Evaluation of geometric and dosimetric inaccuracies of stereotactic irradiation in the rat brain

OBJECTIVE: To evaluate geometric and dosimetric inaccuracies in the irradiation of the rat brain with the Leksell Gamma Knife. MATERIALS AND METHODS: Altogether three types of dosimeters were employed for these measurements: (a) a thermoluminescent dosimeter, (b) a semiconductor detector and (c) a polymer gel dosimeter. The thermoluminescent dosimeter and the semiconductor detector were calibrated using an ion chamber and then implanted in the brain of a rat cadaver and used for absolute dose determination. A special glass phantom mimicking exactly the shape of the rat body filled with the polymer gel was used for measurements of the relative dose distribution and evaluation of geometric inaccuracies during the stereotactic irradiation in the rat brain. RESULTS: Both thermoluminescent and semiconductor detectors, due to their size, measured mean doses. The observed results demonstrated that the Leksell GammaPlan can be employed for the calculation of absorbed doses in irradiation of experimental animals. In our case, it was necessary to apply a correction factor of 1.078 for the absolute absorbed dose to obtain reliable results. A comparison of calculated dose profiles using the treatment planning system in all three axes with those measured by the polymer gel dosimeter demonstrated a very good geometric agreement with the mean deviation in profile position of 0.5 mm. CONCLUSION: The results indicate that this technique can effectively check the geometric and dosimetric accuracy of stereotactic irradiation in the rat brain. The Leksell GammaPlan can be employed for the calculation of absorbed doses, but the correction factor of 1.078 had to be applied for the absolute dose calculations in our irradiation geometry.