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Inclusion of compensator‐induced scatter and beam filtration in pencil beam dose calculations
Author(s) -
du Plessis F. C. P.,
Willemse C. A.
Publication year - 2006
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 - 10.1118/1.2219777
Subject(s) - optics , attenuation , beam (structure) , monte carlo method , percentage depth dose curve , materials science , physics , ionization chamber , mathematics , ionization , ion , statistics , quantum mechanics
Compensators can be used as beam intensity modulation devices for intensity‐modulated radiation therapy applications. In contrast with multileaf collimators, compensators introduce scatter and beam hardening into the therapeutic x‐ray beam. The degree of scatter and beam filtering depends on the compensator material and beam energy. Pencil beam dose calculation models can be used to derive the shape of the compensator. In this study a novel way of incorporating the effect of compensator‐induced scatter and beam filtration is presented. The study was conducted using 6, 8, and 15 MV polyenergetic pencil beams (PBs). The compensator materials that were studied included wax, brass, copper, and lead. The perturbation effects of the compensators on the PB dose profiles were built in the PB dose profiles and tested for regular fields containing a step compensator and benchmarked against DOSXYZnrc Monte Carlo calculated dose profiles. These effects include compensator beam filtration and Compton‐scattered photons generated in the compensator materials that influence the resulting PB dose profiles. These data were obtained from DOSXYZnrc simulations. A Gaussian function was used to model off‐axis scatter and an exponential function was used to model beam hardening at any radius, r . Dose profiles were calculated under a step compensator using the method that can model beam hardening and off‐axis scatter, as well as a conventional method where the PB profiles are not adjusted, but a single effective attenuation coefficient is used instead to best match the dose profiles. Both sets of data were compared to the DOSXYZnrc data. Depth and profile dose data for 10 × 10cm 2and 20 × 20cm 2fields indicated that at 2 cm depth in water the method that takes compensator scatter into account agrees more closely with the DOSXYZnrc data compared to the data using only an effective attenuation coefficient. Further, it was found that the effective attenuation method can only replicate the DOSXYZnrc data at 10 cm depth where it was chosen to do so. At shallower depths the effective attenuation method overestimates the dose and beyond 10 cm depth it causes an underestimation in the dose. The scatter and beam hardening inclusion method does not exhibit such properties. The exclusion of scatter can lead to dose errors of up to 4 percent with a copper compensator at 5 cm depth for a 10 × 10cm 2field under a thickness of 5 cm at 6 MV . For materials such as lead this discrepancy could be as high as 7 to 8 percent at 6 MV . For larger fields ( 20 × 20cm 2 ) the effect of in‐phantom scatter reduces the differences between the dose profiles calculated with the mentioned methods.