Effect of incomplete repair on normal tissue complication probability in the spinal cord.

ABSTRACT

PURPOSE: To incorporate the effects of repair into a model for normal tissue complication probability (NTCP) in the spinal cord. METHODS AND MATERIALS We used an existing model of NTCP for the spinal cord, based on a critical volume concept, into which we incorporated an incomplete repair (IR) scheme. Values for the repair half time were taken from existing experimental data. Repair corrections were expanded to account for the possibility of biphasic repair, namely the existence of long and short components of repair. RESULTS: We found that the model predicts complete repair to occur at approximately 15 hours, consistent with experimental data. The dependence of the model on the value of the dose per fraction was also studied. It was found that there is a sparing effect as the dose per fraction is decreased below 2 Gy. Surface plots of the NTCP as a function of both the interfraction interval (IFI) and the dose per fraction were generated. We investigated "iso-NTCP" curves, which may allow freedom in choice of treatment plans in terms of the optimal IFI and dose per fraction. As for biphasic repair, as the relative weights of the long and short components of repair were varied, the NTCP changed as well. The model showed little difference between mono- and bi-exponential repair in the time to complete repair, due to a dominance of the long component at long IFIs. CONCLUSIONS: Incorporating IR into NTCP modeling of the spinal cord is consistent with current experimental data. The concept of iso-NTCP curves is an approach which may be clinically useful.