Indeterminate Cell Histiocytosis Mimicking Rosacea
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Indeterminate cell histiocytosis (ICH) is a rare proliferative disorder of histiocytes, which display morphologic and immunophenotypic characteristics of both Langerhans cell histiocytosis (LCH) and non-Langerhans cell histiocytosis (NLCH).

A 55-year-old woman in otherwise good health presented with a history of burning and confluent erythematous papules on the face and arms for the preceding six months. Prior to being referred the patient had been treated for rosacea unsuccessfully. A biopsy was taken which suggested Langerhans cells on H&E. Additional biopsies of the chin and the arm were then performed.

Histopathologic examination demonstrated a dense nodular infiltrate of mononuclear cells. Immunoperoxidase staining of this infiltrate was strongly positive for CD1a and S100 (not shown), as well as less densely positive for CD68. Staining for Langerin was performed and was negative. No appreciable neutrophils or eosinophils were present, and there was no significant epidermotropism.

The presence of CD68, a monocyte-macrophage marker, in combination with the S100 and CD1a is typical of LCH; however, the lack of Langerin makes this case consistent with ICH. At this point the patient was referred to the hematology/oncology department.

Indeterminate cell histiocytosis is a rare histiocytosis that features an overlapping immunochemical and cytologic phenotype between Langerhans and non-Langerhans histiocytoses. This case demonstrates that ICH should be considered in the differential diagnosis of progressive, rosacea-like eruptions, which are refractory to treatment, especially when they involve less common sites for rosacea such as the extremities.