Development and Initial Characterization of an MRI-Compatible Multi-Point Dosimetry Array
Abstract
Purpose
Dosimetric quality assurance (QA) is essential for treatment validation on the MR-linac. However, many conventional QA approaches are challenging in the presence of a magnetic field. In this study a plastic scintillator multi-point QA dosimeter array designed for MRI-guided adaptive radiotherapy applications is characterized for the first time.
Methods
A prototype optical read-out system was developed to facilitate the simultaneous read-out of large-scale arrays of plastic scintillation detectors (PSDs), allowing up to 49 points. To demonstrate the dosimetric capabilities of the system, an initial set of seven PSDs (Medscint, Canada) was connected. For basic system characterization, the seven PSDs were embedded in a 3D-printed cassette (210x70x6mm) which was placed between solid water slabs (RW3). All measurements were performed on a 1.5 T Unity MR-linac (Elekta, Sweden). Calibration was carried out to relate the measured light intensity to the absorbed dose for each scintillator element. The PSDs were read out at 10Hz, providing high temporal dose information. The reproducibility was obtained, and field size dependence (between 5x5cm2 and 22x22cm2) was measured and compared to clinical references. Plan QA was performed on two prostate SBRT plans by inserting the cassette in the MRI4D motion phantom (IBA QUASAR, Canada). Additional measurements were performed with an artificial caudal target drift of 1mm/min to investigate motion sensitivity.
Results
For reproducibility the relative SD was <=0.2% for the seven PSDs. For field size dependence the maximum differences with reference doses were 2% or better. The induced subtle motion drift reduced the mean dose across measurement points by 2% as expected.
Conclusion
The prototype multi-point dosimetry array shows good performance in initial QA testing under static and dynamic experimental conditions, with additional work planned to demonstrate the scalability to more PSDs.