Dose-volume effects in rat thoracolumbar spinal cord: An evaluation of NTCP models

To evaluate models for normal-tissue–complication probability (NTCP) on describing the dose–volume effect in rat thoracolumbar spinal cord.Single-dose irradiation of four field lengths (4, 1.5, 1.0, and 0.5 cm) was evaluated by the endpoints paresis and white-matter necrosis. The resulting dose–response data were used to rank phenomenological and tissue architecture NTCP models.The 0.5-cm field length showed a steep increase in radiationtolerance. Statistical analysis of the model fits, which included evaluation of goodness of fit (GOF) and confidence intervals, resulted in the rejection of all the models considered. Excluding the smallest field length, the Schultheiss (D50 = 21.5 Gy, k = 26.5), the relative seriality (D50 = 21.4 Gy, s = 1.6, γ50 = 6.3), and the critical element (D50,FSU = 26.6 Gy, γ50,FSU = 2.3, n = 1.3) model gave the best fit.A thorough statistical analysis resulted in a serial or critical-element behavior for the field lengths of 1.0 cm and greater. Including the 0.5-cm field length, the radiation response markedly diverged from serial properties, but none of the models applied acceptably described this dose–response relationship. This study suggests that the commonly assumed serial behavior of the spinal cord might be valid for daily use in external- beam irradiation. [Copyright &y& Elsevier]