Histiocytic & Dendritic Cell Disorders

Classification

• Group L (Langerhans-related): LCH, Erdheim-Chester disease (ECD), mixed LCH-ECD, indeterminate cell histiocytosis.
• Group C (cutaneous and mucocutaneous non-LCH): xanthogranulomas and related.
• Group R (Rosai-Dorfman / sinus histiocytosis with massive lymphadenopathy).
• Group M (malignant histiocytoses): histiocytic sarcoma, interdigitating dendritic cell sarcoma. Follicular dendritic cell sarcoma is a separate mesenchymal/stromal neoplasm, not a histiocytosis.
• Group H (HLH-related): hemophagocytic lymphohistiocytosis (familial and secondary; see HLH note).

IHC Quick Reference

Langerhans Cell Histiocytosis (LCH)

• Pathogenesis: clonal myeloid neoplasm, not a reactive process. BRAF V600E mutation in ~50 to 60%; MAP2K1 (MEK1) mutations in many BRAF-wild-type cases. Now regarded as a MAPK-pathway disease. LCH lesions share the common histology of CD1a+/CD207+ dendritic cells.
• Demographics: peak age 1 to 3 yo (children) and adults; ~3 to 5 per million children; ~1 per million adults.
• Clinical spectrum:
  • Single-system unifocal: bone (eosinophilic granuloma, most common; skull, femur, vertebrae); may regress spontaneously.
  • Single-system multifocal: multiple bone lesions.
  • Multi-system: with or without "risk organ" involvement (liver, spleen, BM); risk-organ involvement defines high-risk disease.
  • Special sites: pituitary stalk → diabetes insipidus (~25%); CNS (cerebellum, brainstem); skin (seborrheic-dermatitis-like, classically scalp/intertriginous); pulmonary LCH (adult smokers).
• Pathology: CD1a+, S100+, Langerin (CD207)+; Birbeck granules (tennis-racket) on EM; CD68 variable.
• Workup: skeletal survey plus bone MRI/CT; PET-CT (lesions FDG-avid); brain MRI including pituitary; PFTs and HRCT chest if pulmonary; LFTs; CBC; BRAF V600E testing on biopsy.
• Treatment:
  • Isolated/single bone lesion: curettage and/or corticosteroid injection; observation if asymptomatic; low-dose RT for spinal or CNS-adjacent lesions.
  • Bone-only disease: preferred treatment is a bisphosphonate.
  • Single-system multifocal skin disease: MTX or hydroxyurea.
  • Single-system LCH generally: single-agent prednisone, vinblastine plus prednisone, curettage of bone lesions, topical therapy for skin; some can be observed.
  • Multi-system LCH: induction with vinblastine plus prednisolone, or cytarabine alone; continue therapy after response for a total course of ~12 months (reduces relapse). In adults, vinblastine/prednisone has marked toxicity and suboptimal efficacy, so many treat adult multifocal, multisite, or at-risk-organ disease with cytarabine 100 mg/m2 per dose x5 days per month x12 months (CNS or CNS-risk lesions and progressive neurodegeneration: higher dose ~150 mg/m2).
  • BRAF V600E mutated: vemurafenib (dabrafenib also active).
  • MAPK-pathway mutation, undetectable mutation, or testing unavailable: cobimetinib (MEK inhibitor).
  • Irrespective of mutation: cytarabine or cladribine.
  • Pulmonary LCH: smoking cessation may lead to resolution; if symptomatic progression or pulmonary dysfunction, systemic therapy with cladribine (preferred) or cytarabine; corticosteroid monotherapy lacks convincing efficacy; cladribine for progressive disease.
• Historical eponyms: lytic (skull) bone lesions plus diabetes insipidus plus exophthalmos = Hand-Schuller-Christian disease; infants with disseminated inflammatory lesions involving liver, spleen, and bone marrow = Letterer-Siwe disease.

Erdheim-Chester Disease (ECD)

• Adult-onset non-LCH histiocytosis; rare; average age of onset ~50 years. Excessive production of lipid-laden CD68+ (foamy) histiocytes accumulating in multiple tissues and organs.
• BRAF V600E in about 50 to 60% of ECD (MAP2K1, NRAS, KRAS, ARAF in others); also MAPK-driven.
• Distribution/classic findings: predominantly the long bones (arms and legs) with bilateral symmetric osteosclerosis; can infiltrate the retro-orbital region, kidney (perinephric "hairy kidney" on CT), skin, brain, lung, heart (right atrial mass, pericardium), pituitary gland, and retroperitoneum (retroperitoneal fibrosis); aortic periarterial sheathing ("coated aorta"); neurologic (diabetes insipidus, cerebellum).
• IHC: CD68+, factor XIIIa+, CD163+; CD1a-negative (key distinction from LCH); S100 variable.
• Treatment:
  • Vemurafenib for BRAF V600E mutation (FDA Nov 2017, first FDA-approved drug for ECD).
  • Cobimetinib for a MAP-kinase-pathway mutation or no detectable mutation (FDA Oct 2022).
  • Cladribine, or pegylated interferon alpha-2a/2b, can be offered irrespective of mutation; sirolimus.

