Author: Petr Bruza, Megan Clark, David J. Gladstone, Lesley A Jarvis, Allison Matous 👨🔬
Affiliation: Thayer School of Engineering, Dartmouth College, Dartmouth Cancer Center, Dartmouth Health 🌍
Purpose: Scintillation imaging has demonstrated unique advantages to existing clinical in vivo dosimetry (IVD) techniques by enabling full-field visualization, direct comparison to the treatment plan, and real-time readout. However, calibration procedures have yet to be established and standardized, a step critical for clinical translation and routine use. The goal of this work was to extend the calibration formalism outlined by TG-191, intended for luminescent dosimeters, to scintillation imaging dosimetry, requiring additional considerations and calibration factors. This methodology is currently being tested in an ongoing clinical trial.
Methods: A Cherenkov imaging setup (DoseOptics, LLC) was used to collect raw images of scintillators (ø1.5cm) placed on solid water phantoms during radiation delivery. TG-191 was used as a guide to develop a method for robustly calibrating the image intensity, acting as the raw reading, to surface dose. Calibration factors, such as day-to-day variations in camera and scintillator output (signal fading correction factor), angular dependencies (gantry, scintillator, and camera), batch calibration, and beam quality, were identified and characterized using phantoms. Thermoluminesent (Wisconsin TLD Lab) and a thin-window parallel-plate ionization chamber were used for comparison as the gold-standard measurements.
Results: Calibration conditions and setup were established for the imaging system, with a scintillator placed on solid water at 100cm source-to-surface distance using a 6X beam. Image counts were calibrated to dose via a linear relationship, at 1972±16Counts/cGy (R2=1). By implementing the calibration procedure outlined by TG-191, per-day variation in measured dose agreed with reference dosimeters within the clinical standard of ±5%.
Conclusion: Scintillation imaging can be used as an efficient and informative tool to monitor patient treatment and quantify doses received. TG-191 outlines a methodology for calibration of luminescent dosimetry tools, providing the structure needed for robust characterization of optical methods outlined, for the first time, here, an important step for this emerging dosimetry modality.