Quality control of the stereotactic radiosurgery procedure with the polymer-gel dosimetry

To assess the entire geometric and dosimetric (relative) uncertainties of the radiosurgery procedure with the Leksell gamma knife.

The entire Leksell gamma knife stereotactic radiosurgery treatment procedure was simulated with the use of a special water filled head phantom and polymer-gel dosimeter evaluated by nuclear magnetic resonance (NMR). A test vessel filled with the polymer-gel dosimeter was fixed in the head phantom. The phantom underwent stereotactic NMR imaging, treatment planning and then irradiation according to the treatment plan prepared exactly the same way as in the ordinary treatment procedure for a patient. The treatment plan was represented by one isocenter positioned approximately centrally in the head phantom. This procedure was subsequently repeated for all four collimators (4, 8, 14, 18mm) used on the Leksell gamma knife. Evaluation of dosimeters was performed on a Siemens EXPERT 1T NMR scanner. Dose profiles in X, Y and Z axes through the ellipsoidal shaped dose distribution were obtained to compare experimental results from the irradiated phantom with the treatment planning system calculations.

Reasonable agreement was observed between the treatment planning system calculations of relative dose distribution and the measured data. The maximum observed deviation in the spatial position between the center of the measured and calculated dose profiles was 0.6mm. The maximum observed difference in full width of half maximum between calculated and measured profiles was 1.2mm.

The use of polymer-gel dosimetry for a verification of stereotactic procedures has some unique advantages that can be summarized as follows: the dosimeter itself is tissue equivalent, three-dimensional dose distributions can be measured and the dosimeter allows simulation of the patient's procedures without any limitations.