Abstract

Langerhans cell histiocytosis (LCH) is a rare neoplastic disorder that presents more often in children but may occur at any age. While pulmonary and bone involvement of LCH are most common, the skin is affected in 25-40% of cases. Cutaneous involvement of LCH often presents in the scalp or intertriginous regions with a wide range of morphologies. Given its rarity and heterogeneous clinical presentations, LCH can be prone to misdiagnosis.   We present a case of an 85-year-old female who was referred for a pruritic intertriginous rash. An outside biopsy reported an epidermotropic atypical lymphocytic infiltrate with CD30 positivity, concerning for lymphomatoid papulosis or mycosis fungoides. Repeat biopsy demonstrated mononuclear infiltrate of large cells with abundant cytoplasm, prominent nuclear grooves, and epidermotropism. Upon review of clinical photos, the patient had scattered eroded and crusted red papules in intertriginous areas. In the inguinal folds, the papules coalesced into confluent eroded plaques. With an expanded clinical differential including LCH and extramammary Paget disease, immunohistochemical profiling revealed the enlarged cells were positive for S100 and CD207, consonant with LCH. Notably, CD30 was positive in some of the epidermotropic Langerhans cells.   This case demonstrates the challenge in diagnosing LCH. Biopsies with prominent epidermotropism and enlarged cells without classic reniform nuclei may require clinical suspicion for LCH to result in an accurate diagnosis. Given that Langerhans cells show CD3 positivity, some cases may be misdiagnosed as mycosis fungoides. Dermatopathologists should continue to consider LCH in biopsies showing prominent epidermotropism and correlate with clinical appearance