BI11 Squamoid eccrine ductal carcinoma: a case series of an aggressive skin cancer occurring almost exclusively in immunocompromised patients

Abstract Squamoid eccrine ductal carcinoma (SEDC) is a rare cutaneous adnexal carcinoma first described in 1997 (Wong TY, Suster S, Mihm MC. Squamoid eccrine ductal carcinoma. Histopathology 1997; 30: 288–93). It has distinct biphasic features on histology, with both superficial squamous and deeper ductal eccrine differentiation. It shows a characteristic pattern of positive staining for epithelial membrane antigen and carcinoembryonic antigen on immunohistochemistry, highlighting the ducts. This biphasic pattern commonly results in misdiagnosis as Bowen disease or cutaneous squamous cell carcinomas (sSCC) if diagnostic skin biopsies are superficial, and delayed diagnosis is common. It is locally aggressive with significant metastatic potential. We report here a striking association with immuno­compromise, which has not previously been highlighted in the literature. We conducted a single-centre, retrospective cohort study of all patients diagnosed with SEDC over an 11-year period (2014–2025). Clinical and histological features were recorded, and both treatment and clinical outcomes documented. Ten cases were identified (seven male, three female; mean age at diagnosis 75 years, SD 10.8. Nine (90%) were immunocompromised due to solid organ transplantation (n = 6) or haematological malignancy (myeloma, myelodysplasia, monoclonal gammopathy of undetermined significance, n = 3). Seven (70%) had a history of previous cSCCs. Anatomical locations of SEDC included finger (n = 4), head and neck (n = 4), upper chest (n = 1) and calf (n = 1). SEDC was diagnosed on the basis of initial histology in only two cases. In the other eight cases this was only after further surgery. Locoregional recurrence occurred in three of 10 (30%) and distant metastasis in two of 10 (20%). Treatment included surgical excision in all cases, with postoperative radiotherapy due to incomplete or close margins in 50%, and lymphadenectomy for metastasis in two patients. One patient died from widespread metastatic disease and a second patient is currently undergoing chemotherapy for inoperable metastatic disease. This case series is the first to highlight the close association of SEDC with immunocompromise. In the previous, largest series, immune status was not documented (van der Horst MP, Garcia-Herrera A, Markiewicz D et al. Squamoid eccrine ductal carcinoma: a clinicopathologic study of 30 cases. Am J Surg Pathol 2016; 40: 755–60). Our series also demonstrates the challenges posed in diagnosis of SEDC, which is often delayed, underscoring the need for clinicopathological vigilance in immunocompromised individuals in whom other keratinocytes (pre)cancers may simulate SEDC. Currently there is limited evidence regarding optimal treatment. From the high rate of poor outcomes evident in our series, we propose that wide local excision, postoperative adjuvant radiotherapy and follow-up imaging surveillance are justified. However, further studies are now required to better establish treatment guidelines for this rare aggressive adnexal tumour.