Document Type : Original Paper
Department of Physics, Faculty of Science, Laboratory of Theoretical Physics and Matte Radiation Interaction (LPTHIRM), Saad Dahlab University of Blida, Algeria
Introduction: The calibration process is usually limited to the depth of maximum energy. This study aimed to determine the depth dose in a heterogeneous medium using diodes and to evaluate a dose calculation algorithm.
Material and Methods: Measurements were done at three depths (4, 8, and 12 cm) using ten QEDTM diodes on heterogeneous phantoms (HPH), composed of poly(methyl methacrylate) (PMMA) and expanded polystyrene, roughly simulating the rib cage. These phantoms were irradiated with 6-MV and 18-MV photon beams from a Varian linear accelerator by plans calculated by the Eclipse treatment planning system, equipped with the Anisotropic Analytical Algorithm (AAA). The calibration curves were drawn by considering several measurement points in depth by a graphite ionization chamber in the HPH. The diode calibration factor was taken from the curves via interpolation. The measured and calculated values were compared to evaluate the AAA.
Results: Depending on the depth, the deviations between the measurements and calculations predicted by the TPS remained less than 2%. Some measurements had an order of magnitude of nearly 3%. An average deviation of 1.13% was obtained for all measurements, with an average deviation of 0.66% and a standard deviation of 0.80%. The upper bound of the confidence interval was 1.41%.
Conclusion: The deviations obtained in this study remained within the recommended standard range for validation of a dose calculation algorithm in a heterogeneous medium. The calibration method based on dose profiles provided further information about the dose in a heterogeneous medium, based on a single diode reading.