Radiation related prognostic factors in radiation oncology

1. The outcome of a course of radiotherapy is very dependent on the dose per fraction. The smaller the dose per fraction, as a general rule, the better the sparing of the late reacting normal tissues. 2. Overall treatment time is important, especially for tumours with a rapid doubling time. In such a case, the ideal of small doses per fraction (to save late reacting tissues) as well as a short overall treatment time (to offset the effect of repopulating) can be achieved by small doses per fraction applied two to three times per day, including Saturdays or weekends. 3. The BED (biologically effective dose) is a simple to use formula indicating the effects of fractionation. The most important term in the formula is the alpha/beta ratio which is available from experimental work for many tumours and tissues and can be looked up. As a guide, an alpha/beta ratio of 10 for early (acute) reaction and for tumour effects, and an alpha/beta ratio of 2 for late effects plus normal tissue complications can be used. 4. The application of the BED demonstrates that for HDR intracavitary therapy for cervical carcinoma, the biologically relevant dose lateral to point M(A) falls very much more rapidly than the nominal dose. Line sources are shown by comparison with other published reports, not to be intrinsically inferior to tandem ring/tandem ovoid systems and may have advantages the more cumbersome systems do not have, and may have the large advantage of allowing multiple small fractions without anaesthesia. For the particular line source system under discussion, water in a 40 cm3 Foleys bulb is used as the protecting medium for the posterior bladder wall and the anterior rectum. This particular system allows fraction sizes far smaller than 9.1 Gy at point (M)A, e.g. 3 Gy, which bestows an even greater benefit in terms of the therapeutic ratio according to BED10 and BED2 calculations.

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