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Estimates of Fetal Dose with the Mevion S250i Proton Therapy System with HYPERSCAN

K M Hopfensperger, A Paxton, V Sarkar, F Su, R G Price, B J Salter, S St. James*, University of Utah - Huntsman Cancer Institute, Salt Lake City, UT

Presentations

WE-C930-IePD-F3-2 (Wednesday, 7/13/2022) 9:30 AM - 10:00 AM [Eastern Time (GMT-4)]

Exhibit Hall | Forum 3

Purpose: The Mevion S250i Proton Therapy System with HYPERSCAN has a unique proton MLC called the adaptive aperture. This work investigated the potential dose to a fetus for all modes of beam collimation (dynamic adaptive aperture, static adaptive aperture, and no adaptive aperture) and compared the findings with those of other available proton systems.

Methods: Treatment plans were created in RayStation (RaySearch Laboratories, Stockholm, Sweden) for a simulated left-sided brain lesion in a RANDO phantom (Radiology Support Devices, Inc., Long Beach, CA) treated with a vertex field, a left lateral field, and a posterior-anterior field. All measurements were performed using a WENDI neutron detector (Thermo Fisher Scientific, Waltham, MA) placed at representative locations to mimic the fetus position at different gestational stages.

Results: The fetal dose estimate varied with detector position, and the largest measured dose was 64.7 µSv per 2 Gy(RBE) using the dynamic aperture. The smallest estimated fetal dose was 45.0 µSv per 2 Gy(RBE) at the base of the RANDO abdomen (56 cm from the head of the phantom) for the static aperture delivery, 48 cm from isocenter. The vertex fields at all depths had larger contributions to the total fetal dose than the other two fields. The dynamic aperture plans resulted in the highest fetal dose measured for all depths.

Conclusion: Estimated fetal doses ranged from 45.0 to 64.7 µSv per 2 Gy(RBE). These doses are lower than estimated doses using a double-scattering proton system and are comparable to other pencil beam scanning systems. The vertex fields for each beam collimation mode contributed the highest dose to the fetus. If treating a pregnant patient’s brain, vertex fields may be avoided to limit the dose to the fetus and the static aperture may minimize out of field dose.

Keywords

Fetal Dose, Protons

Taxonomy

TH- External Beam- Particle/high LET therapy: Proton therapy – out of field dosimetry/risk analysis

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