Novel air-to-tissue conversion factors for fast, epithermal and thermal photoneutrons in a Siemens ONCOR dual energy 18 MV X-ray medical linear accelerator

Highlights: • Photoneutron air-to-tissue conversion factors in a medical accelerator were determined. • Polycarbonate/¹⁰B (with/without cadmium) dosimeters in air and phantom were used. • Fast/epithermal/thermal photoneutron conversion factors along central axis are reported. • Photoneutron conversion factors at different phantom depths are also reported. • Factors are highly instrumental for photoneutron primary second cancer risk estimations. -- Abstract: A novel "photoneutron air-to-tissue conversion factor" determination method was developed with provision of extensive data matrix for converting photoneutron dose equivalent at a position in air to that of a similar position in a phantom. The conversion factors were determined along the central axis as well as on surface and at depths of a multi-layer polyethylene phantom in a 10 cm × 10 cm field of an 18 MV X-ray Siemens ONCOR medical accelerator. Polycarbonate/¹⁰B neutron dosimeters (with/without cadmium covers) were applied. Fast-photoneutron-induced recoil tracks and thermal/epithermal-photoneutron-induced alpha tracks from ¹⁰B (n_th,α)⁷Li reactions in polycarbonate were processed by ECE method, counted and converted to photoneutron dose equivalents. Fast, epithermal and thermal photoneutron air-to-tissue conversion factors along central axis as well as at different positions from the central axis on the surface and at 12 phantom depths were determined. Air-to-tissue conversion factors for fast, epithermal and thermal photoneutrons along the central axis follow the trends of depth dose equivalent responses. The air-to-tissue conversion factor responses determined at different depths on transverse axis are flat. Air-to-tissue conversion factors determined at the surface of the phantom are 1.89 ± 0.24 for sum of fast, epithermal and thermal; 3.52 ± 0.25 for sum of epithermal and thermal; 1.57 ± 0.13 for fast; 4.40 ± 0.33 for epithermal; and 3.35 ± 0.18 for thermal photoneutrons. In conclusion, novel "photoneutron air-to-tissue conversion factors" versus distance from the central axis were determined for the first time in data profile groups at 12 depths from the phantom surface. These factors are highly instrumental for converting a photoneutron dose equivalent value in air to that of tissue at the same position.