The physical characteristics of the 15 MV Varian Clinac 2100C unflattened beam

• We made a Monte Carlo simulation to study the dosimetric properties of the unflattened beam of the 15 MV Varian Clinac 2100C.
• Removing the flattening filter increase the dose rate of the beam.
• Removing the flattening filter decrease the out-of-field dose.
• Removing the flattening filter decrease the photoneutron production.
• The surface dose to the patient is increased by removing the flattening filter.

A 15 MV photon beam of a Varian Clinac 2100C medical linear accelerator operating with and without a flattening filter was simulated using the Monte Carlo code ‘FLUKA’ in order to calculate differences in their dosimetric properties. These include: the dose rate, the percentage depth dose on the central axis, the beam profile, the out-of-field dose, the surface dose on a 40×40×40 cm3 water phantom and the neutron contamination. The results obtained showed that the unflattened beam has a dose rate 4.86 times higher than the flattened one. The average out-of-field dose from the edge of the field to the edge of the phantom was reduced by 44%, the neutron fluence at the isocentre was reduced by 77% and the surface neutron dose-equivalent was reduced from 2.11±0.05 to 0.40±0.01 mSv(n)  Gy−1(X) after normalising both beams to give the same dose at dmax (the depth of maximum dose). However, the photon surface dose of the unflattened beam increased by 13%. From this information, it can be concluded that the unflattened beam can lead to better treatment outcome and may reduce the beam-on time which may be required for specific cases.