Estimate of photo-nuclear reaction in a medical linear accelerator using a water-equivalent phantom

In recent radiotherapy techniques, neutrons are generated when the incident photon energy is higher than the threshold of (γ, n) reaction. Reactions occur in the various target materials, as well as in the flattening filters and collimators comprising the head structure of the electron linac. The resulting secondary radiation may lead to secondary cancer in patients because of the increased radiation dose. Therefore, evaluation of neutron dose is important in the medical field. We measured the secondary neutrons generated in the linac head, used for radiation therapy at 10 and 15 MV using a water-equivalent phantom, gold foil, and solid state track detectors. We calculated the neutron distribution in a model of the same situation using the PHITS code. In Monte Carlo calculations, the scattering of thermal neutrons in the water phantom was confirmed. The contribution from these neutrons was 0.1% or less of the X-rays at 10 MV. Because the cross-section of the photo-nuclear reaction increased in the high-energy linac, further examination is required. In 15 MV X-rays, the amount of neutrons was 10 times that in 10 MV X-rays. The PHITS code was created to simulate a photo-nuclear reaction and therefore suitable for calculations involving radioactive materials in these reactions. (author)