Warthin-Like Mucoepidermoid Carcinoma: A Case Series Addressing the Need for Diagnostic Criteria Manuscript type: Case series

Abstract Warthin-like Mucoepidermoid carcinoma(WLMEC) is a newly described rare variant of Mucoepidermoid carcinoma(MEC). Rarity, coupled with potential misdiagnoses as the benign Warthin tumor necessitates careful examination and understanding of clinical and histological features. Lack of defining criteria in the current classification adds to the conundrum. Cases of WLMEC were retrieved from the archives of Department of Pathology from 2017-2022. Clinical and imaging data were obtained from the Electronic medical record system. A total of three cases, in two males and one female, aged 25, 49, and 64 years were diagnosed based on the proposed diagnostic criteria. A longstanding painless swelling in all cases, and lymphadenopathy in one case was seen at presentation. Imaging, in all cases, showed a hypodense mass with a mean diameter of 2 cm (range 1.1-2.9cm), involving the superficial lobe of parotid. Histomorphology, only superficially reminiscent of Warthin’s tumor at low power, was of a circumscribed tumour with disorganised cuboidal epithelium instead of oncocytic lining, with epithelial to lymphoid ratio varying from 0.4 to 0.9. Admixed areas with classic MEC morphology were seen (100% cases) comprising 5%, 15% and 25% of the tumour respectively. Atypical mitoses, lymphovascular invasion and perineurial invasion was not seen in any case. Immunohistochemistry for CK7 and P40 was diffusely positive in all. Mucicarmine stain highlighted interspersed mucous cells. As per MSKCC, AFIP and Brandwein systems, all these tumours were low grade, which corroborated with the prognosis as no recurrence or distant metastases was seen in a mean follow-up period of 12 months. Low power resemblance of WLMEC to Warthin tumor can lead to an erroneous diagnosis with prognostic implications, which may be obviated by using the proposed diagnostic criteria. Awareness of occurrence in younger age, presence of goblet cells, and areas with classic MEC morphology in a thoroughly sampled tumor might help avoid this pitfall.