Abstract
A number of treatment-planning systems still use conventional correction methods for body inhomogeneities. Most of these methods (power law method, tissue—air ratio (TAR), etc.) consider only on-axis points, rectangular fields, and inhomogeneous slabs covering the whole irradiating field. A new method is proposed that overcomes the above limitations. The new method uses the principle of the Clarkson method on sector integration to take into account the position and lateral extent of the inhomogeneity with respect to the point of calculation, as well as the shape of the irradiating field. The field is divided into angular sectors, and each sector is then treated separately for the presence of inhomogeneities using a conventional correction method. Applying this method, we can predict the correction factors for Co-60 and 6-MV photon beams for irregular fields that include inhomogeneities of lower or higher densities relative to water. Validation of the predicted corrections factors was made against Monte Carlo calculations for the same geometries. The agreement between the predicted correction factors and the Monte Carlo calculations was within 1.5%. In addition, the new method was able to predict the behavior of the correction factor when the point of calculation was approaching or moving away from the interface between two materials. PACS number(s): 87.53.Bn, 87.53.Wz
Highlights
The clinical experience with radiation therapy has been based on the tissue responses to the planned doses in a homogeneous body even if the irradiated volume contains inhomogeneous volumes.The dramatic increase in computing power at affordable prices has greatly enhanced several technical advances in radiotherapy
Benchmark simulation First, the new method was compared against the original BSM method with the same experimental setup as described in the paper by Kappas and Rosenwald,(12) and the results were in agreement with a minor discrepancy (0.5%), which can be attributed to round-up errors in the interpolation routines
The appropriate choice of a bulk method assures an acceptable correction for situations where the point of calculation lies off the beam axis, and the lateral extent of the inhomogeneity is smaller than the field size
Summary
The clinical experience with radiation therapy has been based on the tissue responses to the planned doses in a homogeneous body even if the irradiated volume contains inhomogeneous volumes (mainly lungs, air cavities, and bones).The dramatic increase in computing power at affordable prices has greatly enhanced several technical advances in radiotherapy. The clinical experience with radiation therapy has been based on the tissue responses to the planned doses in a homogeneous body even if the irradiated volume contains inhomogeneous volumes (mainly lungs, air cavities, and bones). The radiotherapy treatment-planning system (RTPS) that uses 3D patient data is a reality. Several algorithms have been proposed to implement some sort of inhomogeneity correction, from the simplified tissue–air ratio (RTAR), which yields a correction factor for water-based calculations, to superposition/convolution and Monte Carlo methods, which include the inhomogeneity in the calculation of patient dose. The majority of the current commercial RTPS offer the equivalent TAR (ETAR)(1) method and other conventional methods developed more than 20 years ago[2,3,4] as inhomogeneity correction algorithms The shortcomings of these earlier methods are well known, and sometimes calculation differences of 10% from measurements are not uncommon
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