Dosimetric characterization of radiosurgical beams with a diamond detector.

Dosimetric characteristics of small diameter 6 MV photon beams for a commercial radiosurgery system have been measured in a solid water phantom with a new diamond detector and compared with measurements using a p-type Si photon diode, small volume cylindrical and parallel plate ionization chambers, and radiographic films. Tissue maximum ratios (TMR) and total scatter factors (S c.p) were measured with the three ionometric (diamond, diode, ion chamber) detectors for 12 circular beams ranging in diameter from 12.5 to 40 mm. The TMR values obtained with the three ionometric detectors agreed well (+/- 1%) for all cone sizes and depths investigated when the displacement of the sensitive volume of the detector from its front surface is taken into account. The S c.p factors obtained with the ionometric detectors also agreed well (+/- 1.2%) for field sizes greater than 20 mm in diameter. For smaller field sizes, the cylindrical and parallel plate chambers measure a smaller S c.p factor, as a result of the steep dose gradients across their sensitive volumes. Cross-beam profiles acquired with the diamond detector agree well with the measurements with the diode detector and radiographic film. A distortion in the measured profiles in terms of broadened penumbra is observed with a small volume cylindrical ionization chamber.