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SU‐E‐T‐136: Novel Secondary Quality Assurance Procedure for the INTRABEAM System Ion Chamber Using Commercial OSLDs
Author(s) -
Keller J,
Laub W,
Pillai S
Publication year - 2013
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4814571
Subject(s) - ionization chamber , dosimeter , materials science , nanodot , calibration , quality assurance , ion , dosimetry , nuclear medicine , physics , medicine , optoelectronics , external quality assessment , pathology , quantum mechanics , ionization
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.