We consider the potential gains to be expected from improved dose distributions which can, for example, be obtained from lar ge field, low LET, energy modulated proton beams. Increases in tumour control probability which have resulted from improvements in dose distribution in the past are discussed. Limitations on the maximum dose which can be delivered and on possible ways in which normal tissue tolerance can be extended are considered. The impact of improved dose distributions on management of a number of diseases, particularly cancer of the urinary bladder, is discussed. It is concluded that, for sites in which conventional radiotherapy can deliver doses of the order of 7000 rad in 7 weeks, improvements in local control are unlikely and any improvements in results may be in the form of a reduction of morbidity. However, significantly higher frequency of local control may be achieved in those normal tissue-tumour situations in which doses are currently limited to about 5000 rad in 5 weeks but could be increased to levels >6000 rad in 6 weeks by use of improved dose distribution patterns obtainable with proton beams. © 1974.
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