GROSS
Well-circumscribed, yellow, semi-firm 1 cm mass on thumb

MICROSCOPIC
Mixed mononuclear cells within dermis, which consist primarily of histiocyte-type cells with abundant foamy cytoplasm, round to oval nuclei with irregular nuclear contours, fewer lymphocytes and neutrophils. These cells are interposed with numerous Touton-type multinucleated giant cells.

IMMUNOHISTOCHEMISTRY
Positive: CD68, FXIIIa
Negative: S-100, CD1a, myoD1, desmin, smooth muscle actin

DESCRIPTION
- Juvenile xanthogranuloma (JXG) is an uncommon non-Langerhans cell histiocytosis (negative for CD1a and S100).
- The incidence among pediatric tumors is 0.5% (Kiel Pediatric Tumor Registry). The male/female ration is 1.4:1. The mean age is 22.4 months; median age is 5 months; age range 0-244 months.
- Sites of involvement: mostly cutaneous (face or trunk), soft tissue, CNS, eye, visceral organs
- Focality: 81% of cutaneous lesions are solitary lesions; less than 4% of cases with systemic (visceral) involvement
- Morphologic subtypes: early (dense monomorphic histiocytic infiltration without lipidization, generally lacking Touton-type gian cells), classic (foamy histiocytes with variations of fully developed Touton giant cells), transitional (predominance of spindle-shaped cells, with foci of foamy histiocytes and giant cells, seen more in visceral organs) or combined
- Prognosis/Treatment: generally favorable; low relapse rate (7%); conservative excision; involution even after incomplete resection; multimodal chemotherapy (LCH-type chemotherapy) for the very rare systemic JXG

DIFFERENTIAL DIAGNOSIS
- Langerhans cell histiocytosis – positive for S100 and CD1a; negative for FXIIIa; only weak positive for monocyte/macrophage antibodies; lack or small number of Touton giant cells; EM shows Birbeck granules in histiocytes (JXG cells do not)
- Xanthoma – history (associated with hyperlipidemia); no other inflammatory cells