Superfractionation: its rationale and anticipated benefits.

This manuscript discusses the biological bases of radiation therapy dose fractionation with emphasis on the theoretical implications of the quadratic cell survival equation (S = e-(alpha D + beta D2) ). In the development of the analysis, both the therapeutic ratio, defined as the log tumor cell survival divided by the log normal cell survival, and the therapeutic gain, the ratio of the therapeutic ratios for two different doses per treatment, are shown to be useful. From the analysis it follows that the use of small doses per treatment delivered at short intervals (superfractionation), will generally lead to improved local tumor control rates provided the total dose is increased sufficiently to maintain a constant level of normal tissue damage. Improvement may not be seen if the tumor response is dominated by oxygenated cells with a higher capacity to repair sublethal radiation injury than the host normal tissues.