Diagnosis of Adenoid Cystic Carcinoma with Striking Tubular Hypereosinophilia by MYB and EWSR1 Breakapart Fluorescence In Situ Hybridization

Background Adenoid cystic carcinoma (AdCC), associated with MYB/MYBL1 gene rearrangements, shows epithelial and basaloid myoepithelial cells arranged in tubular, cribriform and solid patterns. Variations from this classic morphology make diagnosis challenging, necessitating molecular testing. AdCC with striking tubular hypereosinophilia (AdCC-STE) is one such recently described histological subtype. Methods A 52-year-old female presented with a floor of mouth swelling for two months, diagnosed elsewhere as polymorphous adenocarcinoma (PAC). A biopsy was obtained. With a diagnosis of oncocytic neoplasm, wide excision of the tumor was undertaken. Histological examination, fluorescence in situ hybridization (FISH) and ultrastructural examination were performed. Archival cases of PAC and epithelial myoepithelial carcinoma (EMC) were reviewed, and MYB immunostaining and FISH were performed to identify potential AdCC-STE cases. Results The excised tumor from the index patient showed bilayered tubules, micropapillae and cribriform pattern. Luminal cells with hypereosinophilic to clear cytoplasm were surrounded by flattened abluminal cells. Focally, basophilic matrix was seen within sharply demarcated pseudocystic spaces. FISH revealed MYB and EWSR1 gene rearrangements, confirmatory of AdCC-STE. Electron microscopy showed features consistent with AdCC; however, mitochondria were not prominent. Among 14 archival PACs, two showed MYB immunopositivity; one showed MYB rearrangement but was classical AdCC. Among 35 EMC, one case showed MYB immunoreactivity and eosinophilia of luminal cells but lacked MYB/MYBL1 rearrangement. Conclusion Awareness of unusual histological subtypes of AdCC, such as AdCC-STE, is imperative, as it may be misdiagnosed as PAC and EMC, among others. Presence of basophilic matrix and squamoid morules in a biphasic tumor even with hypereosinophilic rather than basaloid myoepithelial appearance should raise suspicion for AdCC-STE, and prompt molecular testing for confirmation. With wider accessibility, lower cost and significantly shorter turn-around-time when compared to RNA sequencing, FISH can be employed for confirmation of diagnosis, especially in low- and middle-income countries.