Experimental characterization of two-dimensional pencil beam scanning proton spot profiles

Dose calculations of pencil beam scanning treatment plans rely on the accuracy of proton spot profiles; not only the primary component but also the broad tail components. Four films are placed at several locations in air and multiple depths in Solidwater (R) for six selected energies. The films used for the primary components are exposed to 50-200 MU to avoid saturation; the films used for the tail components are exposed to 800, 8000 and 80 000 MU. By applying a pair/magnification method and merging these data, dose kernels down to 10(-4) of the central spot dose can be generated. From these kernels one can calculate the dose-per-MU for different field sizes and shapes. Measurements agree within 1% of dose-kernel-based calculations for output versus field size comparisons. Asymmetric, comet-shaped profile tails have a bigger impact at superficial depths and low energies: the output difference between two orientations at the surface of a rectangular field of 40 mm x 200 mm is about 2% at the isocentre at 100 MeV. Integration of these dose kernels from 0 to 40 mm radius shows that the charge deficit in the Bragg peak chamber varies <2% from entrance to the end of range for energies <180 MeV, but exceeds 5% at 225 MeV.