Rosai-Dorfman Disease (Sinus Histiocytosis with Massive Lymphadenopathy)

• Uncommon histiocytic disorder with over-production of non-Langerhans sinus histiocytes.
• Presentation: massive, often cervical lymphadenopathy, sometimes with organ damage; peripheral LAD, subcutaneous nodules, and extranodal sites (skin, soft tissue, upper respiratory tract, bone, retroperitoneum, orbits); fever, leukocytosis, ↑ ESR. High serum immunoglobulins may require plasmapheresis.
• Pathology: S100+, CD68+, CD163+; CD1a-negative; histiocytes with round nuclei, central nucleoli, and emperipolesis (intact lymphocytes within histiocytes, characteristic).
• Staging/workup: whole-body PET-CT including the distal extremities; send pathology for targeted NGS for MAPK-pathway mutations (KRAS, MAP2K1) and a gene-fusion assay.
• Treatment:
  • No standard therapy; spontaneous remission in about 20 to 50% of nodal/cutaneous RDD. Observe selected uncomplicated asymptomatic disease.
  • Asymptomatic unifocal or multifocal disease: careful observation.
  • Symptomatic resectable unifocal disease: surgical resection.
  • Multifocal disease needing therapy: systemic therapy. Cobimetinib is preferred for MAPK-pathway mutation or no detectable mutation; cladribine, cytarabine, oral MTX, and prednisone can be offered irrespective of mutation; crizotinib for an ALK fusion; sirolimus.
  • Palliative radiation for large tumors with CNS, ocular, or internal-organ involvement.

Histiocytic Sarcoma

• Rare aggressive malignancy of histiocytic lineage; nodal or extranodal (GI, skin, soft tissue).
• IHC: CD68+, CD163+, lysozyme+; CD1a-negative, CD21-negative, CD23-negative, ALK-negative.
• Often arises with a prior lymphoid neoplasm (CLL, FL, MZL; "transdifferentiation").
• Aggressive; anthracycline-based chemotherapy; MAPK inhibitors if BRAF/RAS-mutated.

Dendritic Cell Sarcomas

• Follicular dendritic cell sarcoma (FDCS): arises from FDCs; CD21+, CD23+, CD35+; nodal > extranodal; surgery for localized disease, chemo plus RT for advanced.
• Interdigitating dendritic cell sarcoma: rare; S100+, CD68+; aggressive; chemo-resistant.
• Blastic plasmacytoid dendritic cell neoplasm (BPDCN): distinct WHO 2022 entity; CD4+, CD56+, CD123+, TCL1+, CD303 (BDCA-2)+; cutaneous purple/red lesions, leukemic phase, BM involvement; aggressive. Tagraxofusp (Elzonris), a CD123-targeted toxin, FDA Dec 2018; pivekimab sunirine (Decnupaz), a CD123-directed ADC, FDA May 2026 (frontline and R/R; boxed warning for hepatotoxicity/VOD); allo-HSCT consolidation in CR1. (See the dedicated BPDCN note.)

High-Yield Pearls

• BRAF V600E drives ~50 to 60% of both LCH and ECD; vemurafenib/dabrafenib are highly active.
• LCH IHC: CD1a+ S100+ Langerin (CD207)+, with Birbeck granules.
• ECD IHC: CD68+ factor XIIIa+ CD1a-negative, distinguishing it from LCH.
• Hairy kidney + coated aorta + bilateral symmetric long-bone sclerosis = Erdheim-Chester.
• Emperipolesis + massive (cervical) LAD + S100+ CD1a-negative = Rosai-Dorfman.
• Pituitary stalk lesion + diabetes insipidus in a child: consider LCH.
• Bone-only LCH: bisphosphonate; single-system skin LCH: MTX or hydroxyurea.
• Cobimetinib is the go-to for MAPK-mutant or mutation-negative LCH/ECD/RD; an ALK fusion in an RDD-like lesion indicates ALK-positive histiocytosis (a distinct WHO-HAEM5 entity that mimics RDD, including emperipolesis); treat with an ALK inhibitor (crizotinib).
• FDCS: CD21/CD23/CD35. Pulmonary LCH: smoking adults; HRCT shows centrilobular and cavitating nodules plus bizarre/irregular cysts, upper and mid zone predominant with costophrenic-angle sparing.