Wedges:

 

  1. The Wedge Angle - is defined to be the angle through which an isodose curve is tilted at the central ray of a beam at the 10 cm depth.
  2. The Hinge Angle - is defined to be the angle between the central axes of two beams (such as in a wedged pair treatment).  There is an optimum relationship between the wedge angle and the hinge angle which provides the most uniform distribution of radiation dose in the plateau:
  1.    image025
  1. image027 - is the wedge angle.
  2. image030 - is the hinge angle.

 

image028

 

  1. Individualized Wedges - require a separate wedge position for each beam width, optimally designed to minimize the loss of beam output.  This means that we have to align the thin end of the wedge with the border of the light field (this is incredibly impractical but maximizes dose rate).

 

image029

 

  1. The Dynamic Wedge - works by electronically creating wedge beam profiles through the dynamic motion of an independent jaw within the treatment beam.  This has consequences for the scatter factors and the depth of penetration of the beam (there is no beam hardening as would be seen with a physical transmission wedge and reduced scatter outside the field).

 

Dynamic Wedge

 

  1. The Wedge Transmission Factor - is defined as the ratio of doses with and without the wedge, at a point in phantom along the central axis of the beam.  This factor should be measured at a depth beyond the point of maximum dose.  We further note that the wedge transmission factor is a function of beam energy, field size, and depth of measurement.
  2. Scatter is minimized when using a dynamic wedge, as the beam is open for most of the treatment. If we were to use a universal wedge for a breast field the scatter to the contralateral breast would be about 1.5% higher than with the dynamic wedge relative to the central axis dose.
  3. Note that a universal wedge, as stated above, refers to a design currently in use by Elekta machines.  In this approach, a single wedge of approximately 60 degrees is housed in the gantry head.  This single wedge is used in conjunction with open fields to create the desired wedge angle by changing the ratio of open to wedge monitor units in the treatment field (i.e., the larger the wedge angle needed, the more monitor units with the wedge in the field).

Anonymous
2017-09-29, 14:04
ABRPhysicsHelp is a great resource for preparing for the ABR exams as well as rotation exams during your residency.