BEAM DATA MEASUREMENTS FOR DYNAMIC WEDGES ON VARIAN-600C (6 MV) AND VARIAN-2100C (6 AND 10 MV) LINEAR ACCELERATORS

The measurement of beam data for dynamic wedge dosimetry requires the integration of radiation dose at points across the radiation field dur ing the dose delivery. The different measurement techniques required w hen using a linear diode array to measure beam profiles and when using ionization chambers to measure depth doses and effective wedge factor s are described. The segmented treatment tables (STTs), which specify the delivered dose as a function of jaw position, are used by the cont rol algorithm to deliver dynamic wedge fields. The reproducibility ach ieved using STTs is very consistent (and the stability of the accelera tors is very good) so there is no significant variation in effective w edge factor or profile shape. There is a unique set of 132 STTs for ea ch energy and design of treatment machine, which encompasses all the d ynamic wedge data. There are significant discontinuities of up to 14% in wedge factors at certain field sizes. This means that wedge factors have to be measured at small increments (0.5 cm) in field size, as it is the width of the dynamic wedge field that determines the STT used. Considerable care must be taken when implementing these data on a cur rent generation treatment planning computer.