Initial clinical experience with Epid-based in-vivo dosimetry for VMAT treatments of head-and-neck tumors

We evaluated an EPID-based in-vivo dosimetry algorithm (IVD) for complex VMAT treatments in clinical\r\nroutine. 19 consecutive patients with head-and-neck tumors and treated with Elekta VMAT technique\r\nusing Simultaneous Integrated Boost strategy were enrolled. In-vivo tests were evaluated by means of\r\n(i) ratio R between daily in-vivo isocenter dose and planned dose and (ii) γ-analysis between EPID integral\r\nportal images in terms of percentage of points with γ-value smaller than one (γ%) and mean γ-values\r\n(γmean), using a global 3%–3 mm criteria. Alert criteria of ±5% for R ratio, γ% < 90% and γmean > 0.67 were\r\nchosen. A total of 350 transit EPID images were acquired during the treatment fractions. The overall mean\r\nR ratio was equal to 1.002 ± 0.019 (1 SD), with 95.9% of tests within ±5%. The 2D portal images of γ-analysis\r\nshowed an overall γmean of 0.42 ± 0.16 with 93.3% of tests within alert criteria, and a mean γ% equal to\r\n92.9 ± 5.1% with 85.9% of tests within alert criteria. Relevant discrepancies were observed in three patients:\r\na set-up error was detected for one patient and two patients showed major anatomical variations\r\n(weight loss/tumor shrinkage) in the second half of treatment. The results are supplied in quasi realtime,\r\nwith IVD tests displayed after only 1 minute from the end of arc delivery. This procedure was able\r\nto detect when delivery was inconsistent with the original plans, allowing physics and medical staff to\r\npromptly act in case of major deviations between measured and planned dose.