Commissioning of the asymmetric collimation mode in a 6 MV linear accelerator

Clinical implementation of the independent jaws on the VARIAN 600C and its asymmetric field capability requires the systematic measurement and analysis of the dosimetric characteristics of this mode of operation. Further, comparison of this data with the output from the computerized treatment planning system in use in the clinic is required to ascertain the level of accuracy and the limitations reflected in a plan with asymmetric fields. Lastly, manual verification of computerized treatment plans, dose calculations and monitor unit (MU) settings requires the derivation of a consistent and coherent system of dosimetry, including a clear understanding of the limitations inherent in this system. This entire process is known as commissioning of the asymmetric mode of the VARIAN 600C linear accelerator.;Khan's formalism was used as the model for determining which dosimetric parameters are necessary and sufficient to describe the beam characteristics of an independent jaw collimation system. Depth dose, total scatter factor, and collimator scatter factor were measured for multiple field sizes at multiple radial distances from the central axis. Measurement of the primary off-axis intensity was also made. Comparison of these asymmetric field parameters with central axis symmetric field parameters are made. Cross beam scans were also measured with a scanning 3-D water phantom for multiple field sizes at multiple radial distances from the central axis. These were also compared with central axis symmetric fields. Comparisons were also made between the measured data and output from our treatment planning system. Absolute dose, and monitor unit settings were calculated by our treatment planning system and were compared to relative ionization measurements made for multiple field sizes at multiple radial distances from the central axis.