ESTRO 2025 - Abstract Book

S302

Brachytherapy – Head & neck, skin, eye

ESTRO 2025

2945

Digital Poster Brachytherapy of outer ear skin cancer using individualized 3D-printed applicators versus electron beam and VMAT radiotherapy: a dosimetric comparison Fasco van Ommen, Milena Smolic, Maarten H.G. Kastelijns, Germaine J.L.M. Jongen, Cornelis P.J. Raaijmakers,

Mirjam Willemsen-Bosman, Eric Brand, Homan Dehnad, Mischa de Ridder, Petra S. Kroon Department of Radiation Oncology, University Medical Center Utrecht, Utrecht, Netherlands

Purpose/Objective: Treatment of outer ear skin cancer comprises surgery or radiotherapy. Since surgery often results in poor cosmetic outcome, radiotherapy is a valid alternative. Since 2021 patients have been treated at our institute with brachytherapy using individualized 3D printed applicators (Figure 1). The aim of this study was to review the brachytherapy technique by comparing its dose coverage and tissue sparing ability to electron and VMAT photon beam radiotherapy. Material/Methods: For seven patients treated with brachytherapy in 18 fractions of 3 Gy using 3D-printed applicators (coverage V85% = 100%), electron and VMAT photon beam radiotherapy treatment plans were created retrospectively based on our current clinical treatment guidelines (coverage V90% = 100% and V95% = 100%, respectively). Due to the difference in required coverage between these techniques, additional electron and VMAT treatment plans were created using the same required target coverage as the clinical brachytherapy plans. The plans were compared based on DVH parameters. Target coverage was compared by calculating the target D90% and D98%. Healthy tissue sparing was determined by calculating the volume of dose in the body minus target, and the mean dose and D1% to brain, cochlea, eye, lacrimal gland and lens. Results: The brachytherapy plans achieved a median target D98% of 86.3% and median D90% of 101.2%, compared to an electron target D90% of 94.5% and VMAT target D90% of 91.5%, when using an equivalent planning aim (Table 1). The brachytherapy plans yielded the lowest median 25% and 50% dose volumes to the body minus target, with a median V50% of 9.8 cm 3 compared to 34.0 cm 3 for electrons and 19.8 cm 3 for VMAT. The median D mean to the cochlea was lowest for electrons at 1.3 Gy, followed by 3.7 Gy for brachytherapy and 4.6 Gy for VMAT. The same pattern was observed for the median D mean to the eye, lacrimal gland and lens. For the electron and VMAT plans, the median 25% and 50% dose volumes to the body minus target and the D mean to the brain and cochlea were higher using our current clinical treatment guidelines for target coverage compared to plans made using the brachytherapy planning aim.