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.

Figure 1.

Sketch of the proton accelerator system (a), and treatment room of the Mevion S250 Hyperscan system in Maastricht (b). ACP: Adaptive Coil Positioner, RMS: Range Modulation System, AA: Adaptive Aperture. A: kV-kV flat panels, B: four camera surface guidance system, C: extendable nozzle and exit entrance window (black square), D: multi-energy cone-beam CT system, E: six degrees of freedom robotic couch.

Figure 2.

Example of the AA leaves position verification with two leaves pattern printed on a transparent film attached to the nozzle and photographed by a CCD camera mounted on a tripod.

Figure 3.

a) Uncollimated spot size in air expressed in terms of sigma as a function of the distance from the isocenter for the 227 MeV beam energy; b) uncollimated spot size and penumbra in air for 15 energies at the isocenter plane at a nozzle extension of 10 cm (10 cm air gap); and left penumbra of a 78 MeV spot trimmed on the left side by the AA (cross).

Figure 4.

Experimental and MC-TPS depth dose distributions in water for 15 proton beam energies.

Figure 5.

Comparison of measured (filled markers) and Raystation TPS (unfilled markers) spot sizes in water at three different depths (1/4, 2/4, and 3/4 of the Bragg peak) for the 15 proton beam energies and air gap of 10 cm. (a) Spot profiles in water in X-direction. (b) Spot profiles in water in Y-direction.

Figure 6.

Percentage differences between measurements and TPS of the absorbed dose to water in the middle of monoenergetic squared fields ranging from 3 × 3 cm² to 20 × 20 cm² for the 15 proton beam energies.

Figure 7.

Percentage differences of on-axis and off-axis absorbed dose to water inside 125 cc SOBPs centred at 5 cm, 10 cm and 20 cm depth, and 1000 cc SOBPs centred at 10 cm and 20 cm depth. The oblique beam incidence at 15° and 30° was evaluated for the 125 cc SOBP centred at 5 cm depth. All SOBPs were planned with aperture (Table 1).

Figure 8.

Complex geometry plans used to validate the TPS dose computation accuracy for irregular fields shaped by the adaptive aperture: ‘Hairdryer’ (top) and ‘rocky-hand’ (bottom). From left to right: TPS representation of the beam, TPS dose plane, measured dose plane, line dose profiles: TPS (solid line) and measurements (dots).