Neutron production and transport at a medical linear accelerator

Abstract

The Colorado State University Veterinary Teaching Hospital (VTH) uses a Varian Trilogy™ linear accelerator (linac) for radiation oncology treatment. The high-energy electron beam is used to treat superficial tumors (deep tissues are spared with this modality) or is accelerated against a target to produce high-energy photons that are used to treat deep seated tumors (skin is spared with this modality). Either application might exceed the neutron production threshold for various materials. This study evaluates neutron production and transport in the environment surrounding the VTH trilogy through MCNP modeling and physical measurements of the 10 MV photon and 18 MeV electron beam modalities. MCNP modeling was accomplished in two phases. The first phase involved simulating the linear accelerator and determining the relevant parameters for neutron production for both modalities. This was accomplished by using various target specifications and replicating the geometry of the machine. In the second phase, MCNP modeling of the accelerator vault as well as other locations of interest within the treatment suite was conducted. This phase determined measurable neutron fluence and dose rates at the test locations where physical measurements were taken. The MCNP results (for neutron energies between 0.2 to 10 MeV) were compared with the physical measurements. Physical measurements were performed with a BF3 detector (responsive to energies between 0.2 and 10 MeV) and taken at the same test locations. For both modalities, MCNP and physical measurements demonstrated neutron production. Large uncertainties were associated with the physical measurements for both modalities. For the photon mode, MCNP modeling resulted in neutron equivalent doses per photon Gy up to 0.112 mrem/photon Gy, and physical measurements up to 0.133 mrem/photon Gy. For the electron mode, MCNP modeling resulted in measureable neutron equivalent doses per electron Gy up to 14.88 mrem/electron Gy, and physical measurements up to 3.83 x 10-04 mrem/electron Gy. Taking the entire neutron spectrum into account, MCNP results showed neutron doses up to 347.81 mrem/ photon Gy at the isocenter for the photon beam, and up to 1.77 x 105 mrem/electron Gy at the isocenter for the electron beam. These numbers could not be compared to the physical measurements because the BF3 detector used in this experiment only responded to neutron energies between 0.2 and 10 MeV. The conclusion made from this research is that neutrons are generated at various locations in and outside the room. For the photon modality, the neutron dose to the patient can be considered negligible when compared with the treatment dose. Neutron production does not appear to exceed the tolerance for workers in appropriate locations surrounding the VTH linac vault. Further research is recommended for an accurate analysis of both modalities.