A first order approximation of field-size and depth dependence of wedge transmission.

The Radiological Physics Center, through its dosimetry review visits to participating institutions, is aware that many institutions ignore the field-size and depth dependence of wedge transmission values. Reference wedge transmission values are normally measured by the Radiological Physics Center for a 10 cm x 10 cm field at the calibration depth of 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 calibration depth were included. The transmission under these two conditions was compared with that under reference conditions. The relative transmission values for 138 photon beams from 88 separate linear accelerators (4-25 MV) and 60Co units were measured. Our data suggest that the dependence of the wedge transmission on field-size and depth, in the first approximation, depends on the absolute value of the transmission under reference conditions. For wedges with a transmission value greater than 0.65%, field-size dependence and change in depth dose are typically less than 2%. However, for wedges with transmission values less than 0.65%, field-size dependence increases with decreasing reference wedge transmission. The change in wedge transmission with depth is significant (> 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 field-size dependencies of wedge transmission in patient dosimetry calculations can result in significant tumor-dose discrepancies.