Possible pion sources for radiotherapy

Abstract

From second meeting on fundamental and practical aspects of the application of fast neutrons in clinical radiotherapy; The Hague, Netherlands (3 Oct 1973). Recently great interest has been shown in pi-meson irradiation as a possible modality for cancer radiation therapy. In order for this short-lived particle to be more than an idle laboratory curiosity, it must be demonstrated that economical pion sources can be built, which satisfy the necessary flux requirements for therapy within reasonable space and cost limitations. Protons have a much higher probability for producing pions per unit target mass than do electrons, so if there were no other consideration it is clear that proton accelerators would be superior to electron accelerators as pion sources. Unfortunately, proton accelerators are considerably more difficult to build than electron accelerators, requiring more precise control, more rf power, and probably more length than their counterparts. At 500-MeV the advantage of protons over electrons for production is about a factor of 50, that is, for the same total yield of pions, an electron accelerator must have 50 times the beam power required of an equivalent proton machine. One possible way around this problem is to design a much more efficient magnetic channel to collect the pions and focus them on the patient being treated. Such a system has been designed that utilizes 60 parallel magnetic spectrometers to focus a total of 1/12 the total pions produced in the pion production target on the patient. This system requires a more sophisticated control system, but is certainly an indicator of a direction to go to reduce source cost. This approach also reduces the beam current requirements on a proton accelerator by a factor of 30 making a 30 microamp accelerator a possibility. A brief description of four accelerator systems that might be suitable for hospital use, two electron accelerators, and two proton accelerators, is given to illustrate the problems involved. (CH