Calibration of thermoluminescent dosimetry crystals for patient organ dosimetry in CT

Abstract

Background Patient organ dosimetry is ideally performed via a direct measurement performed in-vivo. As this is impractical, simulations using Monte Carlo (MC) have been established as the gold standard. Such simulations must however be validated against physical dosimeter measurements using anthropomorphic phantoms or patient cadavers. Malta does not currently have a patient physical dosimetry system. This study is part of a larger project to introduce thermoluminescence dosimeters (TLD) systems for optimisation studies in Malta. Objectives To understand the calibration, irradiation, and read-out of TLD crystals specifically for Computed Tomography (CT) irradiation and to establish the first standard-operating-procedure (SOP) for the preparation of TLD crystals for patient dosimetry in CT imaging in Malta using anthropomorphic phantoms. Method Radiation dose measurements were made in 45 organ-relevant locations in a CIRS adult male anthropomorphic phantom. Using two TLD (LiF:Mg,Cu,P) crystals per location, the phantom was irradiated using a neck-chest-abdo-pelvic (NCAP) protocol on a Siemens Somatom Edge Plus. TLDs were calibrated on a Fuji-film D-Evo suite planar radiography system with an energy spectrum matched to that of the CT. Irradiation from scout scanning was measured separately to produce a helical-only set of organ doses. Results Intra-organ TLD deviation ranged from 0.03 - 23.57% (mean of 6.81%). Organ doses from scout-only irradiation ranged from 13.0±0.0 - 116.0±1.0μGy (mean of 46.6μGy) and contributed 0.04% - 0.37% of the scout and helical scan. Organ doses from helical only ranged from 13.372±1.273 - 37.560±1.771mGy (mean of 28.739mGy). Organ doses of TLDs placed in organs considered symmetric (e.g. lungs), had a mean deviation of <10%. The process employed in this work was written up as an SOP. Conclusions and Recommendations Organ dosimetry for 33 organs in an adult male anthropomorphic phantom was measured under CT NCAP examination. Comparison with other works showed mixed levels of agreement, likely due to differences in phantom and clinical protocol setup. To our knowledge, this is the first study to consider this many organ locations and the first to provide an SOP allowing for future research to explore patient dosimetry for different populations (e.g. foetal, female, obese) in a repeatable manner.