Characteristics of the photoneutron contamination present in a high-energy radiotherapy treatment room

The photoneutron contamination arising from a high-energy medical lineal accelerator is calculated using Monte Carlo simulation, as a function of the radiation field size. The information is used to model the neutron propagation in a radiotherapy treatment room and the transmission across concrete mazes. The Monte Carlo code MCNP4C is used to model the main components of a medical lineal accelerator. Simulations were performed to calculate the photoneutron yields and spectra as a function of the radiation field size. The yield of contaminant photoneutrons is observed to increase with the size of the radiation beam, but the energy spectra remain the same, suggesting that the contamination arises from above the movable collimator. The transport of the photoneutrons across a treatment room corroborates the validity of empirical models, but the transmission across a concrete maze produces a dose-equivalent tenth-value layer that differs from previous data.