RBE and clinical response in radiotherapy with neutron beams

Consideration of the clinical results reported, when a cyclotron produced neutron beam was used for treatments in the pelvis region, suggested that a constant RBE of 3 should not have been used for all neutron doses. Instead a variable RBE, which increased from approximately 3 to 8 (with decreasing dose), should have been used. Although some of these RBE values are much higher than 3, they have been observed in clinical practice. An "equivalent photon" isodose plan was produced by employing a variable RBE and, by taking a TDF limit of 86 for bowel, an isoeffect plan was produced. This shows that in the clinical situation under consideration much of the pelvis was overdosed. Doses to tumour cells and late effects are also briefly considered. It is suggested that, in neutron therapy, both an "equivalent photon" isodose plan and an isoeffect plan should be produced prior to treatment.     

Radiobiological intercomparisons of fast neutron beams used in therapy

A research program was performed at Louvain-la-Neuve to systematically determine the RBE of fast neutrons for the growth inhibition in Vicia faba bean roots and for the regeneration of the intestinal crypts in mice. The following neutron beams were compared p(75) + Be, p(65) + Be, p(45) + Be, p(34) + Be, d(20) + Be, and d(50) + Be. The RBE-variation as a function of neutron energy is larger for the Vicia faba system than for the regeneration of the intestinal crypt cells. This can be related to the inherent differency of the biological systems, but also to the different dose ranges involved (0.33 to 0.56 Gy and 7.66 to 8.56 Gy, respectively). In the high energy range explored, defined by the reactions p(75) + Be to p(34) + Be RBE varies only between 0.92 and 1.28 for Vicia faba and 0.96 and 1.12 for crypt cells normalized to the p(65) + Be beam. By contrast the RBE at lower energy beams (d(20) + Be and d(14.5) + Be) reaches values between 1.5 and 1.6 Finally fractionation has shown to be likely more important at the high energy beams.     

Biological intercomparisons of neutron beams used for radiotherapy generated by p(+)-->Be in hospital-based cyclotrons.

The new generation of hospital-based neutron therapy facilities involve cyclotrons using protons on beryllium. The spectrum of neutrons produced includes a large and variable proportion of low-energy neutrons that are poorly penetrating but biologically effective. Cells cultured in vitro were used to compare the three US facilities at Seattle, M.D. Anderson and UCLA, together with the UK facility at Clatterbridge. Cyclotrons were compared within a given experiment on the same day using cells from a common suspension. Among the three US facilities, the relative potency factor at a depth of 25 mm differs by about 11%, with Seattle the least and UCLA the most biologically effective. Clatterbridge was compared directly with M.D. Anderson and found to be less effective by about 5%; it has a slightly lower biological effectiveness than any of the US facilities. There is evidence for an increased biological effectiveness in the build-up region, which reduces the effective skin sparing potential. There is not much difference in build-up between the three US facilities. Using the proton-on-beryllium neutron production process results in a wide spectrum of neutrons with a large but variable low-energy component. The biological effectiveness of the beam depends on target design and thickness as well as the design of the collimating system. Consequently the biological effectiveness of neutron beams generated by this process must be assessed on an individual basis. It cannot be assumed that because cyclotrons have similar accelerating energies that the relative biological effectiveness will be the same.     

CT manifestation in the pararectal area following fast neutron radiotherapy

Sixty-four pelvic CT studies of 38 patients with pelvic malignancies were obtained at various times (21 to 678 days) following fast neutron radiotherapy. Multiple scans (2 to 3) were available in 19 cases. Fractionated neutron radiotherapy was delivered over four to seven weeks to a total dose of 54 Gy to 60 Gy photon equivalent. Radiation-induced changes on CT were symmetrical thickening of the perirectal fascia (87%), widening of the presacral space (82%), and thickening of the walls of the bladder and rectum (21% and 11%, respectively). Serial CT studies showed that these changes became stable after 12 weeks from radiotherapy in 13 of 14 patients (93%). CT manifestations of radiation-induced changes in the pararectal area were rather regular and would be easy to differentiate from local recurrence or secondary disease.     

Prospects for efficient detectors for fast neutron imaging.

A physical model describing in detail the process of fast neutron imaging in luminescent screens is presented. The detection quantum efficiency, luminosity and inherent spatial resolution of the screen were calculated using this model. Properties of transparent and disperse screens were compared. Two imaging systems were suggested to improve the detection efficiency and spatial resolution. A stack consisting of alternating neutron converters and image plates can help in obtaining both high spatial resolution and efficiency. A system containing a screen of special form and a diaphragm can be of use especially in the case of the fan beam.     

Cyclotron fast neutron RBE for various normal tissues.

The relative biological effectiveness (RBE) values of fast neutrons as a function of dose in the range of dose per fraction being used clinically are reported for effects on several normal mouse tissues. The specific tissues examined were: thymus, spleen, white blood cells (WBC), testes, and small intestine. The RBE values for damage to the thymus (1.2), spleen (1.2), WBC (1.0), and testes (3.0) were independent of neutron dose size. In contrast, the RBE for the small intestine varied between 2.7 and 3.4 and was dependent on the size of the neutron dose. The RBE for intestinal death (2.4) and hematopoietic death (1.1) also is reported.     

A fast neutron source for cultured cell irradiation.

Because of recent interest in the use of neutrons for radiotherapy, there has been an increased interest in the radiology of neutrons. In this irradiated cell study, a 1.3 MeV accelerator produced beam currents over 100 muA on the water-cooled 3-mm thick beryllium disk target. The monolayer of irradiated cells was neutron-shielded by about 700 kg of paraffin. The neutron energy spectrum for the 9Be(d,n)10B reaction was obtained, with an average neutron energy calculated to be between 3.3 and 3.5 MeV, and an average linear energy transfer calculated at more than 30 keV/micron.     

Epithermal neutron self-shielding factors in foils for collimated beams.

An epithermal neutron self-shielding factor must be introduced to take into account the absorption of a neutron beam crossing a sample. This factor depends on the geometry and dimension of the sample, as well as on the physical and nuclear properties of the nuclides. On the basis of a dimensionless variable, which includes the relevant characteristics of the sample, universal curves for monoenergetic and 1/E collimated neutron beams are proposed, which enable the determination of the self-shielding factor for isolated resonances of high absorber nuclides.