The influence of target thickness and backstop material on proton-produced neutron beams for radiotherapy

A new fast-neutron source for radiobiological research

A biomedical cyclotron facility primarily dedicated to radionuclide production has been extended by the addition of an experimental fast-neutron source for radiobiological and biophysical studies. Several beams of fast-neutrons with different average energies and LET distributions can now be provided. The neutrons are produced by bombarding berryllium targets with 18-32 McV protons or 8.7-15 MeV deuterons from our K = 32 negative-ion cyclotron. Average neutron energies range from approximately 4 to 15 MeV. Doses at maximum build-up vary from 0.24 to 1.85 cGy microA-1 min-1 at 1 m SSD, i.e. approximately 55 cGy min-1 at 30 microA of proton current at maximum energy. The design of the facility and some dosimetric results are presented.

The lethal effects of Fermilab fast neutrons vary with the depth of cells in a water phantom

Using V79 Chinese hamster lung fibroblast cells grown in culture we have examined the lethal effects of fast neutrons from the Fermilab therapy facility as a function of depth in a water phantom. Exposures and dosimetry were performed from 3 to 24 cm deep in the phantom along the central axis of the neutron beam, using various collimator configurations. The results indicate that the relative biological effect (RBE), using 60Co gamma rays as the standard radiation, varies with depth in the neutron beam. Starting from d-max (approx. 3 cm), the RBE appears to decrease continuously with depth. At 24 cm deep, the relative effectiveness is 10-15% lower than at 3 cm deep. There appears to be no systematic variation of relative effectiveness with shape or size of collimator. If a hydrogenous filter is added before the beam passes into the water phantom, the variation with depth is greatly reduced.

Physical characteristics of the M.D. Anderson Hospital clinical neutron beam

The physical characteristics of the M.D. Anderson Hospital (MDAH) clinical neutron beam are presented. The central-axis percent depth-dose values are intermediate between a 4 and 6 MV X-ray beam. The build-up curves reach a depth of maximum dose at 1.2 cm and have surface dose values of approximately 30%. Teflon flattening filters are employed to flatten the beam at the depth of the 75% dose level. Two wedges are available for shaping the beam; they are made of Teflon and produce wedge angles of 31 degrees and 45 degrees as defined by the ICRU. Output factors ranged from 0.88 for a 4 x 4 cm field to 1.12 for a 20 x 20 cm field. Tungsten blocks reduced the dose received at Dmax to 25% of the unblocked value but only 52% of the unblocked value at a depth of 22.8 cm.

New neutron sources for radiotherapy

All existing neutron sources suffer from disadvantages which would not be tolerated in modern megavoltage X-ray equipment. Experimental work with 30-60 MeV protons on beryllium and other elements has shown how these difficulties can to a large extent be overcome. Angular distributions and kerma rates as a function of proton energy are presented for various targets. Thin targets are found to be a better method than filtration for improving the penetration of the beam. A 237Np fission counter and a GM counter have been used to decompose stray radiation into its neutron and gamma-ray components. A model of a treatment head was found to attenuate the neutron component to less than 1% of its value in the useful beam.

Comparison of neutron therapy beams produced by 50 MeV deuterons and 65 MeV protons on beryllium

Neutron beams produced by bombarding a 10 cm thick beryllium target with 50 MeV deuterons have been used at Louvain-la-Neuve since nearly 4 years for routine therapeutic applications. At the end of 1981 they were replaced by neutron beams produced by 65 MeV protons on beryllium, mainly in order to improve the beam penetration in tissues. However, produced of neutrons from the p leads to Be reaction implies some disadvantages, mainly a lower dose rate and a higher activation level. In order to solve the problem a new target configuration was designed, consisting of a remote handled system which permits the use of 2 different target assemblies. The irradiated target is automatically removed immediately after the irradiation which greatly protects against exposure to the staff. The dosimetric characteristics of the d(50)-Be and p(65)-Be neutrons beams are compared.