AbstractID: 9428 Title: Practical Quality Assurance of the Automatic Positioning System on the Leksell
Gamma Knife
We present a practical scheme for commissioning and routine quality assurance (QA) of
the Leksell Gamma Knife automatic positioning system (APS) using an economical,
novel radiographic-film phantom. The phantom is CT and Leksell-frame compatible.
QA procedures involve imaging, planning and treatment with the APS, allowing
assessment of cumulative and relative positioning uncertainties for these functions. The
QA phantom consists of two light-tight polystyrene plastic cassettes (designed to hold
radiographic film) mounted on a Lucite plate, mated to a Leksell positioning head frame.
Initially, a treatment plan is designed (based on a CT scan of the phantom and frame) to
irradiate select, individual points in the film planes. Multiple points are identified from
the CT scan. Irradiation is then planned and conducted via two distinct shots on each
point. The two shots per irradiation point vary in collimator size and one of the following
parameters: gamma angle, different docking position (using APS extenders) and/or APS
versus trunnion mode. Variations in the center of each shot on the same interest point on
a film plane represent a precision margin associated with the underlying parameter that
was varied. Accuracy is determined from distance measurements of three pins, which
puncture and fix film in the phantom, to the center of the irradiation points. Absolute
errors in positioning were found to be ≤ 0.5 mm, while the precision measured for the
APS was ≤ 0.3 mm.