In‐transit cutaneous squamous cell carcinoma

A 79‐year‐old man presented with painful lesions over his left arm on a background of non‐Hodgkin lymphoma, which was well controlled on rituximab (Figure, A). Two years earlier, he had a poorly differentiated cutaneous squamous cell carcinoma of the left wrist excised (Figure, B, arrow). Fluorodeoxyglucose (FDG)‐positron emission tomography (PET) demonstrated extensive, avid cutaneous and subcutaneous lesions along the left upper limb and an axillary node. Biopsies confirmed poorly differentiated cutaneous squamous cell carcinoma. He commenced first line immunotherapy (cemiplimab 350 mg every 3 weeks) with concurrent radiotherapy, with a near complete resolution in his symptoms after seven cycles (Figure, B). FDG‐PET imaging at this time point confirmed a significant partial response in the left arm (Figure, C, blue arrow) but with progressive nodal disease in the left axilla extending to the supraclavicular region (Figure, C, red arrow). In‐transit disease represents an uncommon form of metastatic skin cancer related to lymphatic spread and is an important differential in immunocompromised patients, in whom it occurs more frequently.1