Although linac radiosurgery has some advantages over gamma knife, required accuracy for this purpose is very strict and extended quality assurance programs are needed. We performed phantom measurements for the accuracy of our positioning method. Tungsten balls of 4 mm diameter were assumed to be intended targets for beam concentration. Four tungsten balls were placed randomly in a head phantom made of styrene foam. The phantom was held in a base frame and a stereotactic localization box for

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... ogram was attached. Two orthogonal x-ray films were exposed for antero-posterior and left-right view of the phantom. The coordinates of the tungsten balls were calculated referring fiducial points on the wall of the box. Displacements of the beam center from the target were measured from the linacgraphy taken at the top and side positions of the gantry head. The targets coincide with the isocenter of the linac within 0.25 mm. However, at the top position of the gantry, relatively large displacements of the beam center from the isocenter in z direction (patient cranio-caudal direction) were observed.