EVALUATION OF LINEAR-ACCELERATOR RADIOSURGICAL TECHNIQUES USING BIOPHYSICAL PARAMETERS (NTCP AND TCP)

Purpose: Several irradiation techniques are compared with regard to normal tissue complication probability and tumor control probability. Methods and Materials: Normal tissue complication probability is calculated using a model based on the ''critical element architecture.'' The probability of controlling an inhomogeneously irradiated tumor is calculated using a model that takes into account the heterogeneity of tumors (different radiosensitivity of clonogens within the tumor and the varying number of clonogens among patients with the same kind of tumor). The ratio of tumor control probability to normal tissue complication probability (therapeutic gain factor) at different levels of dose has been used as a score for the analysis of various irradiation techniques. Results: The best irradiation technique depends on many factors: irradiation geometry, irradiation field size, choice of the reference isodose, and it is dictated by the pathology of the lesions (noninfiltrating radioresistant tumors, infiltrating radioresistant tumors, noninfiltrating radiosensitive tumors, infiltrating radiosensitive tumors, artero-venous malformations). For the irradiation of the artero-venous malformations it is proposed to insert on the supplemental collimator a flattening filter to reduce the probability of inducing a poorly syncronized obliterative effect. Conclusion: We propose that for each kind of pathology to be treated radiosurgically, a proper irradiation strategy should be used.