4.6 - Extended SSD Treatment

What is meant by extended SSD radiation treatment?
How is it achieved?

Extended SSD Treatment

Extended SSD treatment refers to treatment where the SSD is greater than 100 cm (or the distance to the isocentre of the linac). It is achieved by changing the position of the couch relative to the treatment head.


Some electron treatments must be performed at extended SSD due to patient anatomy. A good example is a posterior neck electron field in a patient with a short neck and/or large shoulders, which prevents the applicator from sitting just next to the skin.
Other examples include when a volume is too large to fit in the largest field available (usually 40 x 40 cm). A larger field size can be achieved by extending the SSD.
Finally, total body irradiation requires the entire body to be treated. The patient usually rests against one wall of the bunker with the gantry placed horizontally and the treatment head facing them.