Epid-based in vivo dose verification for lung stereotactic treatments delivered with multiple breath-hold segmented volumetric modulated arc therapy

We evaluated an EPID-based in-vivo dosimetry (IVD) method for the dose verification and the treatment reproducibility of lung SBRT-VMAT treatments in clinical routine. Ten patients with lung metastases treated with Elekta VMAT technique were enrolled. All patients were irradiated in five consecutive fractions, with total doses of 50 Gy. Set-up was carried out with the Elekta stereotactic body frame. Eight patients were simulated and treated using the Active Breath Control (ABC) system, a spirometer enabling patients to maintain a breath-hold at a predetermined lung volume. Two patients were simulated and treated in free-breathing using an abdominal compressor. IVD was performed using the SOFTDISO software. IVD tests were evaluated by means of (a) ratio R between daily in-vivo isocenter dose and planned dose and (b) gamma-analysis between EPID integral portal images in terms of percentage of points with gamma-value smaller than one (gamma(%)) and mean gamma-values (gamma(mean)) using a 3%(global)/3 mm criteria. Alert criteria of +/- 5% for R ratio, gamma(%) < 90%, and gamma(mean) > 0.67 were chosen. 50 transit EPID images were acquired. For the patients treated with ABC spirometer, the results reported a high level of accuracy in dose delivery with 100% of tests within +/- 5%. The gamma-analysis showed a mean value of gamma(mean) equal to 0.21 (range: 0.04-0.56) and a mean gamma(%) equal to 96.9 (range: 78-100). Relevant discrepancies were observed only for the two patients treated without ABC, mainly due to a blurring dose effect due to residual respiratory motion. Our method provided a fast and accurate procedure in clinical routine for verifying delivered dose as well as for detecting errors.