A FIRST-ORDER APPROXIMATION OF FIELD-SIZE AND DEPTH DEPENDENCE OF WEDGE TRANSMISSION

The Radiological Physics Center, through its dosimetry review visits t o participating institutions, is aware that many institutions ignore t he field-size and depth dependence of wedge transmission values. Refer ence wedge transmission values are normally measured by the Radiologic al Physics Center for a 10 cm x 10 cm field at the calibration depth o f 5 or 7 cm. Recently, additional measurements (1) for a 10 cm x 10 cm field at 20-cm depth and (2) for a 20 cm x 20 cm field at the calibra tion depth were included. The transmission under these two conditions was compared with that under reference conditions. The relative transm ission values for 138 photon beams from 88 separate linear accelerator s (4-25 MV) and Co-60 units were measured. Our data suggest that the d ependence of the wedge transmission on field-size and depth, in the fi rst approximation, depends on the absolute value of the transmission u nder reference conditions. For wedges with a transmission value greate r than 0.65%, field-size dependence and change in depth dose are typic ally less than 2%. However, for wedges with transmission values less t han 0.65%, field-size dependence increases with decreasing reference w edge transmission. The change in wedge transmission with depth is sign ificant (>2%) only for photon energies less than or equal to 10 MV and can exceed 5% for thick wedges. Failure to include the depth and fiel d-size dependencies of wedge transmission in patient dosimetry calcula tions can result in significant tumor-dose discrepancies. (C) 1998 Ame rican Association of Physicists in Medicine.