TLD-based organ dosimetry in abdomino-pelvic CT

Abstract

Background Ideally, patient organ dosimetry should follow a direct in-vivo approach. However, owing to a lack of feasibility of such an approach, Monte-Carlo (MC) simulations are the gold standard for estimating organ absorbed doses. Such simulation software must be verified via measurements using physical dosimeters calibrated at the beam energy. However, calibration methods of such dosimeters often use high-energy sources without inclusion of lower-energy scattered photons, which reduces calibration accuracy. A direct calibration using Computed Tomography (CT) spectra is desired. Objectives This research investigated how organ doses within the abdomino-pelvic region are affected by CT beam energy. A calibration method was proposed to include the impact of scatter. Research Methodology Thermoluminescent Dosimeter (TLD) dose measurements were made in 9 separate organs within an adult male anthropomorphic phan tom on an abdomino-pelvis protocol. Readings were taken at two different beam energies (120kVp, 140kVp). For each energy, the crystals were read using 2 different calibration profiles, one developed on a planar radiography system (without scatter) and using a 3D-printed insert and CT Dose Index (CTDI) phantom directly on the CT scanner (with scatter). Results An increase of 11.3% mGy normalized mean was observed in organ doses between the two energy levels. There was a significant proportional bias between results from the two calibration methods. Conclusions and Recommendations To our knowledge, this is the first study to consider calibrating TLD crystals directly at a CT scanner.