Geant4 simulations on medical Linac operation at 18 MV: Experimental validation based on activation foils

• Medical accelerator operation at 18 MeV photon beam simulated using Geant4 code.
• Discrepancies of the photon spectrum due electron beam's energy and spatial spread.
• The influence of the contaminant secondary particles on the skin dose.
• Photons and particles distribution for flattened and unflattened beams.
• Photon and neutron fluencies by Geant4 were compared to data using foil activation.

The operation of a medical linear accelerator was simulated using the Geant4 code regarding to study the characteristics of an 18 MeV photon beam. Simulations showed that (a) the photon spectrum at the isocenter is not influenced by changes of the primary electron beam's energy distribution and spatial spread (b) 98% of the photon energy fluence scored at the isocenter is primary photons that have only interacted with the target (c) the number of contaminant electrons is not negligible since it fluctuated around 5×10−5 per primary electron or 2.40×10−3 per photon at the isocenter (d) the number of neutrons that are created by (γ, n) reactions is 3.13×10−6 per primary electron or 1.50×10−3 per photon at the isocenter (e) a flattening filter free beam needs less primary electrons in order to deliver the same photon fluence at the isocenter than a normal flattening filter operation (f) there is no significant increase of the surface dose due to the contaminant electrons by removing the flattening filter (g) comparing the neutron fluences per incident electron for the flattened and unflattened beam, the neutron fluencies is 7% higher for the unflattened beams. To validate the simulations results, the total neutron and photon fluence at the isocenter field were measured using nickel, indium, and natural uranium activation foils. The percentage difference between simulations and measurements was 1.26% in case of uranium and 2.45% in case of the indium foil regarding photon fluencies while for neutrons the discrepancy is higher up to 8.0%. The photon and neutron fluencies of the simulated experiments fall within a range of ±1 and ±2 sigma error, respectively, compared to the ones obtained experimentally.