Aspiration cytology of pleomorphic adenoma with squamous metaplasia: A case series and literature review illustrating diagnostic challenges

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OBJECTIVE: Squamous metaplasia is not uncommonly observed in salivary gland neoplasms, including pleomorphic adenomas (PAs). It poses as a diagnostic pitfall with cytomorphologic features overlapping with low-grade epithelial malignances and carcinoma ex-PA. We report series of aspirates of PAs with squamous metaplasia and review the literature for the clinicopathological and cytologic features of this diagnostically challenging entity. METHODS: Three cases including index aspiration cytology, post-aspiration specimen cytology, cell block immunohistochemistry (IHC), histologic correlation, and clinical data were reviewed. Literature search was performed for cases of PAs with squamous metaplasia, yielding 22 case reports including 11 with aspiration cytology performed. RESULTS: Metaplastic squamous components were seen in all index aspirations, but foreign body reaction and cystic changes were observed only after aspiration or in irritated lesions. PLAG1 IHC demonstrated reliable immunoreactivity in cell block preparations. Summarizing cases in this series and in the literature, cytologic features reported were variable and no consistent diagnostic feature was identified. Most reported cases were initially diagnosed as suspicious or malignant, but all ran a benign clinical course. Significant morbidity was reported only in one case due extensive surgery based on a malignant diagnosis. CONCLUSIONS: Squamous metaplasia in PAs is not associated with adverse outcomes, but may induce reactive changes upon aspiration/irritation, resulting in worrisome cytologic features. PLAG1 IHC is a useful adjunct when characteristic cytologic features for PAs are absent or obscured. Prudent use of the diagnostic category salivary gland neoplasm of uncertain malignant potential (SUMP) in the Milan system can avoid overtreatment.

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