The wedge technique is most commonly utilized to alter the dose distribution to compensate for missing tissue. For example, in a prone breast lumpectomy boost, wedges may be employed with the thick end of the wedge facing toward the anterior direction as there is a lesser separation in the anterior aspect of the breast relative to the posterior aspect. A wedge is specified by its wedge angle, and it is the angle by which the isodose lines tilt along the central axis at a reference plane of 10 CM depth. A wedge is also specified by its orientation “in” and “out” which represents the direction that the wedge initially faces when equipped onto a field.
In the past, a physical metal wedge was mounted on the linear accelerator (LINAC), but modern LINACs have an enhanced dynamic wedge (EDW) by the use of the collimator jaws to mimic the alteration of the dose distribution obtained from a physical wedge. In using an EDW, one collimator jaw will close to the other collimator jaw and nearly make contact with the other jaw. When the beam turns on, the collimator jaw will then move back out to open the field size which will “spill” photons into the open field. The speed at which the collimator jaw opens will reflect the thickness of the wedge, and the area under the field which is blocked for a longer time period will reflect the direction of the thick end of the wedge.
- Right click on a field and select on properties.
- Go to the accessories tab and select a wedge ID. Depending on the orientation of the wedge, a collimator rotation may be required to have the wedge face in your intended direction. Wedges are most commonly used at collimator rotations of 0° and 90°.
- Use the different planes to verify that the wedge is orientated in the correct direction and proceed to calculate. With every modification to the wedge, a re-calculation will be required.
- Test different wedge angles to determine the ideal wedge angle.