The sensitivity of human tissues to changes in dose fractionation: Deductions from the RCR survey among UK radiotherapists

The dosage prescriptions reported in the Royal College of Radiologists' fractionation survey among radiotherapists have been further analysed using model equations in order to deduce estimates of fractionation-sensitivity parameters for each of the six cases under consideration. For example, using the linear-quadratic model, including a (significant) time factor, with radical treatments of a T2 breast carcinoma or a T1N0 squamous carcinoma of the vocal cord, the (α/β) ratios were 26±20 Gy and 37±46 Gy, respectively. The values of the time factor, expressed as the maximum extra dose required per day to counteract the decrease in effect with increasing overall time (γ/α), were 0.60 and 0.45 Gy/day respectively. Using the Ellis formula, which provided a significantly better fit to the dosage prescription (P=0.003), the exponents of N were calculated to be 0.24±0.05 and 0.27±0.07, respectively. The corresponding values of the T exponent were 0.16±0.06 and 0.014±0.075. About 20% of radiotherapists prescribed doses greater than ±10% from the mean fitted values for the breast treatment, and about 6% of them in the case of the vocal cord.