TLD measurements of the relative output factors for the Leksell Gamma Knife.

The accuracy of the output factor directly affects the accuracy of dose delivery during patient treatment. At our Gamma Knife center, annual output factor measurements have been carried out by using a high accuracy TLD technique, characterized by the group annealing and sorting (GAS) procedure. For each collimator exposure, one to five LiF Thermoluminescent Dosimeter (TLD) 1 x 1 x 1 mm3 cubes were used, depending upon the collimator size, and the process was repeated until approximately 10 TLDs had been exposed for each collimator. Transit radiation dose accumulated during the motion of the treatment couch was measured for each collimator helmet, and the result was subtracted from the uncorrected TLD dose measurements. The mean values of the output factors for the 14, 8 and 4 mm collimators from last 5 years were 0.985 ¿ 0.001, 0.948 ¿ 0.005 and 0.833 ¿ 0.007, respectively, relative to the 18 mm collimator. These measured relative output factors are virtually identical to the values recommended by the manufacturer for the 14 mm and 8 mm collimators. On the other hand, the output factor for the 4 mm collimator was approximately 4.1% larger than that recommended by the manufacturer. The significance of these findings is discussed.