ESTRO 2025 - Abstract Book

S3120

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2025

1916

Digital Poster Implementing VMAT-Based Online Adaptive Radiation Therapy for Prostate Cancer on a Standard Linac Marco Fusella, Francesca Dusi, Mattia Nicoletto, Davide Fiorentin, Alessandro Testolin, Fabio Busato Radiation Oncology, Policlinico Abano Terme, Abano Terme, Italy Purpose/Objective: This study reports our experience delivering Online Adaptive Radiation Therapy (OART) for prostate cancer patients using Halcyon v4.0 with the Hypersight imaging suite, demonstrating that OART is feasible without a dedicated adaptive platform. OART was implemented to manage anatomical changes, particularly in patients with inconsistent bladder and rectum conditions compared to simulation, leading to shifts in prostate and seminal vesicle positioning. Over three months, more than 40% of prostate patients required plan adaptation due to bladder and rectum preparation changes, effectively addressed using our OART workflow. Material/Methods: OART was delivered for 20 fractions across 20 prostate cancer patients. The workflow began with acquiring full field-of-view (FOV) CBCT, reconstructed with the Acuros algorithm. Automatic contours for CTVs and OARs were generated using the LimbusAI DL-based autocontouring system, with manual revisions and editing performed in Varian Eclipse v.18. VMAT treatment planning followed standardized templates (2 complete arcs), using AcurosXB for dose calculation with an Enhanced Leaf Model (ELM) for MLC modeling. Varian RapidPlan supported DVH prediction, and the Aperture Shape Controller coupled with MU limit minimized plan complexity and enhanced deliverability of the plan. Independent plan verification was performed in real time using Varian Mobius 3D. After plan approval, a second CBCT confirmed alignment before treatment delivery. Timing for each workflow step and total in-room time was recorded. Results: The Hypersight imaging suite provided high-quality CBCTs, with HU stability confirmed (standard deviations: 5 HU for bone, 4.2 HU for bladder). Gamma pass rates for doses calculated on CT and CBCT exceeded 98% using a 2%/2mm gamma index. Patient-specific QA analysis let us calculate Action Level (as suggested by AAPM TG-218) of 94.7% (3%/2mm) and 91.3% (2%/2mm), surpassing typical benchmarks. Plans checked with independent calculation showed GPR (3%/2mm) over 98% for all cases. The entire workflow maintained an average in-room time of 18 minutes, with less than 8 minutes required for contour verification, dose optimization, and calculation. Treatment delivery times were comparable or less to those on dedicated adaptive platforms. Conclusion: We successfully implemented OART for prostate cancer without a dedicated adaptive machine by leveraging optimized workflows, high-quality onboard imaging, and efficient treatment delivery. The Halcyon and Hypersight system combined rapid image acquisition with advanced autocontouring and streamlined processes to enable safe and effective adaptation to anatomical changes. This cost-effective approach demonstrates that high-quality adaptive radiation therapy can be achieved on standard Linacs, expanding access to advanced treatments for centers with limited resources.

Keywords: Hypersight, ART, VMAT