Comparison of measured and Monte Carlo calculated dose distributions from the NRC linac

We have benchmarked photon beam simulations with the EGS4 user code BEAM [R ogers et al., Med. Phys. 22, 503-524 (1995)] by comparing calculated and me asured relative ionization distributions in water from the 10 and 20 MV pho ton beams of the NRC linac. Unlike previous calculations, the incident elec tron energy is known independently to 1%, the entire extra-focal radiation is simulated, and electron contamination is accounted for. The full Monte C arlo simulation of the linac includes the electron exit window, target, fla ttening filter, monitor chambers, collimators, as well as the PMMA walls of the water phantom. Dose distributions are calculated using a modified vers ion of the EGS4 user code DOSXYZ which additionally allows scoring of avera ge energy and energy fluence in the phantom. Dose is converted to ionizatio n by accounting for the ((L) over bar/rho)(air)(water) variation in the pha ntom, calculated in an identical geometry for the realistic beams using a n ew EGS4 user code, SPRXYZ. The variation of ((L) over bar/rho)(air)(water) with depth is a 1.25% correction at 10 MV and a 2% correction at 20 MV. At both energies, the calculated and the measured values of ionization on the central axis in the buildup region agree within 1% of maximum ionization re lative to the ionization at 10 cm depth. The agreement is well within stati stics elsewhere. The electron contamination contributes 0.35(+/-0.02) to 1. 37(+/-0.03)% of the maximum dose in the buildup region at 10 MV and 0.26(+/ -0.03) to 3.14(+/-0.07)% of the maximum dose at 20 MV. The penumbrae at 3 d epths in each beam (in g/cm(2)), 1.99 (d(max) 10 MV only), 3.29 (d(max), 20 MV only), 9.79 and 19.79, agree with ionization chamber measurements to be tter than 1 mm. Possible causes for the discrepancy between calculations an d measurements are analyzed and discussed in detail. [S0094-2405(00)00810-5 ].