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Study of unflattened photon beams shaped by multileaf collimator using BEAMnrc code

Abstract Purpose In our study basic dosimetric properties of a flattening filter free 6 MV photon beam shaped by multileaf collimators (MLC) is examined using the Monte Carlo (MC) method. Methods and Materials BEAMnrc code was used to make a MC simulation model for 6 MV photon beam based on Varian Clinic 600 unique performance linac, operated with and without a flattening filter in beam line. Dosimetric features including central axis depth dose, beam profiles, photon and electron spectra were calculated and compared for flattened and unflattened cases. Results Dosimetric field size and penumbra were found to be smaller for unflattened beam, and the decrease in field size was less for MLC shaped in comparison with jaw-shaped unflattened beam. Increase in dose rate of >2·4 times was observed for unflattened beam indicating a shorter beam delivery time for treatment. MLC leakage was found to decrease significantly when the flattening filter was removed from the beam line. The total scatter factor showed slower deviation with field sizes for unflattened beam indicating a reduced head scatter. Conclusions Our study demonstrated that improved accelerator characteristics can be achieved by removing flattening filter from beam line.