Fine-needle aspiration biopsy of a cystic pleomorphic adenoma with extensive adnexa-like differentiation: differential diagnostic pitfall with mucoepidermoid carcinoma

Although fine-needle aspiration biopsy (FNAB) is a highly accurate tool for the diagnosis of pleomorphic adenomas, even this common salivary gland neoplasm can be diagnostically challenging and cause pitfalls in cytodiagnosis. In particular, the presence of either cystic degeneration or squamous and mucinous metaplasia can lead to a false positive diagnosis of malignancy. Here we present the case of a 16-year-old female who presented with a painless, slowly growing mass in the superficial lobe of the right parotid gland. Magnetic resonance imaging of the parotid demonstrated a mass with heterogeneous postcontrast enhancement and a central, nonenhancing area suggestive of necrosis. FNAB of the lesion yielded proteinaceous debris and numerous whorls of keratin, small cohesive clusters of basaloid and squamoid epithelial cells, and many vacuolated and foamy cells. Initially and after consultation at an outside institution, the FNAB was reported as an "atypical neoplasm, cannot exclude mucoepidermoid carcinoma." A total, nerve-sparing parotidectomy and level II neck dissection revealed a pleomorphic adenoma with central cystic degeneration and extensive mixed appendageal differentiation. While foci of squamous metaplastic changes may occur in pleomorphic adenomas, the combination of cystic degeneration and extensive appendageal differentiation with numerous keratotic cysts is rare, and it presents the potential for misinterpretation of the FNAB as indicative of malignancy in general, and mucoepidermoid carcinoma in particular.