Pleomorphic Adenoma with Epithelial Atypia, Apocrine Metaplasia, and/or In situ/Intracapsular Salivary Duct Carcinoma Are Indolent Lesions with Good Prognosis: A Proposal for Unified Nomenclature and Clinical Observation

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Abstract
Purpose: Salivary duct carcinoma (SDC) is the most common malignancy to arise in a pleomorphic adenoma (PA). Intracapsular or non-invasive carcinoma ex PA (CXPA) is defined by the presence of malignant-appearing tumor cells within the PA that do not violate the tumor border. Knowledge regarding the possible morphologic spectrum and prognosis of intratumoral CXPA is scarce. This study aims to evaluate the morphologic, immunohistochemical, and clinical features of PAs with apocrine / salivary-duct-like intratumoral atypia. Methods: Surgical pathology databases were queried for in situ or intracapsular/intratumoral SDC ex PAs and PAs with atypical epithelial cells (AEC). Exclusion criteria included recurrent lesions, invasion, positive margins, atypia only in myoepithelial cells, or other morphologic variants. Chromogenic multiplex (androgen receptor (AR) and HER2) and monoplex (p40) assays were performed on all available cases, as well as on a control group of non-atypical benign PAs and overtly invasive SDCs. Clinical outcomes were recorded. Results: 96 cases were identified: 23 AEC, 6 apocrine metaplasia, 41 benign PA, 8 SDC ex PA. All AEC, apocrine metaplasia, and benign cases were treated with surgery alone, with 3 AEC cases also receiving a neck dissection. No case recurred. Five of 8 SDC ex PA recurred; 3 died of disease. AR and HER2 were respectively expressed in 96% and 22–48% of AEC; 83% and 0% of apocrine metaplasia; 51% and 0% of benign PA; and 86–100% and 38–57% of SDC ex PA. Patients had increasing average age from benign (~ 50 years) to atypical/in situ (60 years) to invasive carcinoma (~ 70 years). Conclusion: The presence of epithelial atypia within a PA (ranging from isolated AR expression to apocrine metaplasia to overtly dysplastic/malignant epithelial cells) does not portend recurrence or metastasis if the atypia is confined within the borders of the adenoma and negative margins are achieved. Therefore, use of the term “in situ/intracapsular/intratumoral salivary duct carcinoma ex pleomorphic adenoma” is discouraged in light of good prognosis and potential for overtreatment by clinical teams. Nomenclature such as pleomorphic adenoma with epithelial “atypia” or “dysplasia” is recommended, followed by a comment regarding the morphologic features and likely indolent behavior.

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