A three-source model for the calculation of head scatter factors

Department of Radiation Oncology, Stanford University, California 94305-5304, USA.
Medical Physics (Impact Factor: 2.64). 10/2002; 29(9):2024-33. DOI: 10.1118/1.1500767
Source: PubMed


Accurate determination of the head scatter factor Sc is an important issue, especially for intensity modulated radiation therapy, where the segmented fields are often very irregular and much less than the collimator jaw settings. In this work, we report an Sc calculation algorithm for symmetric, asymmetric, and irregular open fields shaped by the tertiary collimator (a multileaf collimator or blocks) at different source-to-chamber distance. The algorithm was based on a three-source model, in which the photon radiation to the point of calculation was treated as if it originated from three effective sources: one source for the primary photons from the target and two extra-focal photon sources for the scattered photons from the primary collimator and the flattening filter, respectively. The field mapping method proposed by Kim et al. [Phys. Med. Biol. 43, 1593-1604 (1998)] was extended to two extra-focal source planes and the scatter contributions were integrated over the projected areas (determined by the detector's eye view) in the three source planes considering the source intensity distributions. The algorithm was implemented using Microsoft Visual C/C++ in the MS Windows environment. The only input data required were head scatter factors for symmetric square fields, which are normally acquired during machine commissioning. A large number of different fields were used to evaluate the algorithm and the results were compared with measurements. We found that most of the calculated Sc's agreed with the measured values to within 0.4%. The algorithm can also be easily applied to deal with irregular fields shaped by a multileaf collimator that replaces the upper or lower collimator jaws.

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    • "We attempted to assess the suitability of multisource modeling for computations of out-of field dose distributions related to linear accelerator head scatter and leakage radiation. This approach, introduced by Dunscombe and Nieminen (1992), has already been successfully used by several authors to study the field size dependence of relative output from linear accelerators (Dunscombe and Nieminen 1992, Yu and Sloboda 1995, Jian et al 2001, Yang et al 2002). By demonstrating that the mean difference between measurements and calculations may be less than 9% in out-of-field dose calculations, our work strongly suggests that multisource modeling could provide valuable assistance in developing modern out-of-field dose evaluation algorithms necessary for providing solutions to recent ICRU recommendations regarding the dose delivered to the RVR. "
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    ABSTRACT: Our purpose was to assess the out-of-field dose component related to head scatter radiation in high-energy photon therapy beams and then derive a multisource model for this dose component. For scattered photons, several planar sources have been defined, with number, location and tilt depending on the complexity of the field shape. In the absence of precise knowledge of out-of-field scattering characteristics, several assumptions are made to derive emission spectra and radiation intensity from measurements. Among these, the Compton formula is used to evaluate scattered photon energy and the Henyey-Greenstein phase function is used to evaluate the scattered photon angular distribution. For measured doses under out-of-field conditions, the average local difference between the calculated and measured photon dose is 10%, including doses as low as 0.01% of the maximum dose on the beam axis. This study demonstrates that the multi-plane source approach is suitable for accurate analytical modeling of the out-of-field dose component related to head scatter radiation. These results should be taken into account when evaluating doses to the remaining volume at risk in external beam radiotherapy planning.
    Full-text · Article · Nov 2012 · Physics in Medicine and Biology
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    • "Since individual machines—even of the same vendor and model— have slightly different physical characteristics, the source model should be flexible to allow adaptation to an individual treatment unit. The proposed approaches for source modelling include full Monte Carlo (MC) simulation of the radiation transport through the accelerator head (Rogers et al 1995, Naqvi et al 2005), histogram-based models generated from the MC simulated phase space (Schach von Wittenau et al 1999, Chetty et al 2000, Fix et al 2004) and analytical models (Liu et al 1997b, Jiang et al 2001, Yang et al 2002, Fippel et al 2003, Ahnesjö et al 2005). These models differ in the amount of technical information required, their accuracy and their adaptability to different accelerators. "
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    ABSTRACT: Accurate modelling of the radiation output of a medical linear accelerator is important for radiotherapy treatment planning. The major challenge is the adjustment of the model to a specific treatment unit. One approach is to use a multiple-source model containing a set of physical parameters. In this work, the parameters were derived from standard beam data measurements using optimization methods. The source model used includes sub-sources for bremsstrahlung radiation from the target, extra-focal photon radiation and electron contamination. The cost function includes a gamma error measure between measurements and current dose calculations. The procedure was applied to six beam data sets (6 MV to 23 MV) measured with accelerators from three vendors, but the results focus primarily on Varian accelerators. The obtained average gamma error (1%, 1 mm) between dose calculations and measurements used in optimization was smaller than 0.7 for each studied treatment beam and field size, and a minimum of 83% of measurement points passed the gamma < 1 criterion. For experiments made at different SSDs and for asymmetric fields, the average gamma errors were smaller than 1.1. For irregularly shaped MLC apertures, the differences in point doses were smaller than 1.0%. This work demonstrates that the source model parameters can be automatically derived from simple measurements using optimization methods. The developed procedure is applicable to a wide range of accelerators, and has an acceptable accuracy and processing time.
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