Tissue maximum ratios (and other parameters) of small circular 4, 6, 10, 15 and 24 MV x-ray beams for radiosurgery

Small, circular, x-ray beams are commonly used for radiosurgery applications. Dosimetric characteristics of 4, 6, 10, 15 and 24 MV circular x-ray beams ranging in size from 10 to 40 mm are reported. These characteristics include the measurement of TMR, beam profiles and relative output factors. Measurements of these parameters were performed in a solid water phantom using film, a small diode, small parallel-plate and cylindrical ionization chambers and TLD. Comparison of relative dose measurements of small, circular beams performed using these detectors showed that the small diode, film and TLD results consistently agreed for circular beams as small as 10 mm diameter. Beam profiles were measured using film dosimetry. Comparison of TMR values of a 10 mm diameter beam measured using film and a small parallel-plate ionization chamber showed no significant differences. Tertiary collimators designed with tapered, divergence-matching holes, and straight-drilled holes have been used for radiosurgery applications. Measurement of beam penumbra produced with either of these types of tertiary collimators showed minimal differences between them.