Application of the linear-quadratic-concept for prediction of late complications after combined irradiation of the uterine cervix.

Examining the applicability of the linear-quadratic (LQ) concept for prediction of late rectum and rectosigmoid complications due to combined irradiation of the uterine cervix, it appears that an agreement between clinical experiences and the LQ-based tolerance model can be achieved when relative high tolerance dose values for the separate irradiation techniques are assumed: 70 Gy in 28 fractions for the fractionated irradiation, 70 Gy in 6 days for the continuous irradiation. These tolerance values are derived by assuming a value of 2.5 for the biological parameter a1/a2 (or alpha/beta) as suggested by the LQ-model to be a good approximation for studies of late effects. This investigation shows how fractional tolerance values can be derived for the continuous and fractionated irradiation components. From a simple addition of these values, the total biological effectiveness of a combined irradiation treatment can be determined. This provides a model based on radiobiological parameters for comparison of different treatment schedules, and for dose-adjustments to be made in individual treatments. Especially in cases where the introduction of new afterloading techniques is accompanied by changes in dose rate of the intracavitary irradiation, the LQ-model offers a possibility to prevent mistakes in dosage of critical organs.