ESTRO 2025 - Abstract Book

S13

Invited Speaker

ESTRO 2025

Abstract: Traditionally, CT simulation (simCT) has been essential for the development of immobilization, target delineation, and electron density mapping. However, modern advancements in onboard imaging (OBI), treatment planning techniques, and adaptive radiotherapy (ART) challenge the longstanding requirement for simulation-based radiation therapy (RT). "Direct-to-Unit" (DTU) workflows—often called “simulation-free” or “simulation-omitted”- require high image quality for structure identification, robust planning geometries, and accurate dose calculation methods for successful DTU approaches. Emerging advanced technologies such as HU-accurate CBCT-guided RT (CBCTgRT), MR guided RT (MRgRT), ART, and hybrid approaches now facilitate this transition to DTU. This presentation reviews the physics evidence demonstrating the image quality and dose calculation accuracy of onboard imaging for DTU while highlighting other clinical considerations for the safe delivery of DTU-RT. When assessing the image quality of modern CBCTgRT systems, it has been demonstrated that there is improved visibility for structure delineations while also minimizing the number of difficult-to-delineate areas within CBCTs. Additionally, contouring studies have shown non-inferiority compared to traditional simulation CTs. When preparing CBCTgRT techniques with the planning dataset established as a diagnostic CT, beam geometries that reduce the uncertainty of the delivered dose are encouraged. For dose calculation on a diagnostic CT, these scans provide HU values that are similar to those of planning CTs for most tissues, with minimal impact on target coverage, even if larger differences are present. Studies demonstrate multiple viable strategies for HU-corrected CBCT-based dose calculation, including deformable registration, direct acquisition, and synthetic CT generation. These approaches yielded clinically acceptable dose distributions, with differences typically less than 1%. MRgRT offers enhanced soft-tissue contrast, which can assist in localization for diseases where CT soft-tissue visualization poses challenges. Low-field MR images have demonstrated superior or equivalent visibility compared to earlier generations of CBCT. Delineation studies indicate no significant volume differences when measured against CT for specific disease sites. MRI-based dose calculations utilizing bulk density overrides, deformable CT, or synthetic CTs show minimal differences in PTV and OAR doses (less than 0.5%). Finally, there are examples of multi-modality use cases, such as hippocampal avoidance (HA). For emergent whole brain RT, upstream contouring with diagnostic MRI images, along with synthetic CTs or HU-calibrated CBCTs, enables HA planning without simCT. This workflow has reduced planning time to less than 15 minutes and on-table adaptation time to less than 40 minutes. Since the timeline and processes for DTU change, quality assurance (QA) will need refinement and updates to address the modern challenges encountered during DTU. This includes real-time QA, plan integrity assessment, and comparisons to population-based metrics for these disease sites to ensure high-quality planning approaches. In conclusion, CBCT and MRI now meet or approach clinical requirements for localization, delineation, and dose calculation—key criteria for enabling DTU workflows. However, standardized image quality metrics and robust HU calibration protocols remain essential for broader clinical adoption. As RT workflows evolve beyond simulation, these findings support a paradigm shift toward greater flexibility, efficiency, and patient-centric care in radiotherapy.

4669

Speaker Abstracts Celiac plexus radiosurgery Marcin Miszczyk 1,2 , Yaacov R Lawrence 3,4

1 Comprehensive Cancer Center, Medical University of Vienna, Vienna, Austria. 2 Collegium Medicum - Faculty of Medicine, WSB University, Dąbrowa Górnicza, Poland. 3 Department of Radiation Oncology, Sheba Medical Center, Ramat Gan, Israel. 4 School of Medicine, Tel Aviv University, Tel Aviv, Israel

Abstract:

Retroperitoneal pain syndrome, particularly prevalent among patients with advanced pancreatic cancer, represents an unmet need. This debilitating pain, originating from the upper abdominal viscera and transmitted via the celiac