Verification of the dose to the isocentre in stereotactic plans

The aim of the study was: (a) to develop a simple, reproducible, technique to verify the dose to the isocentre, in a typical stereotactic treatment plan, for collimators from 12.5 to 40 mm in diameter; (b) to investigate a variety of detectors to compare different approaches; and (c) to introduce the technique into a quality assurance programme.

The symmetry, directional response and stability of calibration of a small 0.125 cm(3) ion chamber, a diamond and three types of diode (photon, electron and stereotactic) were tested. Correction factors were calculated to account for directional dependence, where appropriate and calibration factors were obtained to convert each reading to absorbed dose in water. Single arcs and typical four arc treatments were planned on XKnife and the dose to the isocentre verified in phantom with each usable detector.

The ion chamber showed no asymmetry, the stereotactic diodes exhibited 4% and the others 1-2%. Maximum directional dependence was 1% for the ion chamber and diamond and 7-20% for the diodes. Correction factors were calculated to account for this. Only the response of the diodes decreased with cumulative dose; the response of the other detectors remained constant. The ion chamber, electron diode and diamond measured the dose in single arcs to within 1.5% of calculation, in the 40 and 12.5 mm collimators. The photon diode was within 3.5 and 2.5% in the largest and smallest collimators, respectively.

A simple method of verification was developed. The ion chamber, the diamond and the electron diode were found to be the best detectors to verify the dose to the isocentre in a typical multiple arc treatment for collimators between 40 and 12.5 mm in diameter. The technique has been incorporated into a quality assurance programme, using the ion chamber and diamond, on a twice yearly basis.