Radiological physics of pions.

Pions, and in particular negative pions, are of interest to radiology for at least three outstanding reasons: First, a beam of negative pions can produce in tissue depth and isodose distributions which, for treatment of deep-seated tumors, give a considerably better tumor-to-total-dose ratio than any other commonly used therapeutic radiation source (1). This improved ratio with pion irradiation is largely due to the short-ranged, heavily ionizing products resulting from nuclear pion interactions at the end of the pion track. Thus, second, an additional effectiveness of the dose delivered to the tumor relative to that delivered to healthy tissue is likely because of the oxygen effect observed with heavily ionizing radiation (2). Third, a pion beam seems at present to be the best choice in order to create a well-defined region in which nuclear reactions or nuclear stars give a substantial contribution to the radiation dose. Such conditions are essential for dosimetry and radio-biological research of strong nuclear interactions and thus for approaching a deeper understanding of the particular problems of high-energy dosimetry and radiobiology (3). In the following discussion, some experimental studies of the pion beam from the CERN 600-MeV Synchro-Cyclotron are reported. The results are limited to dosimetry and radiation quality measurements in a water phantom exposed to a 70-MeV pion beam; an attempt was also made to evaluate the average local energy deposition per stopped pion. The results are of a preliminary nature, and more extensive experimental investigations along the same lines are continuing.