SU‐E‐T‐136: Novel Secondary Quality Assurance Procedure for the INTRABEAM System Ion Chamber Using Commercial OSLDs

Purpose: In recent events, our clinic has experienced a soft x‐ray ion chamber calibration drift of 8%. For the INTRABEAMTM system which relies on an ion chamber measurement to deduce a dose rate correction factor for treatment, such an ion chamber change results in a large deviation between the planned patient dose and the treatment dose. We present a cross calibration method between an ion chamber and commercially available optically stimulated luminescent dosimeters (OSLDs) to monitor the consistency of the soft x‐ray ion chamber. Methods: Our method uses the INTRABEAM™ system dose rate determined by a PTW soft x‐ray ion chamber (model 23342) under normal quality assurance procedures to produce the counts per dose rate for irradiated nanoDot™ OSLDs. The procedure replaces the ion chamber with an OSLD at the same position which is obtained with a fabricated stage of the same dimension as the ion chamber stage. Upon replacement of the ion chamber, the QA procedure is repeated irradiating a nanoDot™ OSLD for approximately two minutes with low energy kV x‐rays of energy 20 keV with 0.64 mm Al HVL. The QA procedure is repeated three times with a new OSLD for each irradiation. A ten minute waiting period is invoked prior to reading each nanoDot™ OSLD a total of three times. Results: This method produces an average counts per dose rate for all nanoDot™ OSLDs with a 1‐sigma standard deviation of 1.7%. The low standard deviation gives this method the ability to detect an ion chamber measurement change in dose rate greater than 3.5% incorporating the positional uncertainty and sensitivity change. Conclusion: In the advent of complete equipment failure, a system can degrade and report erroneous measurements. Our quick secondary check for equipment drift provides a necessary tool to ensure patient safety.