The use of the cumulative radiation effect (CRE) concept in clinical radiotherapy

Abstract

The aim of radiation treatment is to deliver to the tumour volume dose that would kill the cancer cells. The result is affected by several factors, among which are the total dose, the number of fractions in which the dose is delivered and the overall treatment period. 
Clinical experience to successful radiation treatment is mostly based on a regime which consists of delivering 200 rads/fraction for 5 fractions/week to the tumour volume.
However, for various reasons during a treatment series, the regime may change either by altering the daily tumour dose or by the fact that the patient has to abstain from treatment for a period of time.
It has been a problem to try to quantify in one way or the other what changes in the treatment regime mean and how to complete the treatment series after a change in regime in order to obtain the same successful result.
Since as has been said, the success of a treatment is affected by the total dose, the number of fractions in which the dose is delivered and the overall treatment period, various theories had been developed that incorporate these factors for the purpose of assessing the biological effectiveness of a given treatment regime.
These models serve to quantify a given course of radiation treatment by assigning a single value which represents a certain level of biological effect.
Among the various theories that have so far been developed, however, the concept of the Cumulative Radiation Effect (C.R.E.) by Kirk has become widely accepted.
This study aims to demonstrate how the CRE concept could be used in evaluating the effects of changes in the treatment regime (e.g. alteration in the daily tumour dose, occurrence of gaps, etc.).
This evaluation will then serve as guidance for the radiotherapist to decide if an additional radiation dose should be given and how this dose should be distributed in time.
To facilitate the discussion, the theoretical aspects of the CRE system is discussed on the first part of this paper. Several sample cases are then cited, however the step by step calculations are placed in the appendix.