THE INTEGRAL BIOLOGICALLY EFFECTIVE DOSE TO PREDICT BRAIN-STEM TOXICITY OF HYPOFRACTIONATED STEREOTAXIC RADIOTHERAPY

Objective: The aim of this work was to develop a parameter for use dur ing fractionated stereotactic radiotherapy treatment planning to aid i n the determination of the appropriate treatment volume and fractionat ion regimen that will minimize risk of late damage to normal tissue. M aterials & Methods: We have used the linear quadratic model to assess the biologically effective dose at the periphery of stereotactic radio therapy treatment volumes that impinge on the brain stem. This paper r eports a retrospective study of 77 patients with malignant and benign intracranial lesions, treated between 1987 and 1995, with the dynamic rotation technique in 6 fractions over a period of 2 weeks, to a total dose of 42 Gy prescribed at the 90 % isodose surface. From differenti al dose-volume histograms, we evaluated biologically effective dose-vo lume histograms and obtained an integral biologically-effective dose ( IBED) in each case. Results: Of the 77 patients in the study, 36 had t arget volumes positioned so that the brain stem received more than 1 % of the prescribed dose, and 4 of these, all treated for meningioma, d eveloped serious late damage involving the brain stem. Other than type of lesion, the only significant variable was the volume of brain stem exposed. An analysis of the IBEDs received by these 36 patients shows evidence of a threshold value for late damage to the brain stem consi stent with similar thresholds that have been determined for external b eam radiotherapy. Conclusion: We have introduced a new parameter, the IBED, that may be used to represent the fractional effective dose to s tructures such as the brain stem that are partially irradiated with st ereotactic dose distributions. The IBED is easily calculated prior to treatment and may be used to determine appropriate treatment volumes a nd fractionation regimens minimizing possible toxicity to normal tissu e. (C) 1998 Elsevier Science Inc.