Dose outside the treatment volume for irradiation with negative pions.

Irradiation of humans with negative pions requires a knowledge of the absorbed dose and radiation quality outside the primary pion beam. In conjunction with early clinical trials at LAMPF, experimental data have been obtained with microdosimetric techniques and multiwire proportional counters. Theoretical calculations have been made for the neutron contribution to the dose and are consistent with these data. Measurements were made with in 40 cm x 51 cm x 76 cm water phantom for a negative pion beam of initial momentum of 170 MeV/c, deltap = +/- 3MeV/c. The absorbed dose outside the treatment volume is the result of: (1) neutrons and photons from the pion interactions,(2) treatment room background and (3) peripheral muons, electrons and pions in the primary beam. The first two components are nearly isotropic and are congruent to 0.02% of the peak dose at a distance of 24 cm from the treatment volume; the third component is anisotropic and varies from 0.01 to 0.1% of the peak dose. Collimation of the bean increases the dose outside the treatment volume typically by 50%.

Microdosimetry of negative poins at LAMPF.

Lineal energy distributions and linear energy transfer (LET) spectra are being measured at the biomedical facility of the Los Alamos Scientific Laboratory for a negative pion beam having an initial momentum of 168 MeV/c. The results show that the fraction of the dose for lineal energies greater than 50 keV/mum is approximately 2% in the plateau and approximately 12% at the Bragg peak. Preliminary data indicate that the high lineal energy (and high LET) component increases significantly in the region slightly downstream from the peak.