Accuracy of empirical formulas in evaluation of neutron dose equivalent inside the 60Co vaults reconstructed for medical linear accelerators

In Southeast Europe medical accelerators are sometimes placed in small vaults originally built for 60Co treatment unit. In order to meet shielding requirements for high energy photon beams, the wall thickness had to be increased. Since the vaults are already limited in size, instead of adding more concrete, materials with high-Z elements were used. Limited vault size and addition of high-Z elements can contribute to the neutron dose equivalent for both medical personnel and patients.

The most commonly used empirical equations for estimation of neutron dose equivalent at the maze door in the vault are by Kersey and Wu-McGinley. In order to assess accuracy of these equations, Monte Carlo (MC) simulations of various geometrical and compositional changes of vault were conducted. Neutron ambient dose equivalent (H*n(10)) was observed when dimensions of the vault walls were reduced gradually.

The empirical equations gave results with reasonable accuracy when vaults were of standard size. When the vault was decreased to the size of the usual 60Co unit vault, the most commonly used equations showed significant difference in results (up to 90%) in comparison to MC simulations. MC simulations showed that introducing different materials in shielding can change the neutron dose equivalent in vicinity of accelerators.

For vaults limited in size, new simplified equation for neutron dose equivalent at the maze doors is presented, although performing a MC simulation of the specific vault is suggested.