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SU‐E‐T‐29: Modeling Primary Off‐Axis Ratio and Off‐Axis Beam Softening Effects
Author(s) -
Liang X,
Zhu T
Publication year - 2012
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4735084
Subject(s) - beam (structure) , softening , neutral axis , photon , optics , physics , beam energy , computational physics , quantum mechanics
Purpose: To extract primary dose components from dose measurements, and model primary off‐axis ratio and beam off‐axis softening effects, and to obtain POAR and off‐axis softening parameters for megavoltage beams for different energies and from different manufacturers. Methods: Total dose is modeled as a function of scattering factors (SF) and primary dose. Using previously determined SF, the primary dose is extracted from measured beam data. Based on the primary dose, primary off‐axis ratio and off‐axis softening effects are modeled at off‐axis points. By fitting photon beam profiles, this model provides a method to extract off‐axis parameters and thus characterize primary off‐axis ratio and off‐axis softening effects. Photon beam profiles were measured from 15 different megavoltage x‐ray beams, covering all major linear accelerator manufacturers, and are fitted by this model. The fitting algorithm is implemented by a Matlab based program. Results: The proposed model successfully fits the measured photon beam data for various field sizes. The parameters for primary off‐axis ratio and off‐axis softening were successfully determined for different megavoltage photon beams. The fitting errors are within 1%. Conclusions: The primary off‐axis model has been established, and parameters were determined for different megavoltage x‐ray machines and different energies. Our results suggest that the primary off‐axis ratio and beam off‐axis softening effects are machine and energy dependent, and do not support the generic model from literature.