THE CRITICAL VOLUME TOLERANCE METHOD FOR ESTIMATING THE LIMITS OF DOSE-ESCALATION DURING 3-DIMENSIONAL CONFORMAL RADIOTHERAPY FOR PROSTATE-CANCER

Purpose: To describe the ''Critical Volume Tolerance'' (CVT) method fo r defining normal tissue tolerance during 3D-based dose escalation stu dies for prostate cancer. Methods and Materials: The CVT method predic ts the tolerance to radiation for ''in series''-type functional units based on the assumption that tolerance depends on a critical threshold ''low-volume high-dose region.'' The data used for describing this mo del were generated from 3D analysis of randomly selected patients with prostate cancer. Commonly used coplanar four- and six-field conformal (SFC) techniques were chosen as the comparison techniques. For purpos es of comparison, rectal tolerance was assumed to be reached following whole pelvic irradiation using a four-field box technique to 50 Gy, f ollowed by a conedown boost to 70 Gy using bilateral 9 x 9 cm 120 degr ee arcs as popularized by investigators from Stanford University (SUH) . Results: Based on the average dose volume histograms for the patient s studied, the maximum safe increase in dose for the SFC technique com pared to the SUH technique, would be 10% if 30% of the rectal volume w as the critical dose limiting volume (CVT = 30%), 5% if the CVT = 10%, or greater than 20% if the CVT = 40%. Commonly used four-field confor mal techniques would not be expected to allow significant escalation o f the dose without increasing the risk of complications. Conclusions: The CVT method is relatively simple, and data generated based on it ca n be used to support normal tissue complication probability equations. The CVT method can be verified or modified as partial tolerance data become available. Based on the CVT model, sophisticated treatment tech niques should allow a modest increase in the total dose of radiation d elivered to the prostate without an increase in late complications.