Comparison of the Batho, ETAR and Monte Carlo dose calculation methods in CT based patient models

This paper shows the contribution that Monte Carlo methods make in regard t o dose distribution calculations in CT based patient models and the role it plays as a gold standard to evaluate other dose calculation algorithms. Th e EGS4 based BEAM code was used to construct a generic 8 MV accelerator to obtain a series of x-ray field sources. These were used in the EGS4 based D OSXYZ code to generate beam data in a mathematical water phantom to set up a beam model in a commercial treatment planning system (TPS), CADPLAN V.2.7 .9. Dose distributions were calculated with the Bathe and ETAR inhomogeneit y correction algorithms in head/sinus, lung, and prostate patient models fo r 2X 2, 5X5, and 10X10 cm(2) open x-ray beams. Corresponding dose distribut ions were calculated with DOSXYZ that were used as a benchmark. The dose co mparisons are expressed in terms of 2D isodose distributions, percentage de pth dose data, and dose difference volume histograms (DDVH's). Results indi cated that the Bathe and ETAR methods contained inaccuracies of 20%-70% in the maxillary sinus region in the head model. Large lung inhomogeneities ir radiated with small fields gave rise to absorbed dose deviations of 10%-20% . It is shown for a 10X10 cm(2) field that DOSXYZ models lateral scatter in lung that is not present in the Batho and ETAR methods. The ETAR and Bathe methods are accurate within 3% in a prostate model. We showed how the perf ormance of these inhomogeneity correction methods can be understood in real istic patient models using validated Monte Carlo codes such as BEAM and DOS XYZ. (C) 2001 American Association of Physicists in Medicine.