Dosimetric comparison of three photon radiosurgery techniques for an elongated ellipsoid target

Purpose: To examine the dosimetric differences among three radiosurgery tec hniques: gamma knife, linac multiple arcs, and conformally-shaped static fi elds. Methods and Materials: A simulated target was taken to be a prolate ellipso id, 25 mm in diameter, 35 mm in length, centrally located in a three-dimens ional (3D) model of a patient head taken from MR images. Single isocenter l inac treatment plans were developed, 9 portals for the static shaped field technique, and a 7-arc plan for the multiple are method. A total of 13 isoc enters with 3 different collimators were used in the gamma knife plan. Results: At dose levels from 25% to 50% of the reference dose, multiple are and shaped-field plans treated a greater volume than the gamma knife plan. The linac plans, however, delivered the dose more homogeneously across the target volume as compared to the gamma knife plan. For the dose levels bet ween 50-100%, the shaped fields and gamma knife plan have a similar dose di stribution, and treated slightly less volume than the multiple are plan. Conclusion: For a target of limited volume and essentially any shape, one c an obtain closely conformal dosimetry with the gamma knife. For a regular-s haped target, the single isocenter multiple are technique gives a more homo genous dose distribution within the target. Static shaped fields offer an a lternative radiosurgery technique, with dosimetry similar to the multiple a re method, applicable to targets of any shape. (C) 1999 Elsevier Science In c.