New modalities in cancer treatment: heavy charged particles

Heavy charged particles represent the ultimate that the physicist can contribute to the development of radiation sources for therapy. Of the heavy charged particles, protons are the least expensive to accelerate and can be manipulated to give a sharply defined high-dose volume with a rapid fall-off of dose outside the target area. The biological properties of protons do not differ significantly from X or gamma rays. Negative pi mesons require an elaborate accelerator for their production. Pions offer the possibility of concentrating energy, some of it densely ionizing with a reduced oxygen enhancement ratio and elevated biological effectiveness within the designated tumour volume, while minimizing the dose of sparsely ionizing radiation to the normal tissues traversed. High-energy heavy ions offer the greatest flexibility and allow localized dose distributions and also, with the higher Z particles, a substantial reduction of the oxygen enhancement ratio can be achieved. The ultimate choice of particle depends upon what turns out to be the most important factor in radiotherapy--an improved localization of dose or a reduction in the dependence of cell killing on the presence of molecular oxygen.