Pencil beam scanning dose calibration at reduced source‐to‐axis distance

Abstract

Background Pencil beam scanning (PBS) monitoring chambers use an ionization control signal, monitor units (MUs), or gigaprotons (Gp) to irradiate a pencil beam and normalize dose calculations. The nozzle deflects the beam from the nozzle axis by an angle subtended at the source‐to‐axis distance (τ) from the isocenter. If the angle is not correctly considered in calibrations or calculations, it can lead to systematic errors. Purpose Aspects to consider for machines of various τs are fourfold. First, for the machine, there is a pathlength change of proton tracks in the monitor chamber. Second, for measurements, a uniform‐square irradiation over a plane, with constant Gp per spot, does not deliver uniform dose in a measurement plane. Third, for Monte Carlo (MC) simulations, Gp (and not MU) is proportional to simulating a number of protons. Fourth, for pencil beam algorithms (PBA), MU or Gp may be used for pencil beam weight, but usage needs to be consistent. Another consideration is the beam shape change from circular to oval in the projection onto voxels. Methods Coordinate systems for PBS delivery are described. Results Users of intermediate‐τ machines, corresponding to the onset of 1% pathlength corrections within the scanned field size, must not assume that MUs are proportional to the number of particles in MC simulations, and the PBA may need pathlength corrections. For a field size of 24 × 24 cm2, intermediate‐τ machines correspond to 59 cm ≤ τ < 120 cm. For a field size of 40 × 40 cm2, intermediate‐τ machines correspond to 98 cm ≤ τ < 200 cm. Small‐τ machines correspond to τ < 59 and 98 cm at these field sizes, respectively, which require corrections in projecting the beam shape onto voxels. Conclusions Identifying corrections due to the pencil beam angle and their onset are important for reducing the outer diameter of proton therapy gantries. The use of Gp (or the number of protons) meterset standardizes data interchange and helps to reduce systematic errors due to the angle of the beam.