SSD (Skin to Source Distance): Distance from the source (i.e. head of linear accelerator) to the patient’s skin. This is used if one field is being used and is usually 100 cm. It is used as a reference distance for each treatment setup.
SAD (Skin to Axis Distance): Distance from the source (i.e. head of linear accelerator) to the isocenter of the rotation of the machine (i.e. the imaginary point about which the gantry, collimator and table all rotate). This is where multiple beams intersect within a patient. With SAD setup, the SSD will be <100cm.
Penumbra: The area at the edge of a beam where the dose decreases rapidly.
Isodose lines: A line on the image that joins all areas that receive the same radiation dose. This is very similar to topographical maps that show lines of similar elevations.
D max: The point where the maximum amount of dose from one beam is deposited.
Phantom: A 3 dimensional image model that is used in place of a person to show the effects of a beam and is used in quality assurance by the Medical Physicists.
Wedges: A beam modifying device that will cause a progressive decrease in the intensity of a beam from one edge to the other.
Blocks: A beam modifying device that will cause a significant decrease in the amount of dose through the block.
Attenuation: As the beam interacts with tissue it losses strength and this leads to less dose being administered at increasing depths.