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Eosinophilic polymorphic and pruritic eruption associated with radiotherapy in a patient with primary nodal Merkel cell carcinoma

A 63-year-old gentleman, who was otherwise well, was referred to dermatology following a diagnosis of Merkel cell carcinoma (MCC) on histology of an isolated nodal swelling in the right groin, excised by general surgeons. Meticulous examination revealed no evidence of primary cutaneous MCC, and PET CT showed absence of metastatic disease. Following discussion and review of histology at a specialist skin cancer multidisciplinary team (SSMDT) meeting, the consensus diagnosis was primary nodal MCC. In 2009 Kim et al., for the first time in dermatology literature, reported a case of MCC confined to lymph nodes without an apparent primary site. This is an unusual presentation. These tumors were described as “Merkel cell carcinoma of lymph node with unknown primary” as they exhibit similar morphologic and immunophenotypic features to those in skin MCC. Within the context of such a rare presentation of MCC, a treatment plan was agreed by the SSMDT, combining radical groin dissection and adjuvant radiotherapy. This was delivered to right pelvic and inguinal nodes to a dose of 60 Gy in 30 fractions over 6 weeks with volumetric arc therapy. One week before completing treatment, he developed an intensely itchy, burning, urticated, pustular eruption over the right buttock (Fig. 1). This was most likely related to radiation therapy, however it was not typical of radiation dermatitis, and because of this atypical presentation, a skin biopsy was taken for further evaluation. Histologic examination of a skin biopsy revealed inflammation extending to the epidermal/dermal junction predominantly lymphocytic in type; perivascular infiltrate composed of small mature lymphocytes and histiocytes but with a significant number of eosinophils (Figs. 2 and 3). This was in keeping with eosinophilic, polymorphic, and pruritic eruption associated with radiotherapy (EPPER). The rash cleared after 6 days of treatment with potent topical steroids and oral antihistamines. Skin scrapings identified trichophyton rubrum on culture, and PAS staining showed fungal particles. This was not felt to be clinically significant as the rash settled with topical steroids alone without antifungal therapy. Eosinophilic, polymorphic, and pruritic eruption associated with radiotherapy is an unusual cutaneous complication associated with radiation therapy, first described in 1999. Most cases to date have been in relation to breast and cervical cancer. EPPER can develop during and after treatment. Gallego et al. described EPPER with endometrial cancer developing