ESTRO 2021 Abstract Book

S235

ESTRO 2021

The measurement of clinical plans with the detector within the myQA SRS phantom demonstrated gamma pass rates of 94.9% - 98.5% (2%, 2mm) and 79.6% - 92.7% (1%, 1mm) when compared to Raystation doses. myQA SRS also agreed with the MatriXX detector array for measurement of clinical plans where gamma pass rates for corresponding dose distributions ranged from 97.9% - 100.0% (2%, 2mm) and 47.4% - 59.5% (1%, 1mm). Conclusion Overall, the myQA SRS detector shows good promise for plan verification of stereotactic body radiotherapy treatments, offering an alternative to radiochromic film. PH-0324 Suitability of plastic detector dosimetry in MR-Linacs M.F. Klavsen 1 , K. Boye 2 , R.H. Hansen 2 , C. Ankjærgaard 1 , C.E. Andersen 1 1 Technical University of Denmark, Department of Health Technology, Roskilde, Denmark; 2 Rigshospitalet, Department of Oncology , Copenhagen, Denmark Purpose or Objective We are designing a detector system that can provide time-resolved dosimetry without disturbing MR imaging in magnet resonance linear accelerators (MR-Linacs). -This property is of high value when testing the dosimetric accuracy of the MR-Linac during MRI-based gating techniques. The dosimetry system uses plastic scintillation detectors (PSDs) coupled to optical fiber cables. These all- plastic detectors have a high degree of water equivalence, and they do not disturb MR images in neither static nor dynamic setups. However, a main challenge with PSDs is to separate the scintillator signal from the unwanted Cerenkov light produced by secondary electrons when the optical fiber cable is irradiated. Cerenkov light will change with the magnetic field because the field affects electron trajectories relative to the guiding angle of the fiber. However, the spectral distribution of the Cerenkov light should not be affected to a large degree. Hence it is our hypothesis that the chromatic stem removal procedure (which is well established for scintillator measurements in conventional Linacs) can be used in MR-Linacs. To test the hypothesis, we measured dose profiles by moving the scintillating detector into the field such that the amount of fiber increased as the detector passed though the beam. Without a proper stem-removal procedure, this test will result in strongly asymmetrical dose profiles. Materials and Methods The PSD studied is a BCF-60 scintillator attached to a 15 m long 1 mm diameter PMMA optical fiber cable. The signal was split into a green and a blue channel. Calibration coefficients were found using a standard technique where the PSD was irradiated to the same dose while having different amounts of fiber cable in the beam. The system was studied in a Viewray MRidian 0.35 T MR-Linac where beam profiles were measured for a 5 cm x5 cm FFF 6 MV photon beam: one with a PTW Semiflex 31021 ionization chamber, one with the PSD and one using EBT3 film. The point detector measurements were performed in the PTW BEAMSCAN MR 3D water phantom while measurements with film were done in solid water. Results The FWHM for the PSD, Semiflex and the film was (48.3±0.2) mm, (47.9±0.2) mm and (47.3±0.2) mm, respectively, and the three beam profiles were similar, albeit not identical (Fig. 1, lower panels). The scintillator beam profile was however, found to be almost identical to the Semiflex, and no significant