SU-F-T-165: Daily QA Analysis for Spot Scanning Beamline.

PURPOSE: The dosimetric results of our daily quality assurance over the last 8 years for discrete pencil beam scanning proton therapy will be presented.

METHODS: To perform the dosimetric checks, a multi-ion chamber detector is used, which consists of an array of 5 single parallel plate ion chambers that are aligned as a cross separated by 10cm each. The Tracker is snapped into a jig, which is placed on the tabletop. Different amounts of Solid Water buildup are added to shift the dose distribution. The dosimetric checks consist of 3 parts: position check, range check and volume dose check.

RESULTS: The average deviation of all position-check data were 0.2±1.3%. For the range check, the average deviation was 0.1%±1.2%, which also corresponds to a range stability of better than 1 mm over all measurements. The volumetric dose output readings were all within ±1% with the exception of 2 occasions when the cable to the dose monitor was being repaired.

CONCLUSION: Morning QA using the Tracker device gives very stable dosimetric readings but is also sensitive to mechanical and output changes in the proton therapy delivery system.