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  4. Advance Energy Modulated Arc Therapy (EMAT) with Penumbra Modulation Requires a Novel Multi-Slit Collimation

Advance Energy Modulated Arc Therapy (EMAT) with Penumbra Modulation Requires a Novel Multi-Slit Collimation

R Shaw*, S Luan , University of New Mexico, Albuquerque, NM

Presentations

(Wednesday, 7/17/2019) 4:30 PM - 6:00 PM

Room: 301

Purpose: Energy modulated arc radiation therapy (EMAT) is a paradigm under development that utilizes multiple radiation sources during treatment. With EMAT, treatment plans can be optimized not just with beam orientation, intensity, field shape, but now includes beam penetration and radiological penumbra. The question to answer is whether the historical aperture-based multi-leaf collimator (MLC) is the most appropriate design for EMAT.

Methods: A simple EMAT treatment plan was created that targeted a cylindrical inside a cylindrical phantom. The dose calculation engine was a Monte Carlo and pencil beam hybrid. An inverse optimization algorithm was created—tailored specifically for EMAT—that considered a rotational EMAT delivery with two radiation spectrums: 400keV (low energy spectrum) and 2 MeV (high energy spectrum). The results of the inverse optimization produce fluence maps of beam aperture as a function of gantry angle. Two fluence maps were created, one for each energy.

Results: The contribution of the low energy spectrum covered the dose on the edge of the target, sharpening the dose gradient. The high energy component of the dose distribution contributed to the center of the target providing the bulk of the dose. The planned apertures of both radiation sources are small: the small aperture for the low energy spectrum is due to the need to only treat the edge of the tumor and the small aperture of the high energy spectrum is due to small volume remaining. These apertures are small enough to be considered as slits with varying widths, not apertures.

Conclusion: These results indicate that the conventional MLC design, which is optimized to create apertures, may not be suitable for EMAT where most of the treatments is characterized by the use of the beam’s edge. This is an argument for a new EMAT collimator specifically optimized for variable-width, multi-slit collimation.

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