Surface and peripheral doses of dynamic and physical wedges

The physical and dosimetric differences between three different wedge systems on a multileaf collimator (MLC) equiped linear accelerator are discussed in this report. In particular, the in-field and peripheral surface doses from these wedge systems are measured and their clinical differences discussed.

A parallel-plate chamber was used in a solid water phantom to measure the surface doses of the wedges. Published correction factors were used to convert relative ionization to relative surface dose. Measurements were performed for 6 and 18 MV photon beams for different field sizes, source-surface distances (SSD), and distances outside the field for peripheral dose measurements. Surface-dose profiles across a field in the wedge-gradient direction were measured for the dynamic and upper wedges. Dose profiles in the nonwedge gradient direction were measured for open beam as well as the three wedges using films at depths of maximum dose (d(max)).

At 85 cm SSD, surface doses on the central axis under a dynamic wedge or upper wedges are similar to those of an open field, while those of a lower wedged field are as much as 100% higher. Differences in surface doses due to beam energy are relatively minor compared with differences due to SSD or wedge systems. Dynamic and upper wedges produce similar peripheral doses, much lower than those produced by the lower wedges. The surface dose profile across the field under the dynamic wedge has a higher slope than that under the upper wedge, when the difference in wedge angles is compensated for by normalization to the dose profile at d(max). In the nonwedge gradient direction, the dose profiles at d(max) of both the upper and the lower wedges demonstrate a marked effect of oblique filtration of the primary beam, resulting in an off-axis ratio at 80% of the field width of 0.95, in contrast to the off-axis ratio of 1.05 in the open and the dynamic wedged fields.

The three wedge systems produce significantly different surface and peripheral doses that should be considered in properly choosing a wedge system for clinical use. Dynamic wedge and upper wedge systems deliver surface and peripheral doses similar to those of open fields and much lower than the lower wedge system. Both physical wedge systems degrade beam profiles in the nonwedged direction.