Neutron therapy using medical linacs

Measurements of the neutron flux generated at depth in water by an 18 MV photon beam of a Varian 2100C linac are presented. The thermal neutron flux at depth exhibits a maximum at around 5 cm of 1.37×10 ⁶ n/cm²/s for a 40×40 cm² field size, 100 cm SSD and a photon output of 400 MU/min. Because of the low thermal neutron flux, a large concentration of boron-10 at the tumor is required to achieve significant dose deposition by boron neutron capture reaction. To reduce the concentration required at the tumor it is necessary to increase the irradiation time without a significant increase in the photon dose. This is found to be possible by completely closing the collimator jaws which results in a 1000 times reduction in the photon dose. Also the thermal neutron flux can be further increased by reducing the SSD to the minimum possible (50 cm in the authors´ case). In such conditions, the linac can be considered to be operating in a neutron beam mode. In addition, a 25 MV photon beam will generated 10 times more neutrons than an 18 MV beam. This report demonstrates that boron neutron capture therapy and boron neutron capture enhanced photon therapy can be successfully performed using the neutron beam generated by high energy medical linacs