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      A technique for the quantitative evaluation of dose distributions.

      Medical physics

      Radiotherapy Planning, Computer-Assisted, Gamma Rays, Models, Theoretical, Phantoms, Imaging, Photons, Radiotherapy Dosage

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          Abstract

          The commissioning of a three-dimensional treatment planning system requires comparisons of measured and calculated dose distributions. Techniques have been developed to facilitate quantitative comparisons, including superimposed isodoses, dose-difference, and distance-to-agreement (DTA) distributions. The criterion for acceptable calculation performance is generally defined as a tolerance of the dose and DTA in regions of low and high dose gradients, respectively. The dose difference and DTA distributions complement each other in their useful regions. A composite distribution has recently been developed that presents the dose difference in regions that fail both dose-difference and DTA comparison criteria. Although the composite distribution identifies locations where the calculation fails the preselected criteria, no numerical quality measure is provided for display or analysis. A technique is developed to unify dose distribution comparisons using the acceptance criteria. The measure of acceptability is the multidimensional distance between the measurement and calculation points in both the dose and the physical distance, scaled as a fraction of the acceptance criteria. In a space composed of dose and spatial coordinates, the acceptance criteria form an ellipsoid surface, the major axis scales of which are determined by individual acceptance criteria and the center of which is located at the measurement point in question. When the calculated dose distribution surface passes through the ellipsoid, the calculation passes the acceptance test for the measurement point. The minimum radial distance between the measurement point and the calculation points (expressed as a surface in the dose-distance space) is termed the gamma index. Regions where gamma > 1 correspond to locations where the calculation does not meet the acceptance criteria. The determination of gamma throughout the measured dose distribution provides a presentation that quantitatively indicates the calculation accuracy. Examples of a 6 MV beam penumbra are used to illustrate the gamma index.

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          Most cited references 6

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          Commissioning and quality assurance of treatment planning computers.

          The process of radiation therapy is complex and involves many steps. At each step, comprehensive quality assurance procedures are required to ensure the safe and accurate delivery of a prescribed radiation dose. This report deals with a comprehensive commissioning and ongoing quality assurance program specifically for treatment planning computers. Detailed guidelines are provided under the following topics: (a) computer program and system documentation and user training, (b) sources of uncertainties and suggested tolerances, (c) initial system checks, (d) repeated system checks, (e) quality assurance through manual procedures, and in vivo dosimetry, and (f) some additional considerations including administration and manpower requirements. In the context of commercial computerized treatment planning systems, uncertainty estimates and achievable criteria of acceptability are presented for: (a) external photon beams, (b) electron beams, (c) brachytherapy, and (d) treatment machine setting calculations. Although these criteria of acceptability appear large, they approach the limit achievable with most of today's treatment planning systems. However, developers of new or improved dose calculation algorithms should strive for the goal recommended by the International Commission of Radiation Units and Measurements of 2% in relative dose accuracy in low dose gradients or 2 mm spatial accuracy in regions with high dose gradients. For brachytherapy, the aim should be 3% accuracy in dose at distances of 0.5 cm or more at any point for any radiation source. Details are provided for initial commissioning tests and follow-up reproducibility tests. The final quality assurance for each patient is to perform an independent manual check of at least one point in the dose distributions, as well as the machine setting calculation. As a check of the overall treatment planning process, in vivo dosimetry should be performed on a select number of patients.
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            Dosimetric evaluation of a pencil-beam algorithm for electrons employing a two-dimensional heterogeneity correction.

            The accuracy of a pencil-beam algorithm for electrons employing a two-dimensional heterogeneity correction is demonstrated by comparing calculation with measurement. Ionization measurements have been made in a water phantom for a variety of non-standard geometries. Geometries to demonstrate the effect of an extended treatment distance, a sloping skin surface, and an irregular skin surface have been selected. Additionally, thermoluminescent dosimeters have been used to measure distributions in tissue-substitute phantoms, which were designed from individual patient computerized tomographic scans. Three patient scans have been selected: (1) diffuse hystiocytic lymphoma of the left buccal mucosa and retromolar trigone; (2) squamous cell carcinoma of the nose at the columnella ; and (3) carcinoma of the maxillary antrum. Results demonstrate the algorithm's ability to simultaneously account for the isodose shifting as a result of internal heterogeneities and for sidescatter non-equilibrium caused by lateral discontinuities of the skin surface and internal anatomy. The algorithm is shown to generally be accurate to within +/- 4% in the treatment volume or +/- 4 mm in regions of sharp dose gradients as found in the penumbra and distal edge of the beam. Examples of greater disagreement are shown and their physical interpretation discussed.
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              Experimental evaluation of a 2D and 3D electron pencil beam algorithm

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                Journal
                9608475

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