The evaluation of a gas mixture having an atomic composition similar to that of A-150 TE-plastic has been extended to a high energy neutron therapy beam."A-150" gas, air and methane-based TE gas were each flowed through A-150 plastic-walled ion chambers of different sizes and irradiated with p(66)Be (49) neutrons. A tentative value for W(A-150) of 27.3 +0.5 J c-l was derived for this beam. The W value of the A-150 gas mixture is compared to those of methane-based TE gas and of air for the p(66)Be(49) neutron beam as well as to corresponding values found in similar experiments using 14.8 MeV monoenergetic neutrons.

The hardening effects of hydrogenous and non-hydrogenous filters on a p(66)Be(49) neutron beam have been investigated. It was found that all materials studied, Teflon, aluminum, lead, steel and polyethylene, harden the neutron beam, albeit polyethylene to a greater extent. Relationships were found to exist between the attenuation of a filter and its hardening effect, and also between the build-up characteristics and the depth for half-maximum dose of the hardened beams.

In conventional radiation therapy facilities, radiation doses to medical personnel originate from the leakage radiation of 60 co teletherapy systems or from photoneutrons produced during the operation of X-ray generators at energies over 10 MeV in unsuitably shielded therapy rooms. In neutron therapy facilities, during patient setups and position verifications, medical personnel are exposed to photons from remanent radioactivity induced in the shielding around the neutron producing targets and in the beam collimators. At Fermilab, the use of an elevating platform limits personnel exposure periods to those times when collimators are being exchanged. Comparisons with other facilities are shown.

The dose reduction in phantom due to missing backscattering material was measured on and off the central axis of a p(66)Be(49) neutron beam. The doses at different depths in phantom for various field sizes are found to drop by 4-10% relative to full backscatter conditions when backscattering material is removed. A simple algorithm that predicts the magnitude of this drop on the basis of equivalent square size of the beam is presented. The algorithm may be used to correct the dose (usually computed from data obtained with full backscatter) at all points near the exit side of a phantom or patient.

Central axis depth dose (CADD} and off-axis absorbed dose ratio (OAR) measurements were made in water, muscle and whole skeletal bone TE-solutions, mineral oil and and glycerin with a clinical neutron therapy beam. These measurements show that, for a given neutron beam quality and field size, there is a universal CADD distribution at infinity if the depth in the phantom is expressed in terms of appropriate scaling lengths. These are essentially the kerma-weighted neutron mean free paths in the media. The method used in ICRU #26 to scale the CADD by the ratio of the densities is shown to give incorrect results. The OAR's measured in different media at depths proportional to the respective mean free paths were also found to be independent of the media to a good approximation. Therefore, neutron beam CADD's and OAR's may be measured in either TE-solution (USA practice) or water (European practice), and having determined the respective scaling lengths, all measurements may be scaled from one medium to any other. It is recommended that for general treatment planning purposes, scaling be made to TE-muscle value represents muscle TE-solution of density with a density of 1.04 g cm-3 , since this and other soft tissues better than-3 1.07 g cm. For such a transformation, relative measurements made in water are found to require very TM-1145 2 small corrections. Hence, it is further recommended that relative CADD and OAR measurements be performed in universality and convenience. Finally, water because of its a table of calculated scaling lengths is given for various neutron energy spectra and for various tissues and materials of practical importance in neutron dosimetry.