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Dosimetric Analysis of Inverse-Planned Modulated-Arc Total-Body Irradiation

M Held

M Held*, N Kirby, O Morin, J Pouliot, University of California San Francisco, San Francisco, CA

TH-C-BRB-2 Thursday 10:30:00 AM - 12:30:00 PM Room: Ballroom B

Purpose: To determine the key dosimetric characteristics of a newly developed and clinically implemented inverse-planned modulated-arc total-body irradiation (MATBI) technique, including optimal treatment beam parameters, a redefined beam model, and patient-specific quality assurance (QA).

Methods: During MATBI treatment, the patient is positioned prone and supine underneath the gantry at 2 m source-to-surface distance (SSD). The dose is delivered using open-field beams at different gantry angles. Monitor units per beam are inverse-planned through beam-weight optimization based on the patient's anatomy from full-body computed tomography (CT) images. Beam output characteristics were measured at extended SSD and a new beam model was commissioned. Delivery time and dose uniformity were studied vs. beam field size and geometry. Dose delivery accuracy was determined by comparing ion chamber measurements inside anthropomorphic water phantoms to planning system predictions. A phantom with Gaf-Chromic film inserts was designed for clinical use to streamline patient-specific QA.

Results: MATBI was used on 17 patients so far. The best dose uniformity and shortest delivery time were achieved for a beam field size of 40x40 cm² at isocenter with 5° step size between adjacent beams, using 16 to 28 beams per side depending on patient size. Compared to the original beam model, the new beam model improved the dose delivery accuracy by a fraction of a percent to 1.2%. The largest discrepancy between measurement and prediction was 3.2%. The patient-specific QA procedure showed that QA with a 5% tolerance limit could be routinely performed.

Conclusions: The new inverse-planned MATBI technique permits dose optimization, dose-volume histogram calculation and dosimetric analysis. MATBI beam setup parameters for optimal dose uniformity and treatment duration were determined. The developed phantom allows for a patient-specific quality assurance procedure, which is now used routinely. These results are the basis for the safe and accurate clinical implementation of MATBI.

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