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The convergence of the radiation for each collimator in Leksell coordinate space should be at a position of x = 100.0, y = 100.0, and z = 100.0, which is the RFP. The patient positioning system (PPS) positions the targets within the patient’s brain in reference to this point. To determine the accuracy of the PPS with the radiation profiles, we measure the radiation profiles of each collimator with a precise pin prick located at the RFP. Elekta has a special holder that precisely aligns a sharp needle that punctures the film. We determine the field edge of the radiation profile, then determine the location of the central pin prick on the film, and determine the difference of the pinprick relative to the field center. When done and tabulated for the x:y and x:z orientations, the results can then be processed as:

RFP =√(Δx² +Δy² +Δz²)(tolerance less than 0.4 mm),

where Δx² = displacement of the FWHM value from the pinprick in the x direction.