ESTRO 2025 - Abstract Book

S911

Clinical - Haematology

ESTRO 2025

3936

Mini-Oral Improved organ-sparing using intensity modulated proton therapy combined with breath-hold for mediastinal lymphoma: a four-way plan comparison study Bastiaan D.P. Ta 1 , Richard A.M. Canters 1 , Kim van der Klugt 1 , Gloria Vilches-Freixas 1 , Esther Kneepkens 1 , Fleur Vereijken 1 , Maud Cobben 1 , Maud van den Bosch 1 , Indra Lubken 1 , Cissy Stultiens 1 , Marije Velders 1 , Tina Verstappen 1 , Gyanne Tholen 1 , Anne G.H. Niezink 2 , Dirk K.M. De Ruysscher 1 , Maaike Berbée 1 1 Department of Radiation Oncology (Maastro), GROW School for Oncology, Maastricht University Medical Centre+, Maastricht, Netherlands. 2 Department of Radiation Oncology, University Medical Center Groningen, Groningen, Netherlands Purpose/Objective: In December 2023, a breath-hold (BH) technique was clinically introduced for mediastinal lymphoma (ML) patients receiving intensity modulated proton therapy (IMPT). This resulted in four radiotherapy techniques being clinically available in our center, i.e. photon free-breathing (FB), photon breath-hold, proton free-breathing and proton breath hold. The objectives of this retrospective in silico plan comparison study are to report the dosimetric differences and the normal tissue complication probability (NTCP) differences for late toxicities between the four techniques. Material/Methods: Forty-nine ML patients were included, treated with curative intent from 2019 through May 2024, having both a 4D-CT and BH-CT available. To compare dosimetric results, four plans were made for each patient, i.e. VMAT-FB, VMAT-BH, IMPT-FB and IMPT-BH. Full-arc volumetric arc therapy (VMAT) was used for both photon therapy techniques. IMPT plans were robustly optimized in 3D and evaluated in 4D for IMPT-FB. The current thoracic PTV margins (8 mm for VMAT-FB; 5 mm for VMAT-BH) and robustness optimization margins (8 mm for IMPT-FB and IMPT-BH) were applied for all patients. The mean heart dose (MHD), mean long dose (MLD) and bilateral mean breast doses (MBD-bilateral) were extracted to calculate the risks of acute coronary events (ACE), secondary lung cancer (SLC) and secondary breast cancer (SBC), respectively, with models used for the Dutch model-based selection method.[1-2] An absolute dose difference of ≥1 Gy on any dosimetric endpoint between any technique per patient was considered clinically relevant for this analysis. Results: Of 49 patients 53% were male and the median age was 30 years (17-72). The prescribed radiotherapy doses varied between 20-40 Gy. Dosimetric results and NTCP (or estimated risk) values are shown in Table 1 . The average MHD and MLD were lowest with the IMPT-BH compared to VMAT-FB, VMAT-BH and IMPT-FB, which was statistically significant ( p <0.01), while the bilateral MBD with IMPT was significantly lower than with VMAT ( p <0.01). The average MHD and MLD were statistically comparable between the VMAT-BH and IMPT-FB techniques, while both techniques had significantly lower MHD and MLD compared to VMAT-FB ( p <0.01). IMPT-BH and IMPT-FB are the most optimal techniques for ≥68% of patients (Figure 1) .