Calculation of absorbed dose and biological effectiveness from photonuclear reactions in a bremsstrahlung beam of end point 50 MeV

The absorbed dose due to photonuclear reactions in soft tissue, lung, breas t, adipose tissue and cortical bone has been evaluated for a scanned bremss trahlung beam of end point 50 MeV from a racetrack accelerator. The Monte C arlo code MCNP4B was used to determine the photon source spectrum from the bremsstrahlung target and to simulate the transport of photons through the treatment head and the patient. Photonuclear particle production in tissue was calculated numerically using the energy distributions of photons derive d from the Monte Carlo simulations. The transport of photoneutrons in the p atient and the photoneutron absorbed dose to tissue were determined using M CNP4B; the absorbed dose due to charged photonuclear particles was calculat ed numerically assuming total energy absorption in tissue voxels of 1 cm(3) . The photonuclear absorbed dose to soft tissue, lung, breast and adipose t issue is about (0.11-0.12)+/-0.05% of the maximum photon dose at a depth of 5.5 cm. The absorbed dose to cortical bone is about 45% larger than that t o soft tissue. If the contributions from all photoparticles (n, p, He-3 and He-4 particles and recoils of the residual nuclei) produced in the soft ti ssue and the accelerator, and from positron radiation and gammas due to ind uced radioactivity and excited states of the nuclei, are taken into account the total photonuclear absorbed dose delivered to soft tissue is about 0.1 5 +/- 0.08% of the maximum photon dose. II has been estimated that the RBE of the photon beam of 50 MV acceleration potential is approximately 2% high er than that of conventional Co-60 radiation.