Multifocal Benign Metastasizing Pleomorphic Adenoma Presenting as a Lymphoma: An Atypical Clinical Picture Demystified by an Unusual <scp> <i>PLAG1</i> </scp> Gene Rearrangement Detected on <scp>RNA</scp> Next Generation Sequencing

A 57-year-old female presented to urgent care with exertional dyspnea, back pain, and several months of night sweats. Imaging showed an anterior mediastinal mass with concurrent hepatic and vertebral lesions, raising suspicion for a hematolymphoid malignancy. Fine needle aspiration (FNA) and needle core biopsies were obtained from the liver lesion, with morphologic features compatible with pleomorphic adenoma (PA) without evidence of malignant or high-grade cytologic features, adding to the complexity of interpretation. Revisiting her history revealed a diagnosis of a benign salivary gland tumor in the left parotid, which was surgically removed almost 40 years ago. This piece of clinical information put the morphology into perspective and subsequent molecular testing detected an unusual PLAG1 gene rearrangement, confirming the diagnosis of a benign metastasizing PA in this unique case.