Risk assessment for proton therapy in the central nervous system by assuming small increments in RBE

The fixed relative biological effectiveness (RBE) of 1.1 remains a controversial topic in proton therapy. Clinicians have been reluctant to override this value. A method is proposed for the assessment of proton therapy dose prescriptions in clinical situations where there is little or no critical normal tissue dose reduction. The routinely-used RBE of 1.1 may not then adequately respect critical normal tissue tolerance. The method: A clinical example of a low grade brainstem glioma is followed where there is no effective normal tissues dose sparing. Dose reductions must then be considered if accepted tissue tolerance levels are exceeded to an extent that may produce severe late effects. This can be achieved by lowering the number of fractions and/or reducing the total dose, but maintaining the same dose per fraction, since RBE is generally inversely related to dose per fraction. A reduction of dose is not necessarily deleterious in this specific situation, where cure cannot be achieved, but it is more likely to result in a better quality of life, although with a slightly shorter remission than would otherwise have occurred. Clinicians should find this type of ‘hands on’ radiobiological information useful in decision making, leading to improved complication-free outcomes in proton therapy. Similar considerations apply to ion beam treatments.