Vemurafenib-induced histiocytoid neutrophilic panniculitis simulating myeloid leukaemia cutis

Neutrophilic panniculitis is an infrequent but characteristic adverse event under therapy with BRAF inhibitors (BRAFi). Since the approval of vemurafenib for treatment of metastatic melanoma in 2011, only two cases of neutrophilic panniculitis in malignancies other than melanoma have been published. Histiocytoid infiltrates of immature neutrophils resembling histiocytes or myelocytes have been reported in Sweet's syndrome and rarely in other neutrophilic dermatoses. We describe a novel variant of neutrophilic panniculitis with histiocytoid myeloid cells in an early lesion from a patient treated with vemurafenib in combination with an anti-EGFR (epidermal growth factor receptor) agent for metastatic colon carcinoma, three weeks after initiation of therapy. Recognizing this variant of panniculitis associated to BRAFi can avoid misinterpretation of the atypical subcutaneous infiltrate as myeloid leukaemia cutis.