What is the source-surface distance (SSD)?
How is it measured?
What is it compared against and what actions are taken if it is not correct?
Source Surface Distance (SSD)
The SSD is the distance from the source (the target for photons; a virtual source for electrons) to the surface of the patient or phantom.
Measurement of Source Surface Distance
For everyday measurement, the SSD is measured by the Optical Distance Indicator. This device projects a scale (usually from 80 - 120 cm) onto the surface of the patient/phantom that is variably focussed. The scale is focussed when the distance from the source to the surface matches that distance on the scale.
QA of the optical distance indicator
As part of monthly quality assurance, the medical physicists compare the reading of the ODI with a measurement rod that is suspended from the treatment head. These rods are highly accurate and if there is a discrepancy the ODI can be tweaked until it performs accurately.