3-dimensional conformal radiation therapy (3-dih-MEN-shuh-nul kun-FOR-mul RAY-dee-AY-shun THAYR-uh-pee)
3-dimensional conformal radiation therapy involves the use of computed tomography (CT) imaging in the planning of radiation therapy. The CT scan provides not only 3-dimensional imaging of the target and surrounding normal tissues, but also information about tissue density and tissue depth from the skin to the target. These parameters are critical in calculating the dose distribution. In addition to CT imaging, supplemental imaging modalities, such as magnetic resonance imaging or positron emission tomography, can be used to improve target delineation. With 3-dimensional conformal radiation therapy, conformal beams are used to shape the dose delivered to the target, and wedges or compensators can be used to optimize the dose distribution. Conformal beams are shaped either with a high-density material (e.g., Cerrobend) that allows beam contouring or with multi-leaf collimators, which are an array of high-density leaves (usually tungsten) situated in the head of the linear accelerator (LINAC) whose position is controlled via independent stepping motors that allow beam shaping. Wedges are high-density devices that are placed on the head of the LINAC to act as a tissue compensator and/or beam modifier. The effect of a wedge can be created by a moving jaw at the head of the LINAC. With 3-dimensional conformal radiation therapy, variable field weighting and/or use of different energies (higher energies are more penetrating) are additional tools that enable optimization of the dose distribution. Also called 3-dimensional radiation therapy and 3D-CRT.