Abstract

A 56-year-old woman presented with periocular yellow plaques for 10 years and progressive diffuse erythematous eruptions. A previous skin biopsy showing dermal atypical monohistiocytic proliferation was interpreted as myeloid neoplasm. She was treated with azacitidine with clinical response. She currently presented with marked acral joint deformities and multiple periarticular papules and nodules for a year. Clinical workup detected elevated ANA and markedly increased in IL-2R. Skin biopsies of the largest nodule around the elbow and one of the papules on the thigh revealed scattered mononuclear and multinucleated cells in an interstitial pattern of growth involving mid and deep dermis, more pronounced in the thigh lesion. The elbow lesion also showed scar formation. The histiocytic cells displayed abundant amphophilic cytoplasm with collagen phagocytosis highlighted by trichrome stain but without elastophagocytosis by elastic stain. Stains for microorganisms were negative. The interstitial histiocytes were positive for PAS (weakly positive with diastase digestion) and by immunohistochemistry positive for CD68, CD163, and CD10. A subset of cells is weakly positive for S100, but essentially negative for CD1a and langerin. The lesion cells showed expression of cathepsin K and were weakly positive for pan-TRK, while negative for BRAF V600E and ALK. Combining the clinical and histopathological findings, a diagnosis of multicentric reticulohistiocytosis (MRH) was rendered. The patient was initiated on methotrexate and have been closely followed up. This case underscores the spectrum of histopathological findings in MRH, where cytomorphological appearance of the lesional cells may differ from typical reticulohistiocytoma and the expression of osteoclastic marker, cathepsin K suggests a role in arthritis mutilans.