Recommendations for measurement of tray and wedge factors for high energy photons.

Transmission factors for beam modifers such as wedges and blocking trays in dosimetry calculations are very common, but various methods have been used in measuring these factors. A systematic study to measure these factors as a function of field size and point of measurement was carried out. The beam modifiers were a blocking tray and the standard wedges supplied with a cobalt-60 unit and a medical electron accelerator (6- and 18-MV x rays). Several points of measurement were selected, using several depths: dmax, TG-21 calibration depth, 10 cm (wedge angle defining depth, ICRU), 15 cm and in-air (miniphantom), and various field sizes. The results show a small field size dependence in these transmission factors with more dependence found in the point of measurement.