Beam commissioning of the first compact proton therapy system with spot scanning and dynamic field collimation

Abstract

Objectives: To describe the measurements and to present the results of the beam commissioning and the beam model validation of a compact, gantry-mounted, spot scanning proton accelerator system with dynamic layer-by-layer field collimation. Methods: We performed measurements of depth dose distributions in water, spot and scanned field size in air at different positions from the isocenter plane, spot position over the 20 × 20 cm2 scanned area, beam monitor calibration in terms of absorbed dose to water and specific field collimation measurements at different gantry angles to commission the system. To validate the beam model in the treatment planning system (TPS), we measured spot profiles in water at different depths, absolute dose in water of single energy layers of different field sizes and inversely optimised spread-out Bragg peaks (SOBP) under normal and oblique beam incidence, field size and penumbra in water of SOBPs, and patient treatment specific quality assurance in homogeneous and heterogeneous phantoms. Results: Energy range, spot size, spot position and dose output were consistent at all gantry angles with 0.3 mm, 0.4 mm, 0.6 mm and 0.5% maximum deviations, respectively. Uncollimated spot size (one sigma) in air with an air-gap of 10 cm ranged from 4.1 to 16.4 mm covering a range from 32.2 to 1.9 cm in water, respectively. Absolute dose measurements were within 3% when comparing TPS and experimental data. Gamma pass rates >98% and >96% at 3%/3 mm were obtained when performing 2D dose measurements in homogeneous and in heterogeneous media, respectively. Leaf position was within ±1 mm at all gantry angles and nozzle positions. Conclusions: Beam characterisation and machine commissioning results, and the exhaustive end-to-end tests performed to assess the proper functionality of the system, confirm that it is safe and accurate to treat patients. Advances in knowledge: This is the first paper addressing the beam commissioning and the beam validation of a compact, gantry-mounted, pencil beam scanning proton accelerator system with dynamic layer-by-layer multileaf collimation.

Implementation of a Compact Spot-Scanning Proton Therapy System in a GPU Monte Carlo Code to Support Clinical Routine

“…The components taken into account in FRED, that is, the range shifter, the adaptive aperture, and the nozzle window, are sketched in Figure 2. For more details, we also refer to [24].…”

“…The beam model was calibrated according to the following overall scheme: experimental data were acquired as a part of the facility commissioning or specifically for this work. Details about the commissioning can be found in [24]. The measurement conditions were mimicked in FRED, and the beam model parameters were determined by optimizing the match between measured and simulated data.…”

“…In this work, we present the implementation of our treatment machine in FRED, the optimization of model parameters based on experimental data acquired during the commissioning of the facility [24], and validation based on additional data. Finally, we compare dose distributions recalculated with FRED and with the clinical TPS.…”

Characterization of penumbra sharpening and scattering by adaptive aperture for a compact pencil beam scanning proton therapy system

“…This leads to the larger lateral penumbrae for low energies used clinically for cancer treatment. 9,10 Consequently, reducing the lateral penumbra is highly important for these systems to reduce the dose to critical organs.…”

“…24 Beam commissioning studies for Mevion's PBS system have been recently published. Vilches-Freixas et al 10 measured the penumbra with AA in air at two snout positions: 3.6 and 12 cm. Kang et al 9 measured penumbra in air with and without AA for different beam conditions.…”