Abstract
Purpose: To introduce a practical method of using an Electron Density Phantom (EDP) to evaluate different dose calculation algorithms for photon beams in a treatment planning system (TPS) and to commission the Anisotropic Analytical Algorithm (AAA) with inhomogeneity correction in Varian Eclipse TPS. Methods and Materials: The same EDP with various tissue-equivalent plugs (water, lung exhale, lung inhale, liver, breast, muscle, adipose, dense bone, trabecular bone) used to calibrate the computed tomography (CT) simulator was adopted to evaluate different dose calculation algorithms in a TPS by measuring the actual dose delivered to the EDP. The treatment plans with a 6-Megavolt (MV) single field of 20 × 20, 10 × 10, and 4 × 4 cm2 field sizes were created based on the CT images of the EDP. A dose of 200 cGy was prescribed to the exhale-lung insert. Dose calculations were performed with AAA with inhomogeneity correction, Pencil Beam Convolution (PBC), and AAA without inhomogeneity correction. The plans were delivered and the actual doses were measured using radiation dosimetry devices MapCheck, EDR2-film, and ionization chamber respectively. Measured doses were compared with the calculated doses from the treatment plans. Results: The calculated dose using the AAA with inhomogeneity correction was most consistent with the measured dose. The dose discrepancy for all types of tissues covered by beam fields is at the level of 2%. The effect of AAA inhomogeneity correction for lung tissues is over 14%. Conclusions: The use of EDP and Map Check to evaluate and commission the dose calculation algorithms in a TPS is practical. In Varian Eclipse TPS, the AAA with inhomogeneity correction should be used for treatment planning especially when lung tissues are involved in a small radiation field.
Highlights
The accuracy of dose calculation directly impacts radiation treatment efficacy
Our study indicates that both Analytical Algorithm (AAA) with inhomogeneity correction and Pencil Beam Convolution (PBC) in Varian Eclipse treatment planning system (TPS) (Version 8.1.1.17) can reach the accuracy of 2% suggested by AAPM TG65 [25] for all the types of tissues that we have tested using Electron Density Phantom (EDP) if we consider the uncertainty from ADCL calibration on the ion chamber and the electrometer that we used in this study to be negligible
Using the electron density phantom computed tomography (CT) image that was obtained from the CT Simulator (HU numbers) calibrated by this electron density phantom to do the dose calculation for evaluating the TPS in this study might have reduced the bias in dose calculation that depends on CT HU numbers
Summary
The complexity of human anatomy and the heterogeneous components of the body’s tissues require the sophisticated dose calculation algorithm of a treatment planning system (TPS) to precisely calculate the dose prescribed to targets as well as to organs at risk, for which electron density may vary from low, as in the lungs, to high, as in dense bones. The dose calculation with inhomogeneity correction has greatly improved the accuracy of dose calculation, especially when low-density tissues such as lung tissues are involved in the treatment. From a clinical medical physicist’s point of view, there has not been a simple and practical way to evaluate, verify, and commission different dose calculation algorithms embedded in a TPS before they are used
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