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Abstract. A Monte Carlo model of an Elekta Precise linear accelerator has been built and verified by measured data for a 6 MV and 10 MV photon beam running with and without a flattening filter in the beam line. In this study the flattening filter was replaced with a 6 mm thick copper plate, provided by the linac vendor, in order to stabilize the beam. Several studies have shown that removal of the filter improves some properties of the photon beam, which could be beneficial for radiotherapy treatments. The investigated characteristics of this new beam included output, spectra, mean energy, half value layer and the origin of scattered photons. The results showed an increased dose output per initial electron at the central axis of 1.76 and 2.66 for the 6 MV and 10 MV beams, respectively. The number of scattered photons from the accelerator head was reduced by (31.70.03) % (1 SD) for the 6 MV beam and (47.60.02) % for the 10 MV beam. The photon energy spectrum of the unflattened beam was softer compared to a conventional beam and did not vary significantly with off-axis distance, even for the largest field size (0-20 cm off-axis).

Using TPR20,10 as a beam-quality specifier, for the flattening filter free beams used in this study, gave a maximum difference of 0.39% between L̄/ρair (water) predicted using IAEA TRS-398 and Monte Carlo calculations. An additional parameter for determining L̄/ρair (water) has been presented. This parameter is easy to measure; it requires only an additional dose measurement at 5 cm depth with SSD 95 cm, and provides information for accurate determination of the L̄/ρair (water) ratio for beams both with and without a flattening filter at the investigated energies.