V Heng^{1, 2}*, M Serban², C Freeman², M Renaud³, J Seuntjens^1,2,4, (1) McGill University, Montreal, QC, Canada (2) Mcgill University Health Centre, Montreal, QC, Canada (3) Gray Oncology Solutions, Montreal, QC, Canada (4) Princess Margaret Cancer Centre, Toronto, ON, Canada

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  V Heng

TU-D930-IePD-F4-1 (Tuesday, 7/12/2022) 9:30 AM - 10:00 AM [Eastern Time (GMT-4)]

Exhibit Hall | Forum 4

Purpose: Mixed electron-photon beam radiation therapy (MBRT) utilizes external electron and photon beams to minimize doses to normal tissues by exploiting the electron’s sharp dose falloff. This study demonstrates the clinical benefit of setup error-robust MBRT in the context of soft tissue sarcoma of the extremities (STS).

Methods: A retrospective cohort of 14 STS patients treated using VMAT were replanned with MBRT using a column-generation-based robust optimization method and MC dose calculations. Both VMAT and MBRT plans were planned for the Varian TrueBeam linac with a prescription dose of 50 Gy in 25 fractions. MBRT plans consisted of an electron component, including up to 5 energies (6,9,12,16,20 MeV), and a 6 MV photon component. Both modalities were planned to be collimated with only the Millenium multi-leaf collimator and delivered at standard SAD of 100 cm. The MBRT plan was optimized and normalized such that 95% of the CTV was covered by the prescription dose in the worst-case scenario.

Results: Due to the CTV’s superficial extent, 5 patients required the use of bolus with VMAT. All MBRT plans achieved similar CTV coverage without bolus. The 40% isodose volume was found to be smaller in MBRT than in the clinical VMAT plan (median decrease of 23.2%±2.6%). The median dose to the normal tissue strip was found to be significantly reduced with MBRT with a median decrease of 67.6%±7.4%. For patients with a CTV in contact with bone, V(50Gy) of the ipsilateral bone was found to be 64.5%±19.1% smaller with MBRT

Conclusion: Without sacrificing target coverage, robust MBRT significantly spares organs-at-risk for STS patients compared to standard photon VMAT. MBRT plans can be delivered without the use of applicators, bolus, or reduced SSD. In the context of STS, MBRT is thus a clinically viable modality from both a dosimetric and logistical standpoint.

Funding Support, Disclosures, and Conflict of Interest: Funding Support: Fonds de recherche du Quebec Nature et Technologie (FRQNT), and the Canadian Institute of Health Research Foundation Grant (FDN-143257).

Keywords

Treatment Planning, Treatment Techniques, Electron Therapy

Taxonomy

TH- External Beam- Electrons: Development (new technology and techniques)

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