Evaluation of Geometric and Dosimetric Inaccuracies of Stereotactic Irradiation in the Rat Brain

Abstract

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.

Using Monte-Carlo-Simulated Radiation Transport to Calculate Dose Distribution in Rats before Irradiation with Leksell Gamma Knife® 4C: Technical Note

“…Within the experimental uncertainties, the agreement was rather good. Novotný et al [11] provide additional arguments for the applicability of LGP with the aid of detailed empirical results from TLD, diode and polymer gel measurements.…”

“…Complex geometries and material compositions can only be handled accurately with more sophisticated dose calculation models such as the MCS. In their study, Novotný et al [11] also rightly pointed out the importance of an accurate modeling of the surface of the anatomy in order to obtain accurate predictions of the absolute dose. From the discussion above, it is clear that a lack of information about where the surface is located and about the shape of the surface may result in significant dose inaccuracies for superficial targets.…”

“…The ideal solution would be to physically measure the radiation dose directly, but for such small fields and inside a living animal, physical measurements pose very difficult problems. A formalized experimental study was performed by Novotný et al [11] , who evaluated the dose distribution predicted by LGP both in a rat cadaver and in a polymer gel phantom by using 3 types of dosimeters: a thermoluminescent dosimeter (TLD), a semiconductor detector and a polymer gel dosimeter. A fair agreement was found between the measurements and predictions, but since these measurements were not performed on living animals, and since the rat skull geometry used to calculate the irradiation time was only approximate due to limitations of LGP, it may be difficult to transfer their results to any given target in any given rat.…”