A back-projection algorithm in the presence of an extra attenuating medium: towards EPID dosimetry for the MR-Linac. (17th July 2017)

• Record Type: Journal Article
• Authors: Torres-Xirau, I; Olaciregui-Ruiz, I; Rozendaal, R A; González, P; Mijnheer, B J; Sonke, J-J; van der Heide, U A; Mans, A
• Is Part Of: Physics in medicine & biology. Volume 62:Number 15(2017:Aug.)
• Page Start: 6322
• Page End: 6340
• Publication Date: 2017-07-17
• Subjects: EPID dosimetry -- MR-Linac -- patient-specific -- in vivo -- verification -- QA -- IMRT; Biophysics -- Periodicals; Medical physics -- Periodicals; 610.153
• DOI: 10.1088/1361-6560/aa779e
• Languages: English
• ISSNs: 0031-9155

Abstract

Abstract: In external beam radiotherapy, electronic portal imaging devices (EPIDs) are frequently used for pre-treatment and for in vivo dose verification. Currently, various MR-guided radiotherapy systems are being developed and clinically implemented. Independent dosimetric verification is highly desirable. For this purpose we adapted our EPID-based dose verification system for use with the MR-Linac combination developed by Elekta in cooperation with UMC Utrecht and Philips. In this study we extended our back-projection method to cope with the presence of an extra attenuating medium between the patient and the EPID. Experiments were performed at a conventional linac, using an aluminum mock-up of the MRI scanner housing between the phantom and the EPID. For a 10 cm square field, the attenuation by the mock-up was 72%, while 16% of the remaining EPID signal resulted from scattered radiation. 58 IMRT fields were delivered to a 20 cm slab phantom with and without the mock-up. EPID reconstructed dose distributions were compared to planned dose distributions using the γ -evaluation method (global, 3%, 3 mm). In our adapted back-projection algorithm the averaged γ m e a n was 0.27 ± 0.06, while in the conventional it was 0.28 ± 0.06 . Dose profiles of several square fields reconstructed with our adapted algorithm showed excellent agreement when compared to TPS.