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The use of an extra‐focal electron source to model collimator‐scattered electrons using the pencil‐beam redefinition algorithm
Author(s) -
Boyd Robert A.,
Hogstrom Kenneth R.,
White R. Allen,
Antolak John A.
Publication year - 2002
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 - 10.1118/1.1517293
Subject(s) - collimator , collimated light , electron , physics , photon , monte carlo method , optics , dosimetry , cathode ray , electron scattering , scattering , computational physics , nuclear medicine , nuclear physics , mathematics , statistics , medicine , laser
Currently, the pencil‐beam redefinition algorithm (PBRA) utilizes a single electron source to model clinical electron beams. In the single‐source model, the electrons appear to originate from a virtual source located near the scattering foils. Although this approach may be acceptable for most treatment machines, previous studies have shown dose differences as high as 8% relative to the given dose for small fields for some machines such as the Varian Clinac 1800. In such machines collimation‐scattered electrons originating from the photon jaws and the applicator give rise to extra‐focal electron sources. In this study, we examined the impact of modeling an additional electron source to better account for the collimator‐scattered electrons. The desired dose calculation accuracy in water throughout the dose distribution is 3% or better relative to the given dose. We present here a methodology for determining the electron‐source parameters for the dual‐source model using a minimal set of data, that is, two central‐axis depth‐dose curves and two off‐axis profiles. A Varian Clinac 1800 accelerator was modeled for beam energies of 20 and 9 MeV and applicator sizes of 15 × 15 and 6 × 6 cm 2 . The improvement in the accuracy of PBRA‐calculated dose, evaluated using measured two‐dimensional dose distributions in water, was characterized using the figure of merit, FA 3 %, which represents the fractional area containing dose differences greater than 3%. For the 15 × 15 cm 2field the evaluation was restricted to the penumbral region, and for the 6 × 6 cm 2field the central region of the beam was included as it was impacted by the penumbra. The greatest improvement in dose accuracy was for the 6 × 6 cm 2applicator. At 9 MeV, FA 3 %decreased from 15% to 0% at 100 cm SSD and from 34% to 4% at 110 cm SSD. At 20 MeV, FA 3 %decreased from 17% to 2% at 100 cm SSD and from 41% to 10% at 110 cm SSD. In the penumbra of the 15 × 15 cm 2applicator, the improvement was less, but still significant. At 9 MeV, FA 3 %changed from 11% to 1% at 100 cm SSD and from 10% to 12% at 110 cm SSD. At 20 MeV, FA 3 %decreased from 12% to 8% at 100 cm SSD and from 14% to 5% at 110 cm SSD. Results demonstrate that use of a dual‐source beam model can provide significantly improved accuracy in the PBRA‐calculated dose distribution that was not achievable with a single‐source beam model when modeling the Varian Clinac 1800 electron beams. Time of PBRA dose calculation was approximately doubled; however, dual‐source beam modeling of newer accelerators (e.g., the Varian Clinac 2100) may not be necessary because of less impact of collimator‐scattered electrons on dosimetry.