Evaluation of the Anisotropic Analytical dose calculation Algorithm in heterogeneous medium using in vivo dosimetry for high energy photon beams

Abstract

Introduction: This work was aimed to determine the dose at any depth in heterogeneous medium using diodes to evaluate a dose calculation algorithm. Usually the calibration is limited at the depth of the maximum of energy. The particularity of this work was based on the determination of calibration curves.Material and methods: Measurements at three depths (4cm, 8cm and 12cm) were made by ten QEDTM diodes from Sun Nuclear on heterogeneous phantoms composed of PMMA and expanded polystyrene roughly simulating the rib cage. These phantoms were irradiated under 6MV and 18MV photon beams from a Varian linear accelerator (2100C) by plans calculated from the treatment planning system (TPS) Eclipse equipped with the Anisotropic Analytical Algorithm (AAA) V13.6.The calibration curves were carried out by considering several measurement points in depth by a graphite ionization chamber in a heterogeneous medium composed of expanded polystyrene slabs and PMMA plates. The diode calibration factor is taken from these curves by interpolation.The measured and calculated data were compared to evaluate the AAA.Results: Ninety measurements were performed on three types of heterogeneous phantoms. Depending on the depth of the measurements, the deviations obtained between the measurement and the calculation predicted by the TPS remained less than 2%. Some were in the order of magnitude around 3%.An average of 1.13% is obtained over all the measurements with an average deviation of 0.66% and a standard deviation of 0.80%. The upper limit of the confidence interval is 1.41% for a confidence level of 99.9%.Conclusion: The deviations obtained remain within the standards recommended for the validation of a dose calculation algorithm in a heterogeneous medium.The calibration method based on the dose profiles allowed the completion of the knowledge of the dose in a heterogeneous medium based on a single reading of the diode.