An investigation of the characteristics of a negative pion beam by means of induced chromosome aberration in human peripheral blood lymphocytes.

The depth dose profiles of unstable chromosome aberrations were determined by irradiating blood samples at various depths in a plastic phantom exposed to a beam of negative pi mesons (pions). The pions were derived from an experimental beam line from the Nimrod 7 GeV proton synchrotron. With a momentum of 160 MeV/c and a spread of 15 per cent a Bragg profile was produced peaking at 13-9 cm. An ionization peak to plateau ratio of 1-6 was obtained and for cytogenetic damage this was increased to 2-9. Dicentric and acentric dose-response data from peak and plateau positions were fitted to Y=alphaD+betaD-2 models for comparison with data obtained from 60-Co gamma-rays and fission neutrons. With respect to 150 rads gamma-radiation, the r.b.e. values for the pion peak were 2-1 for dicentrics and 2-3 for acentrics. In the plateau the value for both aberrations was 1-5. At pion doses likely to be used as daily radiotherapy fractions, the aberrations predominantly resulted from single-track events, indicating little likelihood of interfraction recovery, particularly in the peak position. It is concluded that the biological damage characteristics of pion beams, as determined from a cytogenetic end-point, offer several features attractive to the radiotherapist.