NEGATIVE PIONS: THEIR POSSIBLE USE IN RADIOTHERAPY

The negative meson beam has a number of favorable characteristics which, taken collectively, make a high-output beam attractive for radiotherapy. These include: (1) a depth-dose and dose equivalent (dose x RBE) pattern superior to those now available; (2) a relatively low OER in the peak portion of the depth-dose curve; and (3) less repair of sublethal cell damage in the peak portion.

The unique, and readily-tailored superior depth-dose patterns obtainable with negative mesons would provide a distinct advantage in radiotherapy. The clinical gain to be expected from such sharply localized radiation is difficult to assess quantitatively from presently available data, and it is not possible to predict whether the over-all improvement in morbidity and mortality is likely to be 5 per cent, 10 per cent, or more.

The favorable OER is advantageous. This does not in itself constitute a compelling reason for making available a high intensity source of negative mesons, since other currently available sources of high LET radiation, namely fast neutrons, are likely to be suitable for the evaluation of the importance of hypoxic tumor cells in radiotherapy and the degree to which high LET radiations may help to overcome this problem. The densely ionizing peak portion of the ^- meson depth-dose curve can be confined nearly selectively to the tumor volume. The resultant lower fraction of repairable damage in tumor versus most irradiated normal cells will result in favorable therapeutic ratios which become more favorable with lowered dose rate of fractionation.

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