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Olfactory neuroblastoma (ON; Esthesioneuroblastoma) is a malignant tumor that arises from the olfactory neuroepithelium. Very rarely, ON can histologically display a biphenotypic pattern, with only 7 cases reported in the literature to date. We describe a case of this poorly understood entity and review the patient's histology, pathology, and treatment. An 85-year-old man presented with endoscopy and imaging findings of a sinonasal mass arising from the olfactory cleft. Biopsy and further pathological review established Hyams grade 3 and Kadish stage C tumor. The patient underwent combined endoscopic and open-approach resection due to the involvement of the nasal bone and subsequent adjuvant radiation therapy. Specimen contained Flexner-Wintersteiner rosettes and stained positive for the neuroendocrine marker synaptophysin consistent with ON. A second, epithelial component pattern of goblet cells and mucin was observed suggesting divergent differentiation. We are the first group to report next-generation sequencing of this tumor, which revealed a pathogenic mutation in PIK3CA and a likely pathogenic variant in RUNX1 (AML1). ON with divergent differentiation is very rare, and more robust studies characterizing molecular drivers and pathology may aid in clinical management., Competing Interests: Declaration of Conflicting InterestsThe author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Sinonasal small cell neuroendocrine carcinoma (SNEC) is an extremely rare and aggressive neoplasm that can arise in the sinonasal region. These tumors are associated with high morbidity and mortality, are difficult to diagnose, and are hard to treat. We describe 2 cases of this poorly understood malignancy and review imaging, pathology, and treatment decisions. A 41-year-old male and a 64-year-old female presented to a tertiary center in 2019 after developing nasal obstruction and were found to have sinonasal masses on imaging. Both biopsies showed strong expression of pancytokeratin with dot-like reactivity and expression of neuroendocrine markers chromogranin and synaptophysin. The findings were diagnostic of SNEC. Staging positron emission tomography/computed tomography and brain MRI were performed, and patients were discussed at a multidisciplinary tumor board. Neither had distant metastatic disease at presentation. One patient had no intracranial or orbital disease and underwent a subtotal endoscopic resection with adjuvant chemoradiation. The other patient demonstrated middle cranial fossa, dural, and orbital involvement as well as cranial nerve V palsy. This patient was treated with induction chemotherapy, followed by concurrent chemoradiation. Both patients are presently alive at 4 months follow-up, but one with persistent local disease and the other distant metastasis. Sinonasal small cell neuroendocrine carcinoma is a rare and poorly understood malignancy with an aggressive clinical course. Continued careful review of pathology and study of molecular features are needed for improved understanding of SNEC, and particularly for head and neck SNEC, to establish a staging system and better formulate treatment protocols.