A novel concept for a superconducting, fixed-field bending section is presented for use in a proton therapy gantry. The large momentum acceptance of this design allows for treatment over the full proton energy range of 70–220 MeV with fixed field in the superconducting magnets, eliminating the technical risks associated with fast-field ramping to match beam energy changes during treatment. A combined study of beam dynamics and magnet design is shown for such a system in which a simple magnet geometry with straight Nb–Ti racetrack coils is used to produce the desired fields. Particle tracking through this design is compared with clinical requirements for beam spot shape and size at isocenter over the full range of proton energy.
Purpose:
The shape of a single beam in proton PBS influences the resulting dose distribution. Spot profiles are modelled as two‐dimensional Gaussian (single/ double) distributions in treatment planning systems (TPS). Impact of slight deviations from an ideal Gaussian on resulting dose distributions is typically assumed to be small due to alleviation by multiple Coulomb scattering (MCS) in tissue and superposition of many spots. Quantitative limits are however not clear per se.
Methods:
A set of 1250 deliberately deformed profiles with sigma=4 mm for a Gaussian fit were constructed. Profiles and fit were normalized to the same area, resembling output calibration in the TPS. Depth‐dependent MCS was considered. The deviation between deformed and ideal profiles was characterized by root‐mean‐squared deviation (RMSD), skewness/ kurtosis (SK) and full‐width at different percentage of maximum (FWxM). The profiles were convolved with different fluence patterns (regular/ random) resulting in hypothetical dose distributions. The resulting deviations were analyzed by applying a gamma‐test. Results were compared to measured spot profiles.
Results:
A clear correlation between pass‐rate and profile metrics could be determined. The largest impact occurred for a regular fluence‐pattern with increasing distance between single spots, followed by a random distribution of spot weights. The results are strongly dependent on gamma‐analysis dose and distance levels. Pass‐rates of >95% at 2%/2 mm and 40 mm depth (=70 MeV) could only be achieved for RMSD<10%, deviation in FWxM at 20% and root of quadratic sum of SK <0.8. As expected the results improve for larger depths. The trends were well resembled for measured spot profiles.
Conclusion:
All measured profiles from ProBeam sites passed the criteria. Given the fact, that beam‐line tuning can result shape distortions, the derived criteria represent a useful QA tool for commissioning and design of future beam‐line optics.
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