Clinical Outcomes and Prognostic Factors for Extramammary Paget's Disease Treated with Radiation Therapy: A Multi-Institutional Observational Study.

: Extramammary Paget's disease (EMPD) is a rare cutaneous carcinoma that typically affects the elderly and is frequently observed in genital and perianal regions. We analyzed the outcomes and prognostic factors for EMPD after radiation therapy (RT). : We analyzed data from 81 patients with non-metastatic EMPD who received either RT alone or in combination with surgery and/or chemotherapy. The median radiation dose was 56 Gy in 28 fractions. Local control (LC), progression-free survival (PFS), and overall survival (OS) rates were calculated using the Kaplan-Meier method. Multivariate analyses were performed using the Cox proportional hazards model. Late adverse events were evaluated by NCI-CTCAE version 5.0. : The median age was 78 years, and the median follow-up period was 36 months. The three-year LC, PFS, and OS rates were 75%, 52%, and 80%, respectively. Multivariate analyses identified the presence of lymph node (LN) metastasis, the absence of surgery, and the omission of elective nodal irradiation (i.e., local irradiation only) as significant factors for unfavorable LC ( = 0.01, 0.02, and 0.006) and PFS ( = 0.001, 0.04, and 0.03). LN metastasis was also a significant factor for unfavorable OS ( = 0.005). One patient developed grade 2 skin infection, and another developed grade 3 lymphedema; no grade 4 or higher toxicity was observed. : The present results revealed prognostic factors for EMPD after RT and suggest that the absence of surgery and omission of elective nodal irradiation worsened outcomes. A prospective study is needed to establish an optimal treatment strategy for this rare malignancy, which is common in the elderly.