Evaluation of radiosurgery techniques with cumulative dose volume histograms in linac-based stereotactic external beam irradiation.

Radiosurgery at UCSF is performed with a 6-MV linear accelerator with tertiary collimation for improved small field definition. The dose delivery to the target relative to normal tissue is influenced by the number of arcs, the arc geometry, field size, and beam energy. The impact of arc number, arc geometry, and field size on the dose distribution from 6-MV X rays in a 16 cm spherical phantom has been evaluated through the use of cumulative dose volume histograms. Dose volume histograms were calculated for a) 1-5 and 10 arcs, and b) collimator sizes of 1.25, 2.0, and 3.0 cm. Differences between techniques were found at the 5-10% level for field sizes from 1.25 to 2.0 cm. It was shown that the finite dimension of the sphere and, by extension, head diminishes the differences between techniques for the larger field sizes. The effect of treating with two isocenters is also analyzed and an approach for improving the dose distribution is presented.