Cell Survival Description of the Cumulative Radiation Effect

The introduction of the NSD concept by ELLIS in 1967 enabled equation of different irradiation treatment regimes in terms of the clinically assessed damage at tolerance sustained by irradiated normal connective tissue.KIRK et coll.(1971) modified ELLIS' basic equation relating total treatment time, fractionation number and total dosage to include subtolerance damage, measured by their parameter, the Cumulative Radiation Effect (CRE).The NSD and CRE concepts are based ultimately on clinical observations of the reaction of skin to irradiation.Using various schedules of widely differing numbers of fractions per week, TURESSON & NOTTER (1976a), have recently confirmed that CRE is a very good predictor of acute skin erythema and pigmentation.As ELLIS points out (1968), the skin reaction is a guide to the normal connective tissue tolerance, since the healing of skin epithelium depends on the integrity of the underlying dermis or subcutaneous tissues, which are composed of connective tissue stroma.Therefore, if NSD and CRE are to have biologic foundation, the equations in these systems must be explained in terms of the irradiation of the basal cells of the epithelium, and not connective tissue cells, in the first instance.It is reasonable that a particular cell survival fraction should be associated with skin tolerance.WITHERS (1967), for example, points out that, for mouse skin, a surviving population of a mere 10 cells per cm2 are sufficient to prevent ulceration of an area irradiated with a dose of 17 Gy (1700 rad).Skin tolerance is the reference point for both the NSD and CRE systems.