TH‐C‐BRB‐12: Fundamental Technical Performance Acceptance Testing Results of a Commercial High Definition Discrete Pencil Beam Scanning System

Abstract

Purpose: This study reports on the basic physics acceptance testing results of a pre‐clinical proton discrete pencil beam scanning (PBS) system (sometimes also known as modulated scanning). Methods: The PBS system tested consists of a 230 MeV cyclotron, a continuous range selector and a dedicated scanning nozzle mounted on two instances of a rotational gantry. The following basic field/beam qualities of the system were determined: maximum field size, lateral and longitudinal field uniformity of a one liter cubic uniform field, output stability over time, gantry angle invariance, monitoring system linearity and reproducibility. Ionization chambers, film and scintillation dosimetry were used. A test scan pattern was irradiated to determine beam in air transverse properties. Results: A 10 × 10 × 10 cm test field was shown to be homogeneous within +/− 3% laterally and longitudinally at high and low energy taking about 2–3 minutes to deliver an approximate physical dose of 2 Gy. The machine output showed variation of 1 % over one month and 1 % dependency over 5 tested gantry angles. Pencil beam alignment to gantry iso‐center was better than 1 mm. The irradiated test pattern analysis indicated that the delivered fluence map when Gaussian blurred for the measured spot profile agreed within 1.5%/1.5mm gamma factor to the physical measurement. The delivery system resolution exceeded the highest test pattern resolution bars that had been spaced at two line pairs per cm. Conclusions: The results suggest that the pre‐clinical PBS system delivers homogeneous fluence under nominal test conditions and gives high performance with respect to accuracy, precision and resolution. A higher resolution test pattern is called for in order to determine the intrinsic PBS resolution.