The study of radiation fields around the cyclotron room
was carried out for the Proton-Therapy-Center in Prague (Czech Republic).
The 230 MeV proton cyclotron will be installed there as well as
additional components located along the beampath. The ambient doses
were computed by taking into account 3 major sources of secondary
radiation (neutron and photon) – the cyclotron, energy degrader
and its collimator. The fluxes of neutrons/photons produced by protons
were computed using Monte Carlo code MCNPXTM 2.5.0. Energy
and angular distributions were computed for various targets and proton
beam energies. Different physical models of high-energy proton interactions
were studied as well. The annual charge at the cyclotron exit was
based on the “patient model” (distribution of treatment type) established
for a similar therapy center in Essen (Germany). Calculated data
were used as an approximation of the radiation field of the most significant
sources of radiation (graphite degrader, tantalum collimator). Fluence-to-dose
conversion factors from ICRP-74 were used for the annual dose equivalent
evaluation. With these results, the shielding analysis in the Therapy-Center
was performed in the context of the current legislation requirements.