The steepness of the dose response curve both for tumor cure and normal tissue injury.

At the Christie Hospital in Manchester, 4 MeV radiation was introduced in 1955. In consequence of a lack of knowledge of absolute dose definition and R.B.E. factor, patients were treated over a range of dosage. Three hundred fourteen patients with laryngeal cancer, treated over a time interval of three weeks, received doses over a range of 5,000 rad to 5,800 rads. Recurrence, morbidity and necrosis rates are plotted against dose. In T1 tumors there is very little variation in recurrence rate, and such tumors may be safely treated at the lower doses thus avoiding morbidity and necrosis. T3 tumors, however, show a considerable variation in recurrence rate being 68 percent at the lower end of the range, dropping to 30 percent at the high end. Necrosis nil at the lower end rises to some 8 percent at the high end. The balance of advantage of tumor sterilization versus the disadvantage of normal tissue damage is discussed.