Treatment planning for pion radiation therapy must take into account changes in radiation quality within the patient. At the biomedical channelπE3 of SIN (Swiss Institute for Nuclear Research) microdosimetric measurements have been performed to investigate radiation quality within pion irradiated phantoms. Results are presented in terms of microdosimetric spectra and derived quantities. As expected marked differences are observed between dose peak and plateau for “narrow” pion beams. The influence of simulated site diameter on measured spectra has been found to be more pronounced in the plateau region than in the peak. Investigation of the influence of peak width on radiation quality revealed a dilution of the high-LET dose fraction for broader peaks.

Energy deposition spectra of negative pions and their application to radiation therapy.

The biological effectiveness of negative pions from the 590 MeV proton accelerator of the Swiss Institute for Nuclear Research (SIN) has been studied and the results of experiments with different end points obtained since the starting of operation of the biomedicalπ−-beam are presented. The dose rate of theπ−-beam (180 MeV/c) in the Bragg peak was 3–5 rad/min, contamination with electrons and myons 13.5%. 140 kV X-rays with the same dose rate have been used for comparison. To avoid complications introduced by protracted irradiation, only very sensitive systems have been chosen. The normal reaction systems included embryonic damages in mice and inhibition of mitotic activity in mouse jejunum. As a sensitive tumor system Ehrlich carcinoma cells were irradiated in vitro and reinoculated into mice. The RBE-values depending on dose were between 1.3–1.7. The ratios of the effects of same doses in peak and plateau showed clinically desired values of 1.4–1.5. Genetic damages (as possible events leading to cell death) were extensively studied at different stages of development of Drosophila germ cells. Nine different types of mutations such as chromosome loss, loss of definite chromosome regions, lethals, translocations etc. were determined. The RBE values varied between 0.4 and 3.3 depending on mutation type and cell stage. The existence of some RBE values for peak pions under 1 have suggested a new concept of high LET action namely the two system theory which gives more importance to the intrinsic properties of the irradiated biological systems. They can be divided in two groups corresponding to their reaction to high LET radiation: 1. The euoxic and modifiable system which is characterized by intrinsic radiosensitivity, high oxygen tension (euoxic cells), high OER values and low RBE values even under 1; and 2. The anoxic and rigid system with high RBE values for high LET (≫ 1), which is characterized by natural radioresistance and (or) hypoxia.

Different genetic and cellular responses to negative pimesons (peak and plateau) in dependence on intrinsic properties of reaction systems: a new concept of RBE.

Neonatal rats and their radiation-induced cerebral petechial hemorrhages were used as an experimental system for evaluating the effects of negative pions on healthy tissue and especially on the microvasculature. Dose response curves for peak and plateau pions (dose range 150--250 rad and 100--400 rad, respectively) were obtained and compared with those of 200 kV X-rays of corresponding dose rates. The RBE of the peak pions was 1.1, that of the plateau pions 0.6 resulting in a peak/plateau ratio of 1.8. Implications were made as to the importance of this favorable peak/plateau relationship since the response of the capillary endothelium to pion-irradiation might be one of the limiting factors in radiotherapy.

Effect of negative pi mesons on vascular permeability of brain in neonatal rats.

Theπ− flux from the biomedical channel at TRIUMF increases with increasing channel momentum, while the contaminating electron flux decreases. Since the electrons appear to result from conversion of the high energyγ-rays produced byπ0 decay in the production target, the electron contamination can be reduced further by target configurations which minimize gamma conversion.

Pre-clinical studies of the negative pi-meson beam at TRIUMF.

Two pion beams of different momentum width have been used to expose meristems of Vicia Faba roots under aerobic and hypoxic conditions. The measurements of the resulting 10 days growth inhibition after exposures at various locations on the pion beam axes have been made and RBE and OER values evaluated for 50% effects compared to60Co tiγ-rays. The results have been related to the fractional doses from star products defined by telescope measure ments of stopped pions along the same beams. It has been found that the RBE value increases with the fractional “star dose” up to a maximum after which the RBE decreases. The OER values, however, were found to decrease with increas ing “star dose” fraction rather rapidly after which it was found to be independent of the “star dose” contribution.

RBE and OER values of negative pion beams from growth inhibition of Vicia Faba roots

Physical properties of the SIN pion beam πE3 have been measured using time-of-flight techniques. In particular the momentum and the momentum band width were determined with an accuracy of ± 0.5 MeV/c and ± 0.1 MeV/c, respectively. The beam contamination with e− and μ− as well as their local distributions have been measured under various conditions.

V. Böhmer

Papers

π−-Beam Studies Using Time-of-Flight Methods