A method for modeling laterally asymmetric proton beamlets resulting from collimation.
OBJECTIVE
To introduce a method to model the 3D dose distribution of laterally asymmetric proton beamlets resulting from collimation. The model enables rapid beamlet calculation for spot scanning (SS) delivery using a novel penumbra-reducing dynamic collimation system (DCS) with two pairs of trimmers oriented perpendicular to each other.


METHODS
Trimmed beamlet dose distributions in water were simulated with MCNPX and the collimating effects noted in the simulations were validated by experimental measurement. The simulated beamlets were modeled analytically using integral depth dose curves along with an asymmetric Gaussian function to represent fluence in the beam's eye view (BEV). The BEV parameters consisted of Gaussian standard deviations (sigmas) along each primary axis (σ(x1),σ(x2),σ(y1),σ(y2)) together with the spatial location of the maximum dose (μ(x),μ(y)). Percent depth dose variation with trimmer position was accounted for with a depth-dependent correction function. Beamlet growth with depth was accounted for by combining the in-air divergence with Hong's fit of the Highland approximation along each axis in the BEV.


RESULTS
The beamlet model showed excellent agreement with the Monte Carlo simulation data used as a benchmark. The overall passing rate for a 3D gamma test with 3%/3 mm passing criteria was 96.1% between the analytical model and Monte Carlo data in an example treatment plan.


CONCLUSIONS
The analytical model is capable of accurately representing individual asymmetric beamlets resulting from use of the DCS. This method enables integration of the DCS into a treatment planning system to perform dose computation in patient datasets. The method could be generalized for use with any SS collimation system in which blades, leaves, or trimmers are used to laterally sharpen beamlets.
Department of Radiation Oncology, University of Iowa, 200 Hawkins Drive, Iowa City, Iowa 52242.
Edgar
E
Gelover
Department of Radiation Oncology, University of Iowa, 200 Hawkins Drive, Iowa City, Iowa 52242.
Dongxu
D
Wang
Department of Human Oncology, University of Wisconsin, 600 Highland Avenue, Madison, Wisconsin 53792.
Patrick M
PM
Hill
Department of Radiation Oncology, University of Iowa, 200 Hawkins Drive, Iowa City, Iowa 52242.
Ryan T
RT
Flynn
Division of Medical Physics, CDH Proton Center, 4455 Weaver Parkway, Warrenville, Illinois 60555.
Mingcheng
M
Gao
Division of Medical Physics, CDH Proton Center, 4455 Weaver Parkway, Warrenville, Illinois 60555.
Steve
S
Laub
Division of Medical Physics, CDH Proton Center, 4455 Weaver Parkway, Warrenville, Illinois 60555.
Mark
M
Pankuch
Department of Radiation Oncology, University of Iowa, 200 Hawkins Drive, Iowa City, Iowa 52242.
Daniel E
DE
Hyer
D009010:Monte Carlo Method; D061766:Proton Therapy / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted