Accuracy of dose calculations in megavoltage photon beams (in English)

Abstract

In radiotherapy simple procedures for the calculation of dose or monitor units (MU) are required, for example, for direct (fixed SSD) beam irradiation techniques. It can also be used as independent checks of the calculations performed by the more sophisticated treatment planning (TP) systems. The algorithms used in MU calculations must comply with accuracy requirements comparable with those for TP systems. This thesis deals with the accuracy achievable in general in dose calculations for megavoltage photon beams. In the first section, the dose calculation procedure is discussed as part of the full dosimetry chain, starting with beam calibration up to the clinical dose delivery. The different aspects of a MU calculation program are discussed from the viewpoint of a separation of the dose variation with field size into a head scatter and a phantom scatter component. For this purpose, the parameters are defined at the reference depth in the phantom of 10 cm, while for the measurements a mini-phantom is used. A consistent formalism for MU calculation is described. In the second section several aspects of a calculation procedure, such as the concept of equivalent squares, the dose variation due to the presence of blocking trays, and the influence of asymmetric field setting on the head scatter component are dealt with. The third section of the thesis deals with the quality assurance of treatment planning systems. An extension of the AAPM TG 53 test set is proposed, while the results of its application to seven commercially available TP systems are presented. Finally, tolerances to be used with a comparison of dose calculations with measurements in different clinical situations are discussed.