Nonmass‐Forming Type Anocanal Cancer With Pagetoid Spread With a Challenging Differential Diagnosis: A Case Report

Pagetoid spread (PS) of anorectal cancer is relatively rare and is associated with poor prognosis compared with Extramammary Paget disease. It is therefore essential to accurately differentiate between the two entities. In PS cases, the primary tumor lesion is usually obvious; however, this is not the case in nonmass-forming type anorectal cancer, making it difficult to distinguish between the two. We hereby report the case of a 79-year-old woman who presented with a six-month history of perianal skin eruption. Physical examination revealed maceration and erosion around the anus. On endoscopy, no obvious tumor formation was observed; however, an ulcer with a whitish coating extending from the anal region into the anal canal was noted. Biopsy was positive for CK7, CK20, and CDX2, and negative for GCDFP-15. The patient was diagnosed with nonmass-forming type anorectal cancer with PS. Laparoscopic abdominoperineal resection was performed with lymph node dissection and mapping biopsy. Postoperative immunohistochemical staining confirmed the diagnosis. Since primary Paget disease differs from perianal pagetoid spread with respect to treatment strategy as well as prognosis, accurate differentiation between the two is necessary. Diagnosis based on several immunohistochemical stains is highly effective for distinguishing between these entities.