From suspected brain malignancy to Erdheim-Chester disease: the role of PET imaging and bone marrow biopsy in diagnosis.

Erdheim-Chester Disease (ECD) is an extremely rare non-Langerhans cell histiocytosis involving MAPK pathway mutations, notably BRAFV600E. We report a 60-year-old male with a history of BRAF-negative malignant melanoma and a new melanoma in situ, who presented with diabetes insipidus, hypogonadism, ataxia, blurred vision, dysarthria, headache, personality changes, and disturbed sleep patterns with kicking and mumbling. Neurological and systemic features raised suspicion for malignancy. PET imaging was more informative than CT or MRI and identified widespread FDG-avid skeletal and soft tissue lesions. Because of skeletal lesions, a bone marrow biopsy was performed despite a normal complete blood count and proved valuable as compared to biopsy of less accessible tissues. Histopathology from skin and bone marrow confirmed ECD with BRAFV600E mutation. This case highlights the diagnostic role of PET and bone marrow biopsy in ECD diagnosis. Multidisciplinary evaluation and targeted therapy with a BRAF inhibitor led to symptom resolution.