Retrospective audit of patient specific quality assurance results obtained using helical diode arrays

A retrospective audit was performed for existing patient specific quality assurance (PSQA) results measured on two ArcCheck helical diode arrays (Sun Nuclear Corporation). Twenty-five volumetric modulated arc therapy (VMAT) and thirty-two helical tomotherapy (HT) treatment plans were re-analysed using SNC patient software (version 6.2) and in-house gamma analysis code (developed in Python). Global gamma analyses were performed on the measured and calculated data (2%/2 mm) to identify the registration shift which provided the greatest gamma agreement index (GAI). Audit results indicated that when the ArcCheck devices were used for VMAT and HT, 1 mm longitudinal (Y) registration shifts frequently provided better GAI results than no shift. Specifically, the SNC and Python codes both identified a significant trend for longitudinal shifts for both ArcCheck devices. No significant trend was observed for roll (X) registration shifts. Measurements performed with physical shifts of the ArcCheck device improved the GAI results with no shift applied, suggesting that unless there is a co-incidental offset in the position of the radiation isocentre on both the TomoTherapy unit and the linac, there may be an actual displacement of the centre of the diode array and the marking lines on the two ArcCheck devices. This behaviour is dependent on gamma evaluation criteria used. The results of this study confirm the necessity of undertaking regular audits of QA results, as well as the need to consider sources of geometric uncertainty when selecting gamma evaluation criteria and when applying automatic geometric shifts to measured data.