SU-F-T-464: Development of a Secondary Check Procedure to Evaluated Flatness and Symmetry Discrepancies Detected During Daily Morning QA.
PURPOSE
A daily QA device is used to monitor output, flatness and symmetry constancy for all linac photon and electron energies. If large deviations from baseline in flatness or symmetry are reported it becomes necessary to crosscheck the measurements with a second device. Setting up another device such as Matrixx (IBA Dosimetry) can be time consuming, due to its warm-up time, and trained personnel may not be readily available to analyze the results. Furthermore, this discrepancy is frequently isolated to a single energy. Unaffected energies could still be used, avoiding further patient delays, if a method to gather data for offline analysis could be developed. We find that optically stimulated luminescent dosimeters (OSLDs) provide a quick, simple, and inexpensive solution to this important clinical problem.


METHODS
The exact geometry of the detectors on the daily tracker (Keithley Therapy Beam Evaluator) was reproduced by placing nanoDot OSLDs (Landauer) on a solid water phantom. A combination of bolus and solid water was placed on top to provide buildup and prevent air gaps. Standard daily measurements of output, flatness and symmetry were taken for 2 photon energies (6x,10x) and 5 electron energies (6e,9e,12e,15e,18e) using the tracker. These measurements were then repeated with the OSLD phantom.


RESULTS
The time it took to set up the OSLD phantom was comparable to that of the tracker. The inline and crossline OSLD phantom measurements of flatness and symmetry agreed with the tracker results to within 2%.


CONCLUSION
OSLDs provide a good solution for a quick second check when questionable flatness and symmetry results are detected with the tracker during daily QA.