Case Report: staged electron beam radiotherapy for inoperable primary ectopic abdominal extramammary Paget disease.

Extramammary Paget disease (EMPD) is a rare cutaneous adenocarcinoma that usually arises in apocrine gland-bearing areas, whereas primary ectopic EMPD of the abdominal skin is exceptionally uncommon. We report a 79-year-old woman with a five-year history of progressively enlarging eczema-like lower abdominal-pelvic skin lesions, including erosion, exudation, scaling, crusting, and pruritus, initially treated as chronic dermatitis without improvement. Histopathology showed large atypical Paget cells with focal dermal microinvasion; immunohistochemistry was positive for cytokeratin 7 (CK7) and GATA-binding protein 3 (GATA3) and negative for cytokeratin 20 (CK20) and p40, while Ki-67, a proliferation marker, showed a labeling index of approximately 60%. Because radical surgery was unsuitable, she received response-adapted two-stage electron beam radiotherapy. The first stage delivered 43.2 Gy in 12 every-other-day fractions using 12-MeV electrons, a 5-mm tissue-equivalent bolus, and customized lead shielding, followed by a 10 Gy boost in five daily fractions using 9-MeV electrons for residual visible disease. The treated field showed complete clinical resolution approximately one month after radiotherapy, with no in-field recurrence nearly one year later. Residual erythematous scaly plaques persisted outside the original field. This case supports response-adapted electron beam radiotherapy as a feasible non-surgical option for selected inoperable superficial EMPD and emphasizes the practical importance of pretreatment mapping, adequate field coverage, bolus conformity, shielding, and field-edge planning for extensive lesions on curved body surfaces.