M Rahman¹*, K Erhart², D Gladstone^1,3,4, P Bruza¹, L Jarvis^3,4, B Pogue^1,4,5, J Hoopes^1,3,4,5, R Zhang^1,3,4, (1) Thayer School of Engineering, Dartmouth College, Hanover, NH, (2) .decimal LLC, Sanford, FL, (3) Department of Medicine, Radiation Oncology, Geisel School of Medicine, Dartmouth College Hanover NH, (4) Norris Cotton Cancer Center, Dartmouth-Hitchcock Medical Center, Lebanon, NH,(5) Department of Surgery, Geisel School of Medicine, Dartmouth College, Hanover NH

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  M Rahman

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(Saturday, 3/26/2022)   [Central Time (GMT-5)]

Purpose: Electron Intensity modulation may, with proper treatment constraints, improve homogeneity of prescribed dose and reduce hot spots for clinical plans. It is highly desirable, and reasonably achievable to combine with electron FLASH-RT to fully exploit the FLASH effect with conformal dose distributions. In this study, the potential and deployment of intensity modulation is demonstrated in passive electron FLASH radiotherapy for treatment plan optimization.

Methods: A beam model of an electron FLASH irradiator (delivering ~300Gy/s at the isocenter) in the decimal ElectronRT treatment planning system (TPS) was validated with film measured lateral and percent-depth-dose (PDD) profiles. A plan for a phantom with a 15x15cm² field and dose constraints to the volume was developed. CT scan and structures of anonymized patient were imported, and plans were developed for fields conforming to tumor shape without modulation and with modulation considering constraints in prescribed dose to the target volume and minimizing dose to organs-at-risk (OAR).

Results: The film measured profiles and TPS beam model agreed on average within 1% and 2% for lateral profile and PDD, respectively. For a 15x15 cm² field the intensity modulation improved the beam flatness (~30% to <5% at 3 cm depth) while retaining UHDR in the treatment field, with <2% difference in the PDD along the central axis and minimal changes to symmetry, shift, and practical range. In the rib metastasis case, the modulated plans treat the tumor volume with a homogeneity index (HI) improvement by 0.2. In the facial orbital plan, the conformity index improved by 8% with an improvement in HI by 0.05 and comparable dose to OARs.

Conclusion: Depending on the treatment case intensity modulation can increase conformity with greater homogeneity. Future study will focus on demonstrating dosimetric benefits quantified by TCP/NTCP models in a larger number of relevant cases.

Funding Support, Disclosures, and Conflict of Interest: This work was supported by seed funding (Norris Cotton Cancer Center core grant P30 CA023108 and Thayer School of Engineering), Department of Medicine SEAM Award from DHMC and Geisel School of Medicine, and NIH grant R01 EB024498. Brian Pogue is cofounder and Mahbubur Rahman is an employee of DoseOptics LLC.

ePosters

• 174-60671-16011654-182589-1958254668.pdf (M Rahman)

Keywords

Treatment Planning, Electron Therapy, Intensity Modulation

Taxonomy

TH- External Beam- Electrons: Intensity modulated electron beam therapy

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