A case report of delayed-diagnosed adult multisystem Langerhans cell histiocytosis with isolated MAP2K1 somatic mutation and systematic literature review.

Numerous case reports regarding adult multisystem Langerhans cell histiocytosis (MS-LCH) have been published, yet the vast majority only describe single-organ involvement. Cases presenting sequential multi-organ lesions, clinical manifestations mimicking idiopathic hypophysitis and refractory gastritis, accompanied by long-term missed diagnosis and delayed definitive confirmation remain extremely rare. This article reports a 44-year-old female patient with delayed-diagnosed multisystem LCH. Her initial symptom was isolated central diabetes insipidus, which was misdiagnosed as infundibulohypophysitis. Subsequently, she developed intractable gastrointestinal discomfort accompanied by unexplained 10 kg weight loss. The definitive diagnosis was established nearly 10 months after symptom onset until a subcutaneous frontal skull mass emerged. Pathological diagnosis was obtained via surgical resection. The patient received cytarabine induction chemotherapy followed by salvage therapy combining TCD regimen and denosumab, achieving sustained partial remission of systemic lesions. Targeted next-generation sequencing identified a grade II somatic MAP2K1 mutation without the common BRAF V600E hotspot mutation, providing definitive molecular evidence for long-term maintenance targeted therapy with MEK inhibitors. This case is intended to deepen clinicians' recognition on the diagnostic and therapeutic strategies of adult LCH, facilitate early confirmed diagnosis and timely clinical intervention to reduce related complications, and further explore its underlying pathogenesis as well as individualized management regimens.