S2353
Physics - Quality assurance and auditing
ESTRO 2026
Laboratory, 2015 Keywords: Pylinac, CBCT, validation
Digital Poster 1760
Assessing the vendor provided field passing criteria for the IQM fluence detector for VMAT PSQA. Dion Conlon, Sean Gillespie, Margaret Moore Dept. of Medical Physics & Clinical Engineering, University Hospital Galway, Galway, Ireland Purpose/Objective: The Integral Quality Monitor (IQM) (iRT Systems) is a single large area ionisation chamber that is mounted to the linac treatment head. It performs real-time verification of the treatment field fluence. This verification is based on the comparison of a calculated fluence signal, derived from the treatment plan file, and a measured signal.1 To implement the IQM as our clinic’s primary PSQA device, this work aims to assess the field passing criteria implemented in its software. Material/Methods: The IQM software determines the measured signal deviations from calculation on a segment-by-segment (SbS) and cumulative basis as the field is delivered. These deviations are assessed against statistically determined Watch and Action tolerances derived from commissioning.2 The field passing criteria requires all the following metrics to pass their respective criteria (see Table 1):Final cumulative deviationCumulative Watch Segment-weighted pass rate (SPR)Cumulative Action SPRSbS Watch SPRA 2D-array is the established PSQA device in our clinic ( γ criteria: 3%/2mm; >90%). To assess the specificity of the IQM passing criteria, 273 clinical VMAT fields were measured simultaneously by the IQM and 2D-array. To assess the IQMs sensitivity, MLC bank opening errors (both banks; 0.5mm, 1mm, 2mm) were introduced into a sample of clinical fields from 3 sites (Prostate, H&N, and Gynae), and measured with both devices. All measurements were performed on Elekta Versa HD linacs. Results: Of 273 clinical VMAT fields, 3 did not pass the IQM criteria, failing on the Cumulative Watch SPR metric only (SPR: 93%, 90%, and 77%). The cumulative signal deviations of the measured error fields and implemented tolerances are shown in Figure 1. The evaluation metrics for 12 of 18 error fields are provided in Table 1, with failing metrics underlined. All fields except Prostate 2 (+0.5mm) were detected by the IQM, although this field also passed the 2D-array criteria (99.4% passing rate).
Figure 1: Cumulative IQM signal deviations for the error induced fields.Table 1: SPR definition and IQM evaluation metrics for error induced fields.
Conclusion: The SbS Watch SPR is the least sensitive metric for error detection, detecting only 4 of 18 error fields. This
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