Potential Diagnostic Pitfall in Pleomorphic Adenoma: A Case of Squamous and Mucinous Metaplasia with MALAT1::PLAG1 Fusion.

INTRODUCTION

Pleomorphic adenoma (PA) is the most common benign salivary gland tumor, yet its histologic diversity can resemble malignant neoplasms, posing diagnostic challenges. Molecular characterization, including fusion gene analysis, can aid in distinguishing these cases.

CASE PRESENTATION: A 28-year-old male presented with a painless, well-circumscribed 2.5 cm left parotid mass. Imaging showed a lobulated lesion without lymphadenopathy. The patient underwent superficial parotidectomy. The resected mass was encapsulated and tan, with homogeneous consistency. Microscopic analysis revealed squamous and mucinous metaplasia within a typical pleomorphic adenoma background, initially raising concern for mucoepidermoid carcinoma. NGS identified a MALAT1::PLAG1 fusion, confirming the diagnosis of pleomorphic adenoma with squamous and mucinous metaplasia. The patient remains disease-free 21 months postoperatively. This represents only the second reported case of a MALAT1::PLAG1 fusion in pleomorphic adenoma.

CONCLUSION

This case highlights a rare molecular subtype of pleomorphic adenoma associated with the MALAT1::PLAG1 fusion. The presence of this fusion may underlie metaplastic differentiation and mimic malignancy. Recognition of such molecular events is crucial for accurate diagnosis and underscores the importance of integrating histopathology with genomic profiling in salivary gland tumor evaluation.