Estimate of Photonuclear Reaction in a Medical Linear Accelerator Using a Water-Equivalent Phantom

2 In recent radiotherapy techniques, neutrons are generated when the incident photon energy is higher than the thre- shold 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 second- ary cancer in patients because of the increased radiation dose. 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 that from X-rays at 10 MV. Because the cross-section of the photonuclear reaction increased in the high-energy linac, further examination is required. For 15 MV X-rays, the amount of neu- trons was 10 times higher than for 10 MV X-rays. The PHITS code was created to simulate a photonuclear reaction and is therefore suitable for calculations involving these reactions in radioactive materials.