Photo-neutron production of medical linear accelerator Elekta, precise model: A study with the Geant4 MC toolkit

We use Geant4 Monte Carlo (MC) toolkit to simulate a Precise Series Elekta linear accelerator. The simulation is
validated for 6 and 15 MV X-rays with measured dose taken along the central axis of the radiation and off axis.
The measurements are used in a clinical system, The neutrons generated from a (X, n) reaction when the 15 MV
X-ray beam is on, are unwated byproducts that add up to the patient’s dose. Through the MC simulation we
quantify the neutron dose contribution per delivered x-ray dose. Neutron fluence, in the units of neutron/cm2,
contamination of the beam’s central axis was found to be proportionally dependent on the size of radiation field
and decreases with the field size. This indicated that the number of neutrons produced in the head is constant,
and when the field size opened wider, the neutrons distributed to a larger area, hence the fluence decreases. On
the other hand, the neutron contamination outside the boundaries of the beam did not depend on its size and was
found to be constant. This is intuitive, as those are the neutrons leaking from the head of the linac laterally, and
as such are independent of the radiation beam size. Moreover, the maximum dose, dmax, for 15 MV x-rays from
neutrons was found to be at a voxel 2 at the central axis at a depth of 2.5 cm inside the water phantom and it
contributes an amount of 0.0003% to total dose.