S341
Clinical - Breast
ESTRO 2026
1 Radiation Oncology, Istituto Nazionale Tumori - IRCCS Fondazione "G. Pascale", Napoli, Italy. 2 Fondazione Muto, ETS, Napoli, Italy Purpose/Objective: To evaluate and compare cardiac and cardiac substructure dose distributions in left-sided breast cancer radiotherapy including internal mammary chain and supraclavicular nodes, planned with Helical TomoTherapy (HT) and Volumetric Modulated Arc Therapy (VMAT).The study aimed to identify differences in cardiac exposure patterns beyond conventional heart metrics (e.g., mean heart dose, MHD), while maintaining comparable target coverage and overall organ sparing. Material/Methods: Five patients with left-sided breast cancer were retrospectively analyzed and a prescription dose of 40.05 Gy in 15 fractions was prescribed to all target volumes. For each patient, two treatment plans were generated: a dual-arc VMAT (210°–230°; clockwise and counterclockwise) and a HT plan. The VMAT isocenter was placed in the upper-left thoracic region, while the HT isocenter was positioned at the midline in the transverse plane. All plans were optimized in RayStation (Raysearch Laboratories) to ensure comparable planning target volume (PTV) coverage and adherence to institutional dose constraints. Cardiac substructures were delineated, including the left atrium (LA) and ventricle (LV), left anterior descending (LAD), right coronary artery (RCA), left circumflex artery (LCX), ascending aorta (AA), inferior vena cava (IVC) and pulmonary artery (PA). Parameters analyzed included cardiac substructure mean dose (Dmean) to MHD ratio (CSDR), and the homogeneity index (HI). Results: All plans met PTV goals and organs at risk constraints (OARs) . HT provided lower cardiac doses and reduced low-dose exposure vs VMAT. For the whole heart, median Dmean and D1% were 3.0 Gy (2.1–6.0 Gy) and 16.7 Gy (12.7–25.3 Gy), respectively, with V5Gy = 13.3% (6.9–44.3%) and V25Gy = 0.09% (0.01–1.05%). The CSDR for the IVC [0.17 (0.13-0.35)], LV [0.9(0.6-1.02)] and RCA [1.01(0.91-1.86)] were significant lower with HT compared with VMAT (p < 0.02) [0.36 (0.23-0.46); 0.95 (0.71-1.13); 0.94 (0.8-1.55), respectively]. CSDR values for other substructures were comparable between techniques, ranging from 0.44 to 3.39. PTV coverage was equivalent, with HT achieving significantly improved dose homogeneity within the breast PTV (p<0.01) [HIHT = 6.85 (3.65-8.28); vs HIVMAT = 8.98 (7.15-12.54)]. Differences for other OARs were modest except for the thyroid gland.
Conclusion: Preoperative SBRT induced pathological downstaging in the majority of this patient cohort, demonstrating its therapeutic potential. The heterogeneity of DDR markers in non-responders suggests that distinct, potentially dysfunctional, DNA repair mechanisms may contribute to treatment effect/resistance. The preliminary transcriptomic data suggest that a "cold" (immunologically suppressed) tumor microenvironment, along with potentially deficient BRCA2-mediated repair, may be a key molecular signature of non-response to SBRT. Keywords: DNA repair, pre-operative RT, transcriptomic
Digital Poster 4891
Cardiac substructure exposure in left-sided breast radiotherapy including nodal regions and internal mammary chain: Helical Tomotherapy vs VMAT Francesca Buonanno 1 , Rocco Mottareale 1 , Sara Falivene 1 , Gianluca Ametrano 1 , Marcello Serra 1 , Valentina d'Alesio 1 , Cecilia Arrichiello 1 , Piera Ferraioli 1 , Gaetano Gagliardo 2 , Vincenzo Ravo 1
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