Monte Carlo simulations of the differential beam hardening effect of a flattening filter on a therapeutic x-ray beam.

A quantitative study of the differential beam hardening effect of the flattening filter on the 6-MV beam of Clinac 2100C has been conducted with Monte Carlo simulations using EGS4 code. The fluence-weighted photon energy of the unfiltered beam decreases from 1.35 MeV at central axis (CAX) to 1.22 MeV at an off-axis distance (OAD) of 20.0 cm. Compared to the unfiltered beam, the fluence-weighted photon energy of the filtered beam increases to 1.93 MeV at CAX and to 1.36 MeV at an OAD f 20.0 cm, respectively. The beam hardening effect was found to be 2.1 times higher at CAX than at an OAD of 20.0 cm. With the differential filtration of the flattening filter, the photon energy fluence reduced to 44% and 78% at CAX and an OAD of 20.0 cm respectively, resulting in the energy fluence of the filtered beam being flat from CAX to an OAD of 20.0 cm. The differential transmission ratios between the high energy and low energy photons decrease as the OAD increases. The percentage depth doses (PDDs) at field size of 10.0 cm x 10.0 cm for both the filtered and unfiltered 6-MV beams at CAX and at an OAD of 15.0 cm were calculated with a Monte Carlo technique based on the simulated spectra and fluence. The calculated PDDs were found to be consistent with the measured data for the filtered beam at CAX and an OAD of 15.0 cm. The beam quality (BQ) of the filtered beam at CAX is also higher than that of the same beam at an OAD of 15.0 cm. All the above results quantitatively demonstrate the differential beam hardening effects of a flattening filter on a therapeutic x-ray beam.