasal Carcinoma: A Clinicopathologic Report of 2 Cases.

Poorly differentiated sinonasal malignancies are amongst the hardest differential diagnoses in pathology, owing to the large number of rare entities that arise there. Complicating the matter is that most pathologists, including those with experience in head and neck pathology, have little experience in any one of these rare entities. Most patients with sinonasal carcinoma present with locally advanced disease and in the past a combination of chemotherapy, radiotherapy, and surgery would usually be recommended without the specific disease subtype playing a large part of the decision making. However, in the era of "precision medicine" and targeted therapies, the specific tumour subtype and an accurate diagnosis will become increasingly important even for the so-called "undifferentiated carcinoma". Specific entities that tend to enter into the differential diagnosis include olfactory neuroblastoma, sinonasal undifferentiated carcinoma (SNUC), and non-keratinizing squamous cell carcinoma (viral and non-viral). However, recent new entities, such as NUT-midline carcinoma also have to be considered. Recently it was found that a subset of tumours originally diagnosed as one of the aforementioned entities all demonstrated loss of the ubiquitously expressed protein Integrase Interactor 1 (INI1; SMARCB1). These tumours were often basaloid with at least partial rhabdoid differentiation and most were considered a part of the SNUC spectrum. In this report, we describe two additional cases of INI1-deficient sinonasal carcinoma prospectively identified, both of which appeared to have a marked response to neo-adjuvant chemoradiation, a finding not previously described.