Purpose: To maximize healthy tissue sparing in the presence of respiration, we previously developed MRI-guided MLC tracking for the 1.5T Unity MR-linac (Elekta AB, SWE). Dosimetric analyses were performed using film dosimetry. To extend the dosimetric analysis of MLC tracking with time-resolved and absolute dose elements, plastic scintillation dosimetry can be used. In this study, we demonstrate the feasibility of a hybrid experimental setup combining film and scintillator dosimetry in a moving phantom quantifying MLC tracking for lung SBRT.
Methods: Experiments were performed on an MR-linac in research mode. The Quasar MRI4D phantom (ModusQA, CA) was used to generate: Lujan motion (cos4, A=10mm, T=4s, drift=1mm/min), and patient-derived respiratory motion (A=11mm, T=3s, drift=0.6mm/min). The phantom contained a film insert with 3cm spherical target (GTV). The Quasar film cassette, was replaced by a 3D printed film cassette prototype that contained EBT3 film and four Hyperscint scintillation detectors (MedScint, CA). They were positioned at the center, the cranial edge and outside the GTV.Target positions were continuously estimated from 2D cine-MR (8Hz). A linear regression prediction filter compensated for system latency.An 8x7.5Gy IMRT plan with 3mm GTV-to-PTV margins was created following the clinical planning template for lung SBRT.
Results: Comparing the measured scintillator dose during a static delivery to the planned dose gives differences within 2.1-4.9% for all detectors. During induced motion, the difference between planned and measured dose is within 0.16-3.4% (patient-derived motion) and 1.7-4.9% (Lujan motion) with tracking. Due to motion induced blurring differences are 1.5-5.2% and 1.4-26.5% without tracking. Dose differences between film and scintillators were 1.5-7.6%. Following the detectors over time reveals superimposed dose lines for static and tracking, while without tracking the dose decreases over time.
Conclusion: The prototype film cassette is capable of measuring dose during motion experiments, combining film dosimetry with time-resolved and absolute dosimetry.
Funding Support, Disclosures, and Conflict of Interest: The authors acknowledge funding by the Dutch Research Council (NWO) through project no. 17515 (BREATHE EASY).