INVESTIGATION OF THE POTENTIAL THERAPEUTIC APPLICATIONS OF FAST NEUTRONS FROM A LINEAR ELECTRON ACCELERATOR.

Abstract

In the process of calibrating and focusing the beam from an 8.5-Mev linear electron accelerator it was observed that substantial neutron fluxes were obtainedfrom a number of target materials. In view of the recent interest in the possible therapeutic advantages of fast neutrons it was thought worthwhile to investigate neutron production by this method. Accordingly, studies were made of different target materials and geometric arrangements for highest neutron flux, as well as of the problem of reducing the x-ray contamination. A number of publications on radiobiological effects in animals of neutrons of various energies (1, 8, 9, 10) have indicated an increased RBE of a fast neutron beam as compared to the standard x-ray. In some recent publications by Fowler et at. (1–7), working with the fast neutron beam from the Medical Research Council Cyclotron at Hammersmith Hospital in London, the increase of RBE with a fast neutron beam was confirmed, and a relatively more efficient action of a neutron beam on hypoxic or anoxic cells was noted, as compared to other radiation sources. The gain factor with neutrons was of the order of 1.7. This meant that for equal injury to well oxygenated tissue, the effect on anoxic cells was increased as if the dose to those cells alone had been increased by a factor of 1.7 by the use of a fast-neutron beam. This fact may have considerable importance in clinical radiation therapy. Measurement of the neutron flux and spectrum were difficult because the high radiofrequency field around the accelerator made operation of pulse detectors unreliable. For neutrons in this range the theoretical spectrum shows a peak at about 1.5 Mev and lies mostly between 0 and 4 Mev (12). An indium foil detector was employed to measure intensity. This was enclosed in a 6-in. paraffin sphere having a 0.5-mm. cadmium cap similar to that described by Stephens and Smith (II). Dosimeters of this type have been extensively used and shown to be fairly energy-independent over a wide range of energies of fast neutrons. The cadmium serves to reduce the response to thermal neutrons. The osimeter was calibrated against a standardized americium-beryllium source, and its arrangement is shown in Figure 1. X-ray measurements were made with a 100-r Victoreen chamber- having a 2-cm. plastic cap. Where applicable, correction was made for the neutron sensitivity of this detector by subtracting the neutron reading. The linear accelerator is an S-band traveling wave accelerator operating at 8.5 Mev and an average current of 35 microamperes. The I microsecond pulses have a recurrence rate of 300 per second. The beam of electrons is about 4 mm. in diarn- eter and can be used directly by passing it through a nickel window, or allowing it to fall on a gold target for production of bremsstrahlung.