Squamous Cell Carcinoma Ex Pleomorphic Adenoma of the Parotid Gland: Unusual Entity and Diagnostic Pitfalls

Background: Carcinoma ex pleomorphic adenoma (PA) of the salivary gland with squamous cell carcinoma (SCC) component is extremely rare and can be easily misdiagnosed as a benign PA or SCC (primary or metastatic).

Case report: A 75-year-old male who had no significant past medical history, presented with a rapid growing right parotid gland mass. A fine needle aspiration revealed malignant cells. Following partial parotidectomy, a 2.4 cm ill-defined mass was grossly identified. Microscopically, it showed a keratinizing SCC with adjacent component of residual PA. Immunohistochemically, the malignant tumor cells were positive for p40, p63 and CK5. The residual PA was focally positive for CAM5.2, SMA, p63 and S100. The pathological features were consistent with SCC ex PA. The patient was well at the 7 month-follow-up post-surgery.

Conclusion: SCC ex PA is a rare entity that can be mistaken for a benign PA with squamous metaplasia, or primary or metastatic SCC. It behaves aggressively and has high recurrence and metastasis rate. Awareness of this disease and the diagnostic pitfalls are essential to avoid misinterpretation in difficult cases.