Poster - Thur Eve - 41: Effect of beam symmetry on enhanced dynamic wedge quality assurance and tolerance levels.

Wedged fields are common in three dimensional conformal radiation therapy and require appropriate quality assurance (QA). Currently, our centre calculates the angle of enhanced dynamic wedge (EDW) deliveries with the PROFILER at a monthly frequency but Canadian guidelines on the recommended QA of enhanced dynamic wedges are not available yet. TG-142 recommendations include monthly validation of the central axis wedge factors with only annual verification of wedge profiles. Our monthly QA results have demonstrated a sensitivity of the calculated EDW angle to open beam symmetry. The goal of this work is to compare wedge angle and direct profile comparison as QA measures of EDW delivery and determine appropriate parameter tolerances. The impact of open field symmetry variations on EDW profiles was determined by varying beam symmetry with a test potentiometer and measuring the resulting EDW beam profile with the PROFILER. A calculated wedge angle and direct profile comparison were used to describe the deviations in EDW deliveries. The impact of the deviations on typical plans incorporating wedged fields was evaluated by constructing 'equivalent wedges' to the profile deviations using mixed 60 degree and open field beams and performing plan comparisons in Eclipse. Beam symmetry was observed to have a significant impact on wedge angle for small angle wedges, with a 1% symmetry tolerance allowing a 3.4 degree range of wedge angles for a nominal 10 degree wedge. Direct profile comparison allows a more consistent definition of a dose based tolerance range to be applied without angle dependent tolerances.