Abstract
Introduction Due to the increasing complexity of radiotherapy treatments, comprehensive quality assurance programs are mandatory to check the proper functioning of the whole radiotherapy chain. In vivo dosimetry (IVD), a direct method of measuring radiation doses to cancer patients during treatment, has shown unique features to trace deviations between planned and actually delivered dose distributions. Purpose To assess the usefulness of EPID-based IVD for complex VMAT treatments in clinical routine. Materials and methods 80 patients (30 with head-neck tumor, 30 with pelvic tumors and 20 patients with extracranial metastases) treated with Elekta were enrolled. IVD tests were evaluate by means of (i) R ratio between daily in-vivo isocenter dose and planned dose and (ii) γ -analysis between EPID integral portal images in terms of percentage of points with γ -value smaller than one ( γ %) and mean γ -values ( γ mean), using a global 3%-3 mm criteria. Alert criteria of ±5% for R ratio, γ % γ mean >0.67 were chosen. Results A total of 980 transit EPID images were acquired. The mean R ratio was equal to 1.001 ± 0.015 for all patients with more than 95% of tests were within 5%. For HN, pelvic and SBRT treatments, γ % and γ mean metrics reported 85.9%, 83.1% and 95.4%, and 93.3%, 92.1% and 98.1% of tests within alert criteria, respectively. The systematic use of IVD revealed relevant discrepancies in a few H–N patients due to major anatomical variations and variations due to random anatomical changes in terms of filling of rectum/bladder in pelvic tumors. No discrepancies were detected in SBRT patients. Conclusion IVD was able to detect when delivery was inconsistent with the original plans, allowing physics and medical staff to promptly act in case of major deviations.
Published Version
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