Читать книгу Leksell Radiosurgery - Группа авторов - Страница 91

The source pattern defined as rings cannot be measured directly as blocking these individual rings is not feasible. Each source in the ring for each collimator setting has a path to the RFP that varies. The contribution of the sources to the dose rate varies in each ring for each collimator setting. To accurately calculate the contribution of each source to the dose, a Monte Carlo calculation is performed to determine each ring and each collimator output factor as shown in Table 1 for the 16 mm collimator. These factors cannot be changed in the treatment planning software and are fixed for all Gamma Knife units. The dose rate calibration of the unit is performed at the largest field size, and when a smaller collimator is selected, the dose rate at the RFP changes as a function of field size. The recommended output factors from Elekta that have been derived from Monte Carlo calculations for the Perfexion and ICON units are 16 mm = 1.0, 8 mm = 0.9, and 4 mm = 0.814. To verify the effective output factor for each collimator setting, methods to determine the output at different collimator sizes have been extensively studied, and Gafchromic film dosimetry delivers the most accurate results [15, 16].