2H06-The Physics of Wedge

Chen-Shou Chui

Memorial Sloan-Kettering Cancer Center, New York City, New York.

Wedge is a device commonly used in radiation therapy to shape the dose distribution from external photon beams. It is available on all major manufacturers of radiation therapy machines.

The most basic form of wedge is the physical wedge, made of metals such as lead or stainless steel. On Varian and Siemens machines, the physical wedge is mounted outside the machine head below the collimating jaws, called the ‘external’ wedge. Standard wedge angles are 15°, 30°, 45°, and 60°. On Elekta machines, a single wedge of 60° is mounted inside the machine head, called the ‘internal’ wedge.

Dose calculation for wedged field commonly uses empirical-based methods. With this method, dose is first calculated for the open field; then modified by empirically obtained profiles which account for the wedge shape. These profiles are measured for each wedge angle at a set of selected depths during machine commissioning. The wedge factor, defined as the ratio of the dose at a depth on the central axis of the wedged field to that of the corresponding open field, increases with depth and field size. This increase is primarily due to beam hardening through the metal wedge, and secondarily due to extra scatter from the wedge itself.

For Elekta machines, a single internal 60° wedge is available. Other wedge angles less than 60° can be obtained by combining the 60° wedge field and the open field with proper weights. This is called the ‘universal’ wedge. The respective weights of the wedged and the open fields are determined by the desired wedge angle.

The physical wedge has some inherent undesirable features. Its depth-dose characteristics differ from that of the open field due to beam hardening. Its field size is limited by the size and weight of the wedge. And for external wedges, it’s cumbersome to load and unload. A new form of wedge is the ‘dynamic’ or ‘virtual’ wedge. With this method, the wedge shape is generated not by a physical wedge, but rather by moving one jaw while the beam is on. With proper design of the jaw motion, any arbitrary wedge angle can be produced. The resultant wedged beam is clean, more flexible in terms of field size and wedge angle, and does not require manual loading/unloading.

In some applications, it is desirable to have the wedge oriented in a direction other than the X- or the Y-axis. This can be achieved by combining the wedged fields along the X- and the Y-axis with proper weights, determined by the desired wedge orientation. This is called the ‘omni’ wedge.

The idea of ‘omni’ wedge can be extended further to produce a wedged field of any angle along any orientation. This is accomplished by combining the open field, the wedged fields along the X- and the Y-axis, all with proper weights. This is called the ‘super omni’ wedge. This method greatly simplifies the problem of optimization of wedged fields.