DYNAMIC FIELD SHAPING FOR STEREOTAXIC RADIOSURGERY - A MODELING STUDY

Purpose: This work assesses the relative field shaping advantages of d ynamic field shaping devices for stereotactic radiosurgery using a lin ear accelerator.Methods and Materials: We selected 43 intracranial tum ors (2.0-4.2 cm maximum dimension, 1.5-25.5 cc tumor volume) out of th e first 64 intracranial tumors treated with radiosurgery at the Joint Center for Radiation Therapy. We modeled five field shaping devices, e ach including a fixed auxilliary circular collimator: (a) fixed circul ar collimator alone; (b) two independent parallel jaws; (c) four indep endent rectangular jaws; (d) four independent rotatable jaws; and (e) ''ideal'' multileaf collimator. We adjusted the model parameters until the minimum target isodose was 80% of the dose delivered to isocenter . We defined the treatment volume ratio as the target volume divided b y the treatment volume (volume receiving at least the minimum target d ose). We used the treatment volume ratio to compare the five models an d the actual patient treatments. Results: For 34 tumors originally tre ated with one isocenter, the median Treatment Volume Ratio was higher for all of the device models except the fixed circular collimator comp ared to the actual patient treatments. For the nine tumors originally treated with multiple isocenters, the median Treatment Volume Ratio fo r the actual multiple isocenter treatments was similar to that for two parallel jaws, four rectangular jaws and four rotatable jaws. Only th e median ''ideal'' collimator treatment volume ratio was higher for th ese nine tumors. Conclusion: Simple field shaping devices have approxi mately 50% of the conformal advantage of an ''ideal'' multileaf collim ator. Approximately 50% of typical radiosurgical tumors between 2 and 4 cm have field shaping advantages which exceed the geometrical uncert ainties inherent in linear accelerator radiosurgery treatments. The th ree models, two parallel, four rectangular, or four rotatable independ ent jaws would improve current linear accelerator technology by provid ing homogeneous doses with equivalent field shaping for most tumors or iginally treated with inhomogeneous multiple isocenter plans (6/9 tumo rs in the current series).