Computer generation of dose distributions for a fast-neutron therapy beam.

A system has been developed for the computer generation of dose distributions for the MANTA-NRL fast-neutron radiotherapy beam. This program is based on scatter-air ratio (SAR) techniques. A method has been developed to unfold the effect of the neutron-beam profile in the derivation of SARs so that the SARs obtained are those which would be derived if the beam profile were flat. Tables of zero-area tissue-air ratios and SARs are presented. Comparisons of calculated and measured dose distributions are shown. An empirical correction to the usual SAR methods was required to obtain agreement between calculated and measured dose distributions at source-to-skin distances (SSD) which are different from the SSD at which the SAR are derived.

Dosimetry measurements of 26, 35 and 45 MeV p-Be and p-Li neutron beams.

Dosimetric properties of neutron beams produced by stopping 26, 35 and 45 MeV protons in beryllium and lithium have been measured. The effects of filtering the p-Be beam with 6 cm of polyethylene have been investigated. The tissue kerma rate in air exhibited an energy dependence of approximately E3 and the rate for p-Be beams was approximately one-fifth of the rate for d-Be beams. The penetrability of the neutrons was significnatly enhanced by the use of the filter, but with a 50% attentuation in tissue kerma rate. The tissue kerma rate for the p-Li beam was nearly the same as that for the p-Be beam.

Physical characterization of neutron beams produced by protons and deuterons of various energies bombarding beryllium and lithium targets of several thicknesses.

Protons of 35 and 65 MeV and deuterons of 35 MeV were used to bombard beryllium and lithium targets of various thicknesses. Four types of experiments were conducted in order to characterize the neutron fields. They were (1) central axis depth-dose measurements in a water phantom, (2) dose buildup at small depths in tissue-equivalent plastic, (3) microdosimetric measurements and LET distributions, and (4) neutron yields and energy spectra at an angle of 0 deg. The results generally show that (a) the central axis depth doses for the 35 and 65 MeV particles roughly approximate those of 60Co and 4-MeV bremsstrahlung photons, respectively, (B) the neutron dose buildups are more rapid than those of the above-mentioned photon sources, (C) the microdosimetric spectra show differences which are consistent with the measured neutron energy spectra, and (D) P-Li compared to p-Be neutron spectra have larger high-energy particle flux for similar target and beam configurations.

Stripping-theory analysis of thick-target neutron production for D + Be.

The Serber theory for deuteron stripping is employed to predict the shape of the neutron energy spectrum produced by 35 MeV deuterons (D+) on a thick beryllium target. In particular, the observation that the maximum of the neutron energy spectrum (at 0degrees relative to the deuteron beam direction) occurs at approximately 0-4Ed, where Ed is the incident deuteron energy, is explained reasonably well by the calculations. The explanation stems mainly from the fact that the stripping theory for thin targets predicts a narrow maximum at 0-5Ed, and thick target effects shift the maximum downward in energy to approximately 0-4Ed. A number of recent spectral measurements are in agreement with these predictions for a wide range of target materials and incident deuteron energies. The application of this theory also accounts for the previously observed Dd2-99 dependence of the absorbed dose in tissue,per unit charge of D+ ions on target, in the direction of the incident beam. This approximate Ed3 dependence is shown to be a characteristic property of deuteron stripping in a thick target and follows directly from the calculations that predict the neutron energy spectrum.

Attenuation measurements of a fast neutron radiotherapy beam.

Attenuation measurements have been made for pressed wood, steel and lead in the neutron beam generated by 35 MeV deuterons on a thick Be target, using narrow-beam, semibroad-beam and broad-beam geometries. In addition, the measurements were extended to the neutron beams from 25 and 15 MeV D+ on Be, for semibroad-beam geometry only. A tissue-equivalent plastic ionization chamber was used as the detector; hence the measured parameter approximated the total (n + gamma) dose in tissue. After correcting for radiation scattering around the absorbers, the remaining curves were nearly exponential in most cases, and a build-up factor could be employed to describe the varying degree of scattered radiation dose as a function of absorber thickness and geometry. The results given should be applicable in designing primary-beam shields and collimators, as well as providing a useful basis for extending calculations to other geometries through Monte Carlo techniques.

Displacement correction factor for fast-neutron dosimetry in a tissue-equivalent phantom.

The displacement correction factor to be used for analysis of fast-neutron dosimetric measurements using air-filled EG and G tissue-equivalent ion chambers in a tissue-equivalent phantom has been investigated using the MANTA neutron radiotherapy beam generated by 35-MeV deuterons on a thick Be target. The displacement correction factor inferred from these measurements is 0.970 for the EG and G IC-17 (1.0-cm3) ion chamber, and is 0.989 for the EG and G IC-18 (0.1-cm3 ion chamber. This multiplicative displacement correction factor has no significant dependence on depth in the phantom or on neutron beam size.

Radiobiological properties of high-energy cyclotron-produced neutrons used for radiotherapy.

Radiobiological properties of high-energy cyclotron-produced neutrons were investigated. The survival curve for mammalian cells exposed to 35 MeV neutrons has an appreciable shoulder, but exhibits a significant initial slope at low doses. split-dose experiments, using doses comparable to the daily fractions of neutron therapy, indicate no repair of sublethal damage. The nitroimidazoles, and particularly Flagyl, have been shown to selectively sensitize hypoxic cells to the effects of x-rays. Experiments demonstrated that Flagyl is equally effective in reducing the oxygen enhancement ratio for high-energy neutrons. Hypoxic sensitizers must by regarded as an adjunct to neutron therapy, rather than as a competitor.