Measurement of Y90 Radioembolization Tubing Using Scintillation Detector, a Monte-Carlo Simulation

Objectives: Measurement of activity within Yitrium-90 infusion tubing may prove to be a valuable method of accurately monitoring the delivery of Y90 radioembolization therapy. Typical Y90 imaging with PET or SPECT relies on the generation of bremsstrahlung radiation as beta radiation interacts with the patient’s body. We recently have used external radiation detectors to successfully monitor dose delivery of Y90 therapies, however, without appreciable high Z materials surrounding the tubing or within the detector unit, and the relatively small crystal size in the detectors used, we question to what extent the bremsstrahlung process plays a role in measurement of Y90 spheres through the tubing compared to direct detector interaction. The goal of this study was to understand where within the detector and through what radioactive processes the Y90 beta particles are detected by this small external detector system.

Methods: In this work we used the GATE Monte-Carlo simulation framework and 3D models of the detector geometry. The detector geometry is based on a commercial topical scintillation detector and consists of a ABS plastic case enclosing a circuit board with silicon photo-multiplier (SiPM), 3x3x3mm BGO crystal, and a brass EMI shielding can. Simulated tubing consisted of a 200mm length of polyurethane tubing (internal diameter of 0.5mm and wall thickness of 0.23mm) positioned in contact with and centered under the detector face. Y90 activity was modeled as an ion source within the tubing using GATE’s EM Standard physics models. To understand where along the tube detector counts are coming from, we first simulated 100 MBq uniformly distributed within the tubing. Using the simulated detector output, we generated a cumulative frequency graph of the 3D origin points of those particles which deposited energy in the detector’s crystal. This graph allowed us to calculate the distance away from center that corresponds with percentage of the detector’s output. To understand the physics processes involved in detection, we used GATE to generate the locations of each of the electromagnetic physics processes. Using this data, we visualized in 3D where each process tends to take place. Additionally, we ran the simulation with and without the bremsstrahlung process enabled to show its specific impact in absolute detector output.

Results: Using the cumulative frequency graph, we determined that 50% of detector counts originate from within 1.5mm of center. Likewise, 90% originate from within 8.75mm of center and 99% originate from within 40.5mm of center. Through visual inspection of physics process locations in 3D simulation output, elastic scattering occurred mostly within the tubing itself, the detector’s plastic case, and the detector’s brass EMI shield can. Electron ionization and bremsstrahlung both occurred most often within the tubing and the detector’s plastic case. Neither the photoelectric effect nor Compton scattering occurred in a significant portion of the interactions. Simulations with and without the bremsstrahlung process enabled indicated that 0.2% of the detector output result from bremsstrahlung radiation.

Conclusions: In this work, we simulated the use of a scintillation detector to measure Y90 activity in a typical radioembolization tube. We found that the mass of the detector’s housing, brass EMI shield, and other components was not enough to generate significant bremsstrahlung radiation. In contrast to typical Y90 imaging, this application relies on detection of beta radiation directly and is thus quite limited in detection radius. These results are useful for understanding the underlying physics behind this method of detecting and measuring radiation from Y90 radioembolization therapy infusions which provides additional understanding of the minimum activity limits and detection depths for accurate monitoring as well as enabling improvements in quantification when using this type of dose delivery monitoring technique.

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