A Monte Carlo simulation of out-of-field radiation from an 18-MV beam

Purpose: Patients undergoing radiation therapy are exposed to out‐of‐field radiation in the form of scattered and leakage photons, and at photon energies greater than 10‐MV, neutrons. This secondary radiation may pose a health risk to the patient by inducing secondary cancers in long‐term survivors; it is also a great concern for pregnant patients. Although estimations and measurements of out‐of‐field doses are possible, they are typically crude or time consuming and measurements may be only applicable to the particular treatment examined. A more robust tool is needed that can calculate accurately, and with relative ease, out‐of‐field photon and neutron dose equivalents. Such a tool has been developed with Monte Carlo. Method and Materials: A Varian 2100 accelerator and treatment vault, operated at 18‐MV, have been simulated in detail with MCNPX (including head shielding and structure, gantry, and couch). The out‐of‐field photon dose and neutron fluence were simulated. Simulated photon doses were compared with those measured with TLD‐700 in an acrylic phantom. Simulated neutron fluences were compared with fast neutron fluences measured with moderated gold foils in the patient plane. Neutron fluence can be converted to dose equivalent via conversion factors provided by the ICRP Report 74. Results: From a 10cm × 10cm field, the simulated out‐of‐field photon doses were found to agree, on average, within 10% of measured data over the range of 5cm to 40cm from the edge of the treatment field. Simulated neutron fluences were found to agree within 22% of measured data. Conclusion: A Monte Carlo model has been developed that is capable of calculating both the photon and neutron dose equivalent out‐of‐field. This tool is a substantial improvement on conventional calculations of the out‐of‐field dose from radiation treatments. © 2005, American Association of Physicists in Medicine. All rights reserved.