[Dose-volume histogram comparison among techniques of linac stereotactic radiosurgery]

The aim of this study was to analyze the three-dimensional dose distributions produced by various techniques used in current radiosurgery treatments by means of dose-volume histograms (DVH). Off-center ratio and tissue-peak ratio from 6-MV X-rays through additional cylindrical collimators were measured in advance for dose calculation. The use of the diamond detector and beam profile film dosimetry in small fields were certified. The impact of arc number, arc geometry, and field size on the dose distribution in a spherical phantom was evaluated through the use of DVH. These were calculated for a) four arc irradiations with collimator sizes of 5, 10, 20 and 30 mm in diameter, and b) for single-plane rotation, four or eleven multiple non-coplanar convergent arcs, dynamic rotation and precessional converging radio-therapy (PCR) with a field size of 20 mm. PCR is performed in the sitting position with the patient seated on a special treatment chair, which is rotating continuously while the gantry moves from the top to the center of the chair rotation. Most of the differences between techniques were found in a range of less than 40% of the maximum dose. Multiple non-coplanar convergent arcs with four or eleven arcs and PCR performed similar result in DVH, while single-plane rotation revealed almost unacceptably shallow dose falloffs. The DVH of dynamic rotation was between that of single-plane rotation and the other three methods.