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

Abstract

Dose calculation for pencil beam scanning proton therapy requires accurate measurement of the broad tails of the proton spot profiles for every nozzle in clinical use. By applying a pair/magnification method and merging film data, 200 mm × 240 mm dose kernels extending to 10(-4) of the central spot dose are generated for six selected energies of the IBA dedicated and universal nozzles (DN and UN). One-dimensional, circular profiles up to 100 mm in radius are generated from the asymmetric profiles to facilitate spot profile comparison. For the highest energy, 225 MeV, the output of both the DN and the UN for field sizes from 40 to 200 mm increases in parallel, slowest at the surface (∼1%) and fastest at a depth of 150 mm (∼9%). In contrast, at the lowest energy, 100 MeV, the output of the DN across the same range of field sizes increases 3-4% versus 6-7% for the UN throughout all the depths. The charge deficits in the measured depth-dose of Bragg peaks are similar between the UN and the DN. At 100 MeV, the field size factor difference at the surface between two orientations of a rectangular 40 mm × 200 mm field is 1.4% at isocentre for the DN versus 2% for the UN. Though the one-dimensional distributions are similar for the primary and tail components at different positions, the primary components of the DN spots are more elliptical 270 mm upstream than at isocentre.

Evaluation of detectors for acquisition of pristine depth‐dose curves in pencil beam scanning

“…The individual depth‐dose curves have been normalized at the water surface to 2.0% (1.2%), 1.3% (0.8%), and 1.3% (0.8%) for 100, 180, and 225 MeV, respectively, for the Bragg peak chamber (Stingray) (6) . (L. Lin, personal communication, October 24, 2014.)…”

“…In another recent publication, a plane‐parallel chamber segmented into concentric rings has been employed to characterize large‐angle scattering in scanned carbon‐ion therapy (5) . In the work by Lin et al, (6) which deals with an identical nozzle to that used in this study, the authors present the corresponding lateral dose profiles and show that the ‘halo’ produced in the nozzle is small, albeit not negligible.…”

“…Note that in the IBA PBS beam delivery system, the dipole magnets in the nozzle adjust the lateral beam position (see Fig. 1 in Lin et al (6) ). This positional correction is switched off in the service mode operation described here.…”

“…This quantity needs normalization, because the effect of the dose halo from the nozzle, which leads to a reduced collection efficiency at water surface, cannot be determined through measurement of depth‐dose curves. Thus, we combined the collection efficiencies at depth $\mathrm{z}=0$ (6) with the results of the current work.…”

Use of a radial projection to reduce the statistical uncertainty of spot lateral profiles generated by Monte Carlo simulation

“…The dose calculation of proton spot scanning treatment plans relies on the accuracy of these lateral profiles. These profiles generally have a Gaussian distribution except at the low‐dose region where a broad tail is observed 3. They can be obtained by measurement4 or generated by Monte Carlo (MC) simulation 5…”