P020 A rare phenomenon in a rare disease: reverse Koebner phenomenon in cutaneous Rosai–Dorfman disease

Abstract A 52-year-old South Indian woman presented to the dermatology clinic with an asymptomatic reddish brown skin lesion over the left cheek. The lesion had progressively evolved to a nodular plaque over 2 months. There was no history of trauma. Her medical history was remarkable for vitiligo vulgaris, for which she was taking native topical medications. She did not report any systemic complaints like fever, weight loss or malaise. Physical examination revealed a well-defined reddish brown plaque of size 4 × 5 cm located over the left preauricular region, with the surface showing nodularity and minimal scaling. Dermoscopy showed yellow ovoid structures resembling apple jelly nodules against an erythematous background with areas of adherent scaling. Preliminary investigations with complete blood counts, erythrocyte sedimentation rate, and renal and liver function tests were normal. Tissue biopsy for mycobacterial, fungal and bacterial cultures was negative. A 5-mm submental lymph node was visualized on ultrasonography, which was not amenable to fine needle aspiration cytology. A skin biopsy from the plaque was obtained. Histopathology revealed dense and diffuse infiltrates of foamy histiocytes extending from the papillary dermis to the subcutis admixed with many plasma cells, lymphocytes and neutrophils. The histiocytes showed abundant eosinophilic cytoplasm with round-to-oval vesicular nucleus with prominent nucleoli. A few of the histiocytes showed engulfed plasma cells and neutrophils, suggesting emperipolesis. Histocytes stained positively with S100, while stains for microorganisms were negative. The patient was lost to follow-up due to the COVID-19 pandemic. When the patient came to us 4 months later, to our surprise, the lesion had completely regressed spontaneously. Owing to the clinical and histopathological features, a diagnosis of cutaneous Rosai–Dorfman disease was established. Rosai–Dorfman disease, also known as sinus histiocytosis with massive lymphadenopathy, is a rare, benign, histiocytic proliferative disorder of unclear aetiology defined originally by Rosai and Dorfman in 1969. The disease is limited to lymph nodes; however, extranodal involvement can occur in 25–40% of patients, with the skin being affected frequently and being the sole affected site in 3% of cases. The purely cutaneous form shows an older age of onset with a female predominance and a predilection for Asian populations. Reverse Koebner phenomenon refers to the disappearance of certain dermatoses following injury at the site. This phenomenon is commonly described in vitiligo vulgaris. This present case highlights the spontaneous resolution of the skin lesion following biopsy, indicating reverse Koebner phenomenon in a case of cutaneous Rosai–Dorfman disease with concurrent vitiligo vulgaris.