Spinal cord re-treatments using photon and proton based radiotherapy: LQ-derived tolerance doses

Re-treatment, using megavoltage photon radiotherapy, can benefit carefully selected patients with new or recurrent tumours. Such re-treatments may involve the further exposure of tissues such as the brain or spinal cord. A time-dependent model has been developed, which incorporates data from all published radiobiological experiments concerned with the in vivo re-irradiation of the spinal cord using photons. It allows an estimation of the increasing recovery in tissue tolerance with elapsed time after the initial treatment course. In accordance with the experimental evidence, the recovery rate depends on the biological effective dose (BED) of the initial treatment. Various degrees of conservatism have been introduced in the model to allow for potential changes in CNS tissue tolerance due to patient age, chemotherapy, surgery etc. An estimation of the re-treatment dose-fractionation schedule is made easier by the use of a downloadable Graphical User Interface (GUI). Worked examples of its use are given for conventional photon (X-ray) based treatments, and also for protons, where relative biological effectiveness (RBE) considerations must be respected within the BED estimates. The model provides boundary conditions for clinical practice. The responsible clinician can choose to use more 'forgiving' BED values and from this to calculate the re-irradiation dose-fractionation schedule. For protons, greater care is required since the inter-relationship between linear energy transfer (LET) and RBE can lead to significant over-dosage relative to accepted CNS tolerance doses, especially with the use of scanned proton beams. LET and RBE factors are important in order to deliver safe and effective re-treatment doses.