Role of immunohistochemistry in diagnosing round-cell tumours affecting the oral and maxillofacial regions.

Background: One of the most challenging spectra of lesions in the oral and maxillofacial region (OMFR) are round-cell tumours (RCTs). They show a considerable degree of overlap in microscopy and immunophenotypes. The main aim of this study is to analyse the spectrum of RCTs encountered in the oral and maxillofacial regions. We emphasise the role of immunohistochemistry (IHC) which in conjunction with histological, clinical, and imaging findings is necessary for their correct characterisation. The secondary objectives are to discuss differential diagnosis, workflow, and diagnostic algorithm for round-cell lesions affecting the OMFR. Methods: Formalin-fixed, paraffin-embedded sections of RCTs were retrieved from the archives of the Department of Oral Pathology (January 2018 to March 2020). These cases were analysed by three pathologists independently by evaluating haematoxylin and eosin-stained sections, and immunohistochemical markers employed to characterise these lesions. Results: Under the spectrum of RCTs, 11 cases (0.53%) were diagnosed with a predominance of non-Hodgkin lymphoma (55%) followed by Ewing sarcoma (18%). The remaining were Langerhans cell histiocytosis (9%), neuroendocrine carcinoma (9%), and sinonasal undifferentiated carcinoma (9%). Except for one case, in all cases, the final diagnosis was established with the use of adjunctive IHC. Conclusion: RCTs can pose a diagnostic challenge for inexperienced oral pathologists. Thorough knowledge of the differentials of RCT occurring in oral and maxillofacial is helpful. An algorithm-based diagnostic approach incorporating the clinical, imaging, and histomorphological findings and immunohistochemical evaluation can help in minimizing diagnostic confusion and errors.