RELATION OF DOSE-FRACTIONATION OF X-RAYS AND ITS SPACING IN TIME TO SKIN DAMAGE

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Abstract
Prolongation of fractionation times greatly modifies the intensity of the reations, irrespective of the number and size of fractions delivered. This is due to a perturbation of cell population dynamics and the time factors involved in the onset of reactive hyperplasia. The results indicate that iso-effect dose formulae for fractionated treatments in radiotherapy, based on experimental animal and cell survival data, are of doubtful validity if dose-time factors which modify repopulation dynamics are ignored. This particularly applies to treatments in which a limited number of large fractions are used. For large dose increments in the range 1,200-1,800 rads, spacings in excess of 14 days between successive fractions do not improve tissue tolerance but tissue tolerance decreases progressively as the interval is reduced to approximately 6 days, below which a further decrease has little effect in modifying damage. Doses of 600 rads produced similar effects but prolongation of such treatments with spacings between fractions in excess of 3 days did not improve tissue tolerance further. In the various tissues of different species, the results of fractionation in respect to time will differ according to cell turnover rates, the capacity of tissues to regenerate and undergo hyperplasia in reaction to damage. These factors invalidate an over-simplification of tissue tolerance and effects in terms of any single "iso-effect- dose" formula designed to provide the radiotherapist with a " ready-reckoner" for calculation of doses.

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