The dosimetry of bone irradiated by fast neutrons

The previous work on the dosimetry of bone is briefly reviewed. A dosimetric theory for the response of detectors irradiated by fast neutrons is applied to the problem of bone dosimetry. In the theory the detector or cavity shape is characterised by distributions of chord lengths along which the neutron-produced charged particles travel and deposit energy. Cavities of different convex geometries can be treated. A simplified version of the theory uses a single mean chord length to characterise the cavity. The absorbed dose to individual marrow cavities in trabecular bone is calculated over a large range of marrow cavity size for monoenergetic neutrons ranging from 0.5 to 7.0 MeV and for 252Cf neutrons. The influence of cavity shape is explored by considering spheres and cylinders of different elongation. The difference in absorbed dose is not great. Also the simplified model using a single mean chord length gives results in close agreement with the results obtained with chord length distributions. The mean marrow dose to different human bones has been calculated in three ways. First by using measured chord length distributions for the marrow cavities in the bones, second by using a sphere with the same mean chord length as the measured distribution and third by applying the measured single mean chord length. The difference between the three approaches is small and the agreement is good with results obtained by other workers who used the Monte Carlo technique. The dose to the endosteal cell layer has also been calculated by approximating the layer with an infinite slab.