ESTRO 2025 - Abstract Book

S25

Invited Speaker

ESTRO 2025

investigations (6–10 MeV) have generated important preclinical data but remain constrained by limited beam penetration.

VHEE (50–250 MeV) overcomes these depth limitations, enabling highly conformal dose distributions with superior motion robustness, particularly relevant for thoracic and abdominal targets. Two major VHEE delivery modalities— open-field and pencil beam scanning—offer complementary advantages: open-field for large, homogeneous tumors and scanning for complex anatomies. Early indications suggest the potential to treat challenging cases like brain, pancreas, lung, and abdominal tumors, where FLASH-induced sparing may broaden the therapeutic window. Moving VHEE-FLASH into the clinic requires dedicated linac designs capable of ultra-high dose rates, stringent beam control, and new treatment planning approaches that accurately model FLASH conditions. Standardized protocols and robust clinical trials will be essential to demonstrate safety, refine dosimetry guidelines, and facilitate regulatory approval. By bridging lessons from IOeRT to emerging FLASH-capable linacs, VHEE stands poised to revolutionize radiotherapy for deep-seated malignancies, pushing the boundaries of what is achievable in cancer treatment. Speaker Abstracts Next generation: dose delivery optimization for VHEE treatment Gaia Franciosini 1,2 , Angelica De Gregorio 1,2 , Daniele Carlotti 3 , michela marafini 4 , Alberto Burattini 1 , Vincenzo Patera 1 , flaminia quattrini 1 , marco toppi 1 , angelo schiavi 1 , giacomo traini 2 , alessio sarti 1 1 Basic and Applied Engineering Sciences, Sapienza University of Rome, Rome, Italy. 2 INFN, Section of Rome I, Rome, Italy. 3 Operative Research Unit of Radiation Oncology, Fondazione Policlinico Universitario Campus Bio-Medico, Rome, Italy. 4 Museo Storico della Fisica e Centro Studi e Ricerche, "E. Fermi, roma, Italy Very-High Energy Electron (VHEE) therapy is emerging as a promising alternative to conventional radiotherapy (RT) and proton therapy (PT), particularly due to its potential for delivery at FLASH dose rates. The unique interaction properties of electrons with matter enable precise tumor targeting while minimizing damage to surrounding healthy tissues. However, optimizing dose delivery techniques remains a key challenge, particularly when balancing dose conformity with ultra-high dose rates. In this study, we explore advanced treatment delivery methods for VHEE therapy, focusing on Intensity-Modulated Radiation Therapy (IMRT) delivered with double scattering or pencil beam scanning technique, Volumetric Modulated Arc Therapy (VMAT), and Spatially Fractionated Radiotherapy (SFRT). IMRT and VMAT provide precise dose shaping, with IMRT showing promise for FLASH applications due to its ability to deliver ultra-high dose rates. SFRT, leveraging high Peak-to-Valley Dose Ratios (PVDRs), enhances normal tissue sparing while ensuring effective tumor coverage. To evaluate these techniques, we developed a dedicated VHEE Treatment Planning System (TPS) based on Monte Carlo simulations, incorporating a model for the FLASH effect. A cohort of lung cancer patients with non-small cell lung cancer was selected due to their favorable pathology for FLASH radiotherapy, including small tumor volumes, suitability for hypo-fractionated treatments, and reduced risk of radiation-induced lung fibrosis at UHDR and use to assess the feasibility of these techniques in clinical scenarios, comparing their performance against state-of-the-art RT treatments. Our analysis considers dose conformity, OAR sparing, and tumor coverage to identify the most effective delivery strategy in both conventional and FLASH regimes. The results provide insights into the optimal integration of VHEE with advanced delivery techniques, paving the way for next-generation radiotherapy solutions that maximize therapeutic efficacy while minimizing adverse effects on healthy tissues. Abstract: 4685