When Erdheim-Chester Disease Reaches the Pectoral Muscle.

Erdheim-Chester disease (ECD) is a rare histiocytic disorder. The underlying etiology has not yet been fully elucidated. Recent technological and molecular advances have identified aberrant activation of the MAPK/ERK signalling pathway as a central driver of disease, leading to its reclassification as a neoplastic entity rather than solely a multisystem fibroinflammatory condition. These insights have significantly advanced the understanding of disease pathogenesis and are fundamental to the development and implementation of targeted therapeutic strategies. We describe a case of KRAS-mutant ECD with pleural, pericardial, diaphragmatic, and pectoral muscle involvement in a patient with a prior hematologic history. The disease was refractory to corticosteroid therapy, prompting escalation to second-line treatment with a MEK inhibitor. LEARNING POINTS: This case underscores the clinical heterogeneity of Erdheim-Chester disease. The disease can affect virtually any organ system, although skeletal muscle involvement remains exceedingly rare.The present case highlights the critical role of comprehensive whole-body imaging and the acquisition of multiple tissue biopsies from different affected sites to establish an early and definitive diagnosis.Molecular profiling is essential to identify alterations in the MAPK/ERK signalling pathway, as this enables the use of targeted therapies that are associated with substantial improvements in clinical outcomes and overall survival.