Incidental finding of extramammary Paget's disease during active surveillance for early‐stage prostate cancer in a prostate biopsy

INTRODUCTION: Skin tissue contamination within transcutaneous visceral organ biopsies is seldom found. We encountered a rare case of extramammary Paget's disease incidentally diagnosed by prostate biopsy during active surveillance for prostate cancer. CASE PRESENTATION: A 71‐year‐old Japanese patient was diagnosed with prostate cancer, and active surveillance was selected. After 1 year, prostate biopsy was performed by a transperitoneal approach, and 16 biopsy cores were taken. One biopsy core contained skin tissue showing extramammary Paget's disease. Careful skin examination confirmed the presence of an extramammary Paget's disease lesion in the left perineum, and curative surgical resection was performed. Recurrence and metastasis did not occur after 6 months of follow‐up. CONCLUSION: Although the perianal region is a common site of extramammary Paget's disease, early‐stage extramammary Paget's disease is often asymptomatic. Thus, during a transcutaneous biopsy, it is important to consider the appearance of the skin and the pathological features of migrating skin tissue.