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 during the dose delivery. The different measurement techniques required when using a linear diode array to measure beam profiles and when using ionization chambers to measure depth doses and effective wedge factors are described. The segmented treatment tables (STTs), which specify the delivered dose as a function of jaw position, are used by the control algorithm to deliver dynamic wedge fields. The reproducibility achieved using STTs is very consistent (and the stability of the accelerators is very good) so there is no significant variation in effective wedge factor or profile shape. There is a unique set of 132 STTs for each energy and design of treatment machine, which encompasses all the dynamic 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 current generation treatment planning computer.