A 57-year-old woman with no significant medical history presented for routine screening colonoscopy. An incidental ulcer was found 50 cm from the anus, and biopsy revealed colonic mucosa with an underlying proliferation of atypical cells with large plump oval nuclei with overt grooves, open chromatin, inconspicuous or small nucleoli, and abundant pale eosinophilic cytoplasm with numerous admixed eosinophils (panels A-B; hematoxylin and eosin stain, original magnification ×20 [A] and ×100 [B]). The atypical cells expressed CD1a (panel C; original magnification ×10), S100 (panel D; original magnification ×10), CD68 (small subset, not shown), BRAF V600E (panel E; Ventana, clone VE1, original magnification ×40), and cyclin D1 (panel F; original magnification ×10), consistent with Langerhans cells. The overall findings were consistent with Langerhans cell histiocytosis (LCH).

LCH in adults is an uncommon histiocytic disorder, associated with BRAF mutations in ∼50% of cases, that may either be limited to 1 organ, most commonly the bones, or present as a multisystem disorder. Gastrointestinal tract involvement is extremely rare, most commonly found in children with widespread disease. In adults, the predominant presentation is as a solitary polyp or ulcer in the colorectal tract, often as an incidental finding in middle-aged women with no systemic involvement. Despite the indolent behavior, this lesion may be characterized by mutations in BRAF p.V600E, as seen in this case. The activation of BRAF was further confirmed by the downstream overexpression of cyclin D1.