A COMPARISON OF 3 STEREOTACTIC RADIOTHERAPY TECHNIQUES - ARCS VS. NONCOPLANAR FIXED FIELDS VS. INTENSITY MODULATION

Purpose: Linac are based stereotactic radiotherapy is being used with increasing frequency to treat brain tumors. This approach can be used for single or fractionated treatments, and is typically carried out wi th circular collimators which are optimal for small, spherical targets . Treatment planning using fixed noncoplanar beams or intensity-modula ted beams may enhance the ability to conform to irregularly shaped and /or large tumors, especially when combined with stereotactic localizat ion. We compare the dose conformity and normal brain dose characterist ics of three stereotactic techniques for various nonspherical target s hapes. Methods and Materials: Three intracranial test targets were con structed using a 3D treatment planning system after a patient underwen t CT simulation. The targets included an ellipsoid with major axis dim ensions of 4.0, 2.0, and 2.0 cm, a hemisphere with a diameter of 4.0 c m, and an irregularly shaped patient tumor with a maximum dimension of 5.3 cm. The following stereotactic techniques mere compared for each target: a) 5 arcs as used in traditional linac radiosurgery/radiothera py (noncoplanar arcs [ARCS]), b) 6 fixed noncoplanar custom blocked fi elds (3D), c) intensity modulation using 6 noncoplanar beams and a min i-multileaf collimator (intensity-modulated radiation therapy [IMRT]). Dose volume histograms were performed for each target/technique combi nation. Results: For the ellipsoid, dose conformity is similar for all three techniques and normal brain isodose distributions are more favo rable with the ARCS plan. For the hemisphere and irregular tumor targe ts, dose conformity and high/low isodose normal brain volumes are more favorable with the IMRT technique. Conclusions: For the targets descr ibed above, the intensity-modulated technique results in improved dose conformity and decreased dose to nontarget brain in high and low isod ose regions as compared to the standard noncoplanar are technique or n oncoplanar fixed fields for the hemisphere and tumor targets. Intensit y-modulated treatment delivery may allow for an increase in the therap eutic ratio for treating stereotactically defined large and/or irregul arly shaped intracranial targets. (C) 1998 Elsevier Science Inc.