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Clinical Results of 3D in Vivo Dose Verification of VMAT

B Mijnheer

B Mijnheer1*, I Olaciregui-Ruiz1, R Rozendaal1, J Sonke1, J Stroom2, R Tielenburg1, M van Herk1, R Vijlbrief1, A Mans1, (1) Netherlands Cancer Institute, Amsterdam, ,(2) Fundação Champalimaud, Lisboa

SU-E-T-346 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose: To elucidate our approach of 3D in vivo dose verification of VMAT and to present the clinical results since its introduction in 2009.
Methods: An EPID-based method using a back-projection algorithm is clinically applied for 3D in vivo dose verification of the delivery of VMAT plans, created using the SmartArc module of Pinnacle. Single-arc treatment is applied for VMAT of prostate cancer, while two arcs are used for most other VMAT treatments (head-and-neck, lung-SBRT, and brain-SRS). EPID 'movies' are acquired during VMAT delivery on Elekta linacs using in-house developed software. Our routine clinical procedure is to perform 3D in vivo dose verification of the first three fractions. The comparison of the EPID-reconstructed and planned dose distributions is done by a 3D γ-evaluation method, based on 3%/3mm γ-criteria. Since August 2011 dose-reconstruction and γ-evaluation software runs automatically.
Results: The analysis of the results of about 4000 VMAT arcs (February 2012) showed that γ-evaluation data are comparable to the values found previously for 2D IMRT verification. The average difference between measured and predicted isocenter dose was (-0.2±3.7)%. More than 90% of the in vivo VMAT analyses were directly approved (no interaction of any kind needed); the remaining required further inspection by a clinical physicist. The limitations of the model and anatomical changes of the patient, confirmed by cone-beam CT scans, were the main reasons for the observed deviations. Examples will be given of differences between (in vivo) measured and planned dose distributions that were not observed during phantom measurements. Using the automatical software, a dosimetry report is available for inspection within minutes after treatment delivery without any manual intervention.
Conclusions: EPID dosimetry is a simple and accurate tool for 3D in vivo dose verification of VMAT delivery. It is faster than pre-treatment verification and provides clinically more relevant information.

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