ESTRO 2024 - Abstract Book

S313

Brachytherapy - Head & neck, skin, eye

ESTRO 2024

1 Sunnybrook Odette Cancer Centre, Radiation Oncology, Toronto, Canada. 2 Sunnybrook Odette Cancer Centre, Medical Physics, Toronto, Canada. 3 Sunnybrook Odette Cancer Centre, Dermatology, Toronto, Canada

Purpose/Objective:

Extramammary Paget disease (EMPD) is a rare neoplasm with uncertain histogenesis, that usually presents in the anogenital area. The neoplasm is composed of malignant epithelioid cells within the epithelium and is characterized by slow growth with high recurrence rates. Surgical resection with wide local excision currently is first line therapy but repeat surgery can result in mutilation and functional impairment. Surface mould brachytherapy (SMBT) could be an effective treatment option which can deliver radiation to the perineum and treat superficial skin lesions located over curved surfaces and at locations where a sharp dose fall-off is required. We describe the clinicopathologic findings of eight cases of EMPD and our initial experience of using SMBT treatment using novel 3D printed custom applicators to deliver high-dose-rate brachytherapy to the perineum.

Material/Methods:

Eight patients with EMPD were treated with SMBT from November 2019 to April 2023. CT images acquired with and without a thin metal wire were used for marking target extent to contour target volumes and designing 1cm thick applicator, respectively, in MIM treatment planning system (MIM Software Inc., Cleveland, OH, USA). CTV was delineated in CT images aided with clinical exams and a uniform lateral margin of 7mm was applied to define Target. The standard triangle language (STL) files for the applicator and the target were generated and further processed in Autodesk Inventor 2021, Autodesk MeshMixer 2017, and Autodesk MeshEnabler 2021 (Autodesk, Inc. San Rafael, California, USA) to design the applicator. Customized catheter positioning regarding spacing and depth from the skin surface (5-7mm, depending upon the patient anatomy) were carried out to cover the target volume and minimize dose to organs at risk (OAR). Applicators were then 3D printed using the Accura ClearVue, a USP class VI transparent resin. Plastic catheters were inserted in the applicator and QA test completed to ensure a safe radiation source delivery before using them to acquire planning CT images. Treatment plans were generated with a uniform prescription (Rx) of 42.5Gy in 10 daily fractions prescribed at the target surface with the aim of Target D90% >35Gy and maximum surface dose (defined by D0.3cc) <130% (55.25 Gy) of Rx. Of the 8 EMPD cases, the median age at presentation was 78.5 years (range 77-94) and the majority of patients were male (6; 2 female). All patients presented with characteristic expanding erythematous plaques with typical histopathology and supportive immunohistochemical and clinical findings for EMPD. All patients received SMBT dose of 42.5 Gy in 10 fractions prescribed to the surface. Mean target D90 was 34.5 Gy (range 31.7-36), mean target D0.1cc 53.1 Gy (range 46.2-58.4), mean target D0.3cc 51.3 Gy (range 45.3-56.4), and mean target D0.5cc was 50.3 Gy (range 44.9-55.4). Mean follow-up was 13.3 months (range 2.4-48.4) with a complete response in 8/8 patients and a single relapse in 1 patient which was marginal and outside of the treatment field at 13.6 months post-treatment. Acute dermatitis grade 3 was noted in all 8 patients with resolution at 53.9 days (mean, range 35 72 days) and late toxicity hypopigmentation (grade 1; 3 patients) and telangiectasia (grade 1; 3 patients) was noted in a subset of patients treated. Seven of 8 patients rated long term cosmesis very good to excellent and one patient with grade 1 telangiectasia rated her skin cosmesis poor to good. She developed thin skin due to excessive use of topical steroids over the vulva. Results:

Conclusion: