Author: Thomas I. Banks, Tsuicheng D. Chiu, Viktor M. Iakovenko, Christopher Kabat, Chang-Shiun Lin, Mu-Han Lin, Arnold Pompos 👨🔬
Affiliation: Department of Radiation Oncology, UT Southwestern Medical Center, UT Southwestern Medical Center, Medical Artificial Intelligence and Automation (MAIA) Lab & Department of Radiation Oncology, UT Southwestern Medical Center, Department of Radiation Oncology, UT Southwestern Medical Center, Dallas, TX 🌍
Purpose: The superior soft-tissue contrast provided by MR imaging offers favorable conditions for the effective application of stereotactic body radiation therapy (SBRT). Accurate small field dosimetry is essential for precise dose calculations in SBRT. TRS-483 does not cover small field dosimetry in the presence of magnetic field, nor is provides the recommendation for equipment. Due to Lorentz force the profiles exhibit asymmetrical shape that can manifest a challenge in positioning air-filled detectors. In contrast, scintillators, being water-equivalent, capable of being miniaturized and not sensitive to magnetic field, are a promising detector for small field dosimetry in MR-Linac systems. In this work, an assessment of commercially available scintillator detector for small dosimetry application in 1.5T MRgRT system is presented.
Methods: Measurements were performed in a 1.5T MR-Linac (Elekta,Unity system) using the following detectors: small volume ion chambers:Semiflex_3D MR(TW31024) [IC1], Semiflex(31010) [IC2], microDiamond, and commercially available scintillator detector (MedScint_HS-RP200). Output factors (OF) were measured in water at a source-to-axis (SAD=143.5 cm), depth=10cm, and field size(FS)=10x10 cm². The results were compared with Monte-Carlo treatment planning system (TPS). To assess any effect of MRI acquisition on small field output factors, IC1 and scintillator data was obtained with MRI acquiring and without.
Results: For FS>2x2, all detector are within 2% tolerance. Difference increased significantly with FS decrease for IC (1x1cm2:18.3%[IC1] and 12.5%[IC2]). However, for scintillator, agreement is within 1.5%, except for 0.7x0.7cm2 (5.7%). Interplay effect of magnetic field during image acquisition and radiation delivery is negligible for small fields as the demonstrated by the measured output factors with scintillator.
Conclusion: Performance of commercially available scintillator is promising for small field dosimetry in MR-Linac systems and can be integrated into end-to-end testing during acceptance and quality control of gating system performance. Further measurements are necessary to evaluate the temporal performance of the scintillator during gating delivery.