Wedge

 
Three types of wedge filter are currently in use: manual, motorized and dynamic.

A physical or external wedge is an angled piece of lead or steel that is placed in the beam to produce a gradient in radiation intensity. Manual intervention is required to place physical wedges on the treatment unit’s collimator assembly.
The wedge angle is defined as the angle through which an isodose curve at given depth in water (usually 10 cm) is tilted at the central beam axis underthe condition of normal beam incidence.

 

 
The wedge (transmission) factor (WF) is defined as the ratio of doses at zmax in a water phantom on the beam central axis with and without the wedge.
Physical wedges are usually available with wedge angles of 15º, 30º, 45ºand 60º .

 

 
A motorized or internal wedge is a similar device, a physical wedge integrated into the head of the unit and controlled remotely.
A dynamic or virtual wedge produces the same wedged intensity gradient by having one jaw close gradually while the beam is on. Dynamic wedges are used to achieve any arbitrary wedge angle in the range 0°–60º.
Virtual wedge (Siemens GmbH) – the speed of collimator is constant and the dose rate is changeable. The smallest opening distance of collimator is 1 cm. The dose rate varies for every 2 mm movement of collimator jaws. The monitor units appropriate for prescribed dose are calculated on the base of algorithm.
Enhanced dynamic wedge (Varian ) – the dose rate is constant and the speed of collimator is changeable. The Multiple Asyncronous Parallel Processing controlled the dynamic wedge system. The treatment monitor unit is determined on the base of Segmented Treatment Tables.
The following figure shows the comparison of Varian dynamic and Siemens virtual wedge.

 

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