Technical Note: Quality assurance and relative dosimetry testing of a ⁶⁰Co total body irradiator using optical imaging

Abstract

Purpose The aim of this study was to create an optical imaging‐based system for quality assurance (QA) testing of a dedicated Co‐60 total body irradiation (TBI) machine. Our goal is to streamline the QA process by minimizing the amount time necessary for tests such as verification of dose rate and field homogeneity. Methods Plastic scintillating rods were placed directly on the patient treatment couch of a dedicated TBI 60Co irradiator. A tripod‐mounted intensified camera was placed directly adjacent to the couch. Images were acquired over a 30‐s period once the cobalt source was fully exposed. Real‐time image filtering was used; cumulative images were flatfield corrected as well as background and darkfield subtracted. Scintillators were used to measure light‐radiation field correspondence, dose rate, field homogeneity, and symmetry. Dose rate effects were measured by modifying the height of the treatment couch and scintillator response was compared to ionization chamber (IC) measurements. Optically stimulated luminesce detector (OSLD) used as reference dosimeters during field symmetry and homogeneity testing. Results The scintillator‐based system accurately reported changes in dose rate. When comparing normalized output values for IC vs scintillators over a range of source‐to‐surface distances, a linear relationship (R2 = 0.99) was observed. Normalized scintillator signal matched OSLD measurements with <1.5% difference during field homogeneity and symmetry testing. Beam symmetry across both axes of the field was within 2%. The light field was found to correspond to 90 ± 3% of the isodose maximum along the longitudinal and latitudinal axis, respectively. Scintillator imaging output results using a single image stack requiring no postexposure processing (needed for OSLD) or repeat manual measurements (needed for IC). Conclusion Imaging of scintillation light emission from plastic rods is a viable and efficient method for carrying out TBI 60Co irradiator QA. We have shown that this technique can accurately measure field homogeneity, symmetry, light‐radiation field correspondence, and dose rate effects.

Improvements to an optical scintillator imaging-based tissue dosimetry system

“…Scintillator dosimeters were originally designed to have a nonsoluble reflective coating on the rear face and side edges to allow for high light emissivity out of the front face and minimize angular dependency of the detected scintillation light. 1 , 9 – 12 A water-based solvent compatible with the specific plastic composition of the scintillator was used to paint the scintillators. As such, the coated layer is mechanically fragile and chemically very susceptible to common medical-grade cleaning agents.…”