Uncovering Uncommon Pleural Complications of RET Tyrosine Kinase Inhibitor Therapy: A Case of Selpercatinib-Induced Chylothorax in a Patient with Metastatic Medullary Thyroid Carcinoma

INTRODUCTION: Selpercatinib, a highly selective RET tyrosine kinase inhibitor (TKI), was recently approved for RET-altered lung and thyroid cancers, showing substantial objective response rates and a favorable safety profile. However, with increased clinical use, novel treatment-emergent adverse events (TEAEs), including pleural effusion (PE), have been reported. These can be misinterpreted as disease progression, complicating management and treatment decisions. We present a significant case of selpercatinib-induced chylothorax in a patient with metastatic medullary thyroid carcinoma (MTC).