No abstract

This paper focuses on the accuracy, in absolute dose measurements, with GafChromicTM EBT film achievable in water for a 6 MV photon beam up to a dose of 2.3 Gy. Motivation is to get an absolute dose detection system to measure up dose distributions in a (water) phantom, to check dose calculations. An Epson 1680 color (red green blue) transmission flatbed scanner has been used as film scanning system, where the response in the red color channel has been extracted and used for the analyses. The influence of the flatbed film scanner on the film based dose detection process was investigated. The scan procedure has been optimized; i.e. for instance a lateral correction curve was derived to correct the scan value, up to 10%, as a function of optical density and lateral position. Sensitometric curves of different film batches were evaluated in portrait and landscape scan mode. Between various batches important variations in sensitometric curve were observed. Energy dependence of the film is negligible, while a slight variation in dose response is observed for very large angles between film surface and incident photon beam. Improved accuracy in absolute dose detection can be obtained by repetition of a film measurement to tackle at least the inherent presence of film inhomogeneous construction. We state that the overall uncertainty is random in absolute EBT film dose detection and of the order of 1.3% (1 SD) under the condition that the film is scanned in a limited centered area on the scanner and at least two films have been applied. At last we advise to check a new film batch on its characteristics compared to available information, before using that batch for absolute dose measurements.

The 0.35 T Co MRIdian system (ViewRay Inc., Mountain View) has been in clinical use in our institution since May 2016. For quality assurance (QA) of dose delivery and end-to-end testing for this machine, a reliable dosimeter is required. However, it is possible that a magnetic field may cause perturbations to dosimetry measurements. For static magnetic fields, there is conflicting information in the literature concerning EBT film behaviour, while for real-time MR imaging such information is not available at all. The purpose of this study was to investigate the suitability of EBT3 GafChromic film for MRIdian QA, both with and without real-time MR imaging. EBT3 film sheets were irradiated in water using the MRIdian and a conventional linear accelerator (Linac) for reference. Dose calibration measurements were first performed up to 8 Gy for both machines. The MRIdian measurements were performed with and without real-time MR imaging. Second, film sheets were irradiated at seven different angles with respect to the B-field. Optical density and dose values were analysed for the three colour channels. In both the film dose-response and B-field orientation measurements, the mean dose values were within the 1% uncertainty range of prescribed dose values for the red and green channels, for both machines. There were no dose deviations detected between the MRIdian and Linac film measurements, nor for different B-field orientations. In addition, the film dose-response measurements during real-time imaging were within 1.5% of the reference Linac measurements. EBT3 GafChromic film can be used for absolute dosimetry during real-time MR imaging independent of its orientation in the B-field. This makes it a suitable dosimeter for patient-specific QA measurements and end-to-end testing of 0.35 T MRI-radiotherapy devices.

In the evolving field of adaptive MR guided radiotherapy, the need for dedicated procedures for acceptance and quality assurance is increasing. Research has been devoted to MR compatible dosimeters and phantoms, but to date no end-to-end test has been presented that covers an MRgRT workflow. Such an end-to-end test should comprise each step of the workflow and include all associated uncertainties. The purpose of this study was to investigate the usability of an anthropomorphic deformable and multimodal pelvis (ADAM-pelvis) phantom in combination with film dosimetry for end-to-end testing of an MRgRT adaptive workflow. The ADAM-pelvis phantom included surrogates for muscle tissue, adipose and bone, as well as deformable silicone organs mimicking a prostate patient. At the interfaces of the critical structures (bladder and rectum), small pieces of GafChromic EBT3 films were placed to measure delivered dose. Pre-treatment MR imaging of the phantom was used to delineate the prostate, rectum and bladder and to generate a treatment plan to deliver 2 Gy to the prostate. Electron density (ED) map from CT imaging was used for dose calculation after deformable image registration (DIR) to the pre-treatment MR scan. At each fraction, bladder- and rectum filling was varied and a new adapted plan was generated. Dose calculation was performed using both a DIR-based ED map and a CT-based ED map after acquisition of a new CT scan of the phantom at each fraction. All dose calculations were performed taking into account the magnetic field. A good agreement between measured and calculated dose was found using both, the CT-derived and the DIR-based ED map (2.0% and 2.8% dose difference, respectively). The gamma index pass-rate (3%/2 mm) varied from 96.4% to 100%.The ADAM-pelvis phantom was suitable for end-to-end testing in MR-guided radiotherapy and a very good agreement with the calculated dose was achieved.