Evaluation of the analytical anisotropic algorithm in an extreme water-lung interface phantom using Monte Carlo dose calculations

Our study compares the performance of the analytical anisotropic algorithm ( AAA), a new superposition-convolution algorithm recently implemented in the Eclipse (Varian Medical Systems, Palo Alto, CA) Integrated Treatment Planning System (TPS), to that of the pencil beam convolution (PBC) algorithm in an extreme (C-shaped, horizontal and vertical boundaries) water-lung interface phantom. Monte Carlo ( MC) calculated dose distributions for a variety of clinical beam configurations at nominal energies of 6-MV and 18-MV are used as benchmarks in the comparison. Dose profiles extracted at three depths ( 4, 10, and 16 cm), two-dimensional (2D) maps of the dose differences, and dose difference statistics are used to quantify the accuracy of both photon-dose calculation algorithms. Results show that the AAA is considerably more accurate than the PBC, with the standard deviation of the dose differences within a region encompassing the lung block reduced by a factor of 2 and more. Confidence limits with the AAA were 4% or less for all beam configurations investigated; with the PBC, confidence limits ranged from 3.5% to 11.2%. Finally, AAA calculations for the small 4 x 4 18-MV beam, which is poorly modeled by PBC (dose differences as high as 16.1%), provided the same accuracy as the PBC model of the 6-MV beams commonly acceptable in clinical situations.