Fenestrating Adenocarcinoma: A Novel Salivary Gland Cancer With EWSR1::BEND2 Fusion and Predilection for the Pharynx.

Most salivary gland tumors are found to have genetic fusions driving their oncogenesis, with an explosion of such findings in recent years. They typically represent previously identified and well-characterized tumors and the fusions are generally not necessary for diagnosis but rather serve as diagnostic arbitrators in challenging cases. That said, occasional unexpected molecular findings are broadening our understanding of these common tumors and have led to the emergence of new entities, some of which were "hiding in plain sight" and are quite common, like secretory carcinoma. In contrast, others are sufficiently rare that molecular testing was required to identify them in small cohorts, combining cases from multiple institutions, such as microcribriform adenocarcinoma. Recently, we identified a rare case of tongue adenocarcinoma with EWSR1::BEND2 fusion with an unusual morphology, and 2 additional cases were subsequently briefly reported in the literature. In this study, we collected and analyzed a total of 7 cases that show variable but consistent morphology, including a fenestrating glandular appearance, occasional squamous or basaloid growth, and focal mucinous cells. The tumors showed a predilection for the pharynx with 4 cases in the base of the tongue, one case in the tonsil/oropharyngeal wall, one case in the nasopharynx, and only one nonpharyngeal case in the trachea. No oral cavity or major salivary gland tumors were found with this set of findings to date. The tumors showed some aggressive tendencies, with 2/4 with follow-up information having adverse outcomes, including lymph node metastases in 2 cases and 1 of these cases also having brain metastases, recurrence, and death from disease. All cases tested showed EWSR1::BEND2 fusion by NGS, as well as CK7/BEND2 positivity, and S100 negativity by immunohistochemistry. The tumors showed variable p63 staining depending on whether squamous or basaloid features were present. A tissue microarray (TMA) stained with an antibody directed against the BEND2 protein showed no significant staining in any other salivary tumor type. The constellation of findings in this cohort is highly suggestive of a specific entity, and we propose the appellation "Fenestrating Adenocarcinoma (FAC)" of the salivary gland.