[en] The physical characteristics of beams of negative pions relevant to radiotherapy cannot be characterized by a single parameter such as the absorbed dose. In a therapeutic situation, such beams contain many types of particles (contaminants and secondaries) distributed over a wide range in energy. One of the problems confronting radiation physicists is to establish that set of physical parameters which is sufficient to uniquely define biological response. An overall program addressing this problem is presently being pursued at Los Alamos