Evaluating angular accuracy and dosimetric impact of an inclinometer on patient specific QA results

The Octavius 4D QA device uses an inclinometer mounted on the linear accelerator (linac) gantry stand to maintain detector alignment during VMAT delivery. Due to linac acceleration, combined with accuracy limitations of the inclinometer, the Octavius system may record incorrect gantry angles, potentially affecting the accuracy of reconstructed dose distributions. The purpose of this study was to evaluate the angular accuracy of the Octavius. Inclinometer angles were corrected using 2 methods: interpolation of corrections for a set of static gantry angles (Table-corrected), and data extracted from the linac log files (Log-corrected). Corrections were then applied to PSQA measurements, and their impact on PSQA results was assessed. The Table-correction ranged from - 0.2° to 1.1°, while the Log-correction ranged from - 2.0° to + 2.8°. Lag values increased with gantry rotation velocity. Both short (50° to 100°) and long (> 100°) arcs showed similar lag magnitudes and distributions. Although Log-correction improved PSQA results for both short and long arcs (p < 0.001 for both), the effect was more pronounced for short arcs (r = 0.7) compared to long arcs (r = 0.4). The effect of Table-correction on PSQA was statistically significant for long arcs (p = 0.009) but weak (r = 0.3), and it did not significantly affect short arcs (p = 0.546, r = 0.09). The magnitude and effect of corrections on measurements was investigated and possible explanation for lower passing rates in short arc measurements was provided.