Modeling the extrafocal radiation and monitor chamber backscatter for photon beam dose calculation

A simple analytical approach has been developed to model extrafocal radiati on and monitor chamber backscatter for clinical photon beams. Model paramet ers for both the extrafocal source and monitor chamber backscatter are dete rmined simultaneously using conventional measured data, i.e., in-air output factors for square and rectangular fields defined by the photon jaws. The model has been applied to 6 MV and 15 MV photon beams produced by a Varian Clinac 2300C/D accelerator. Contributions to the in-air output factor from the extrafocal radiation and monitor chamber backscatter, as predicted by t he model, are in good agreement with the measurements. The model can be use d to calculate the in-air output factors analytically, with an accuracy of 0.2% for symmetric or asymmetric rectangular fields defined by jaws when th e calculation point is at the isocenter and 0.5% when the calculation point is at an extended SSD. For MLC-defined fields, with the jaws at the recomm ended positions, calculated in-air output factors agree with the measured d ata to within 0.3% at the isocenter and 0.7% at off-axis positions. The mod el has been incorporated into a Monte Carlo dose algorithm to calculate the absolute dose distributions in patients or phantoms. For three MLC-defined irregular fields (triangle shape, C-shape, and L-shape), the calculations agree with the measurements to about 1% even for points at off-axis positio ns. The model will be particularly useful for IMRT dose calculations becaus e it accurately predicts beam output and penumbra dose. (C) 2001 American A ssociation of Physicists in Medicine.