Leksell gamma knife and quality assurance in stereotactic radiotherapy

The aim of the Leksell Gamma Knife stereotactic radiotherapy (LGK SRT) is to deliver a high single dose of radiation to a small complex-shaped intracranial lesion, often located close to critical structures. The main requirements on LGK SRT are to define the geometrical position of the lesion with high accuracy, to verify dose plans in order to reflect real physical dose distributions within the patient's head and to perform with an extraordinary reliability because errors cannot be corrected (single fraction therapy) and interrupted treatments cannot be completed later without negative effects on the distribution of the dose applied to the patient. For minimal risk, it is necessary that geometrical positions are set precisely with an overall deviation of less than 1 mm. This paper presents a test for verifying the accuracy of LGK SRT. The chain of items in terms of complete patient simulation was followed and stereotactic MRI and CT data were verified against a reference, which were stereotactically defined radiochromic films. A marked radiochromic film, situated between inserts of the spherical fantom was fixed parallel to either the xy, yz or zx plane of the Leksell stereotactic coordinate system. After imaging and planning, the fantom was adjusted and irradiated. At the end, the film, dyed by the irradiation field around the pre-marked cross, was evaluated. The measured distance between the target point (centre of the shadow) and the centre of the film is the geometrical deviation of LGK SRT. In 10 months, 20 system test were performed and deviations were found to be less than 0.5 mm.(author)