Assessing MLC performance of beam-matched linacs using a sliding window output factor on the ArcCheck

Abstract

Introduction The Mid-North Coast and Northern NSW Cancer Institute's consist of 3 identical integrated physical sites with 6 beam matched Elekta Synergy linacs with AgilityTM. Maintaining the beam matching accuracy, including the MLCs is important and is a challenging task for remote sites. The SNC ArcCheck is an ideal tool for assessing MLC systems due to its cylindrical shape and acquisition of ''beam on'' time. Using the sliding window output factor (SWOF) [1, 2] test, the consistency and coherence of remote linac MLC performance was assessed. Method Over the last 18 months 72 measurement samples of the SWOF test were collected. Using a 10 TM 10cm² with 200 MUs delivered at gantry 0degree and a CC04 IBA ionisation chamber placed on the central axis, the ratio of the open field to a 2 cm sliding dynamic MLC field (463 MUs) was measured at 3 different gantry angles i.e. G= 0degree, 90degree and 270degree. This tests the MLC system under different orientations with respect to gravity. The ionisation chamber ratio with the open field should equal 1 if the 2 cm leaf gap is maintained. The ''beam on'' time is a parameter which can determine if an MLC motor is inhibiting the performance of the system when compared against other linacs and the linacs own historical measurements. Results Results indicated that all 6 linacs maintained a 2 cm leaf gap within 0.5 mm tolerance with less than 3% variation of the output ratio from unity. The maximum difference in delivery time between linacs was 18 seconds. Conclusion Combining the ArcCheck with the SWOF test has shown to be a useful mechanism for testing and checking the consistency of beam matched linacs across multiple sites, exploiting the key performance indicators of leaf gap and machine delivery time.