Checking monitor unit calculations using coordinate transformations to calculate off-axis distances in the collimator frame of reference.

An important part of treatment-planning QA is to check Monitor Units (MUs) calculated by treatment-planning programs. This is generally straightforward, unless the central axis is blocked. One way to check MUs in this case is to select a reference point in the open portion of the field and use the off-axis distance (OAD), as well as other relevant data, to verify the dose. If wedges are employed in the treatment, the OAD must be specified in the collimator frame-of-reference because one must know where the calculation point is with respect to the wedge to calculate the dose correctly. The purpose of this paper is to describe a method of calculating the OAD in the collimator frame-of-reference using the system of coordinate transformations described by Siddon