A Back-Projection Algorithm in the Presence of an Extra Attenuating Medium: Towards EPID Dosimetry for the MR-Linac

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 gamma-evaluation method (global, 3%, 3 mm). In our adapted back-projection algorithm the averaged gamma(mean) 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.