PHOTONEUTRON DOSE IN SOFT-TISSUE PHANTOMS IRRADIATED BY 25 MV X-RAYS

The photoneutron production and the absorbed dose in two soft tissue p hantoms irradiated with 25 MV x-ray beams were estimated by means of t he MCNP4 Monte Carlo code. Both single-beam and four-field box irradia tions were considered. The neutron and photon therapy dose was calcula ted in various regions of the phantoms. An experimental verification o f the photoneutron yield as a function of depth in a water phantom was also carried out by means of photoactivation reactions on O-16. An av erage photoneutron and x-ray integral dose ratio of 3.7 x 10(-5) (phot oneutron Gy kg-1)/(x-ray therapy Gy kg-1) was estimated inside the tar get volume of the four-field box irradiation phantom, whilst outside t he integral dose ratio is 9.2 x 10(-6) (photoneutron Gy kg-1)/(x-ray t herapy Gy kg-1).