CPC09 Recurrent extramammary Paget disease and colorectal adenocarcinoma

Abstract A man in his 80s was referred to dermatology in 2023 with a recurrence of left groin extramammary Paget disease (EMPD). In 2016, he had biopsy-confirmed EMPD, which was treated by surgical excision with close margins and skin grafting. Following negative investigation for underlying malignancy, including flexible sigmoidoscopy, the disease was classified as primary, with confirmatory immunohistochemistry (IHC) showing CK7 positivity and CK20 negativity. In 2023, he presented with erythematous plaques on the scrotum and in the groin, along the scar lines of his previous excision, highly suspicious for recurrent EPMD, which was confirmed by biopsy. We regarded this as a recurrence of primary EPMD. IHC showed CK7 positivity and CDX2 negativity, and CK20 was requested to exclude secondary EMPD. This revealed CK20 positivity in one of the two biopsies. Following review with the urology team, a computed tomography scan identified a suspicious lesion in the descending colon. The patient underwent hemicolectomy and was given a confirmed tissue diagnosis of colorectal adenocarcinoma (pT4aN0M0, stage IIA). In our case, aberrant CK20 expression on the latest biopsy raised potential concerns about a secondary malignancy. However, the CDX2 negativity on histopathology and the absence of a primary tumour elsewhere over the 7-year period supported the diagnosis of recurrent primary EMPD. This would suggest that the bowel cancer was incidental, being common in this population (incidence rate 464 per 100 000 as per Cancer Research UK). Texts assert that CK20 expression distinguishes between primary and secondary Paget disease, with primary cases being typically CK20 negative and cases secondary to colorectal or urothelial malignancy CK20 positive. However, evidence from recent literature shows that this is not always reliable (Liao X, Liu X, Fan X et al. Perianal Paget’s disease: a clinicopathological and immunohistochemical study of 13 cases. Diagn Pathol 2020; 15: 29). While most primary cases are CK20 negative, only 50% of secondary cases exhibit CK20 positivity (Perrotto J, Abbott JJ, Ceilley RI et al. The role of immunohistochemistry in discriminating primary from secondary extramammary Paget disease. Am J Dermatopathol 2010; 32: 137–43). This variability in cytokine profiles limits their use in differentiation and clinical decisions. This case illustrates the diagnostic challenges in distinguishing primary from secondary EMPD, especially with conflicting IHC markers. Aberrant CK20 expression can be seen in primary Paget disease, and CDX2 IHC may be more useful in excluding a colorectal primary. The case emphasizes the importance of correlating clinical, histopathological and radiological findings. Rescreening for malignancy may be prudent in recurrent primary Paget disease.