AbstractID: 9715 Title: Monte Carlo Dose Calculation and Pre-Treatment Dose Verification
for Highly Modulated Step and Shoot MLC Beams
Complex indications like H&N tumors frequently result in highly modulated IMRT plans consisting of many small off-axis segments. For
dose computations, this requires a very accurate model of the head scatter contribution because a significant fraction of the total dose to any
point in the irradiated volume stems from extrafocal scattered radiation and the output factors of the off-axis segments are dependent on the
softening of the photon spectrum and the extrafocal radiation. These effects will also compromise the accuracy of verification measurements.
In this work, comparisons of Monte Carlo dose computations with the XVMC code and measurements with a multi-chamber dose detector
system and radiographic films are presented. To allow these comparisons, IMRT patient plans were applied to a solid phantom.
Although film measurements offer the advantage of 2D verification, their accuracy is compromised by an overresponse to low energy scatter
radiation. Also, deviations cannot be traced back to single segments within a single beam measurement. At contrast, the linear detector array
provides point only measurements. But its advantage is the ease of online data analysis and the possibility to reconstruct a dose profile from
single segment doses. For high dose regions, calculations and point measurement were found to agree within 1.5 %, showing that the output
factors as one important aspect of the Monte Carlo modeling were correctly reproduced by the calculations. In the low dose regions, the
global 3 % dose-to-agreement criterion was met.