Effects of variation of overall treatment time on the radiation response of normal human skin

The validity of the treatment-time exponent of the Ellis Nominal Standard Dose (NSD) equation has been evaluated by assessment of the early radiation responses of erythema, pigmentation and desquamation of small areas of normal human skin included in the treatment of basal cell carcinomata. Treatments were given as five fractions over 4, 10 and 28 days. Total doses were calculated using an NSD of 1930 rads equivalent therapy with a time component of 0.11. The number of treatment fractions, and thus the fractions exponent, was constant. The temporal patterns of increasing and declining early skin reactions were similar in the three treatment groups but, using an arbitrary scoring system, there was a statistically significant difference in the desquamation reaction. These early results are not consistent with the view that a single time exponent can be used to predict equivalent biological effects over the period 4 to 28 days. (author)