Accuracy of calculations for stereotactic radiotherapy of the lung. Comparison of dose calculation algorithms

The purpose of this study is to evaluate the accuracy of dose calculations by three algorithms. Depth dose, OPF (Output Factor) and dose profiles were measured in a heterogeneous phantom. These values were also calculated by three algorithms of the Batho power law (BPL), Equivalent-Tissue Air Ratio (ETAR) and Convolution superposition (CS). The data were obtained for 4, 6 and 10 MV photon beams with a linear accelerator (Varian 21EX). Field size ranged from 3 x 3 cm2 to 10 x 10 cm2. Dose profiles of beam penumbra were also measured by a 0.125 ml ionization chamber at the point of 8, 13 and 18 cm from the surface of the phantom at intervals of 1 mm. Differences between measured and calculated depth doses were within 2% in BPL and CS, but depth doses were overestimated in ETAR. OPFs were also overestimated with the error of more than 4% in ETAR. Absorbed dose calculated by CS were in agreement with the values measured by the ionization chamber. The absorbed dose by BPL and ETAR, however, were overestimated in particular in absorbed dose of 85-95%. Although isodose lines by CS are precisely correct, the measured absorbed dose may be smaller than the prescribed doses calculated by BPL and ETAR. It should be noticed that calculated isodose lines may be significantly different from measured values in the treatment of lung tumors. (author)