Organ-specific dose coefficients derived from Monte Carlo simulations for historical (1930s to 1960s) fluoroscopic and radiographic examinations of tuberculosis patients

Abstract

This work provides dose coefficients necessary to reconstruct doses used in epidemiological studies of tuberculosis patients treated from the 1930s through the 1960s, who were exposed to diagnostic imaging while undergoing treatment. We made use of averaged imaging parameters from measurement data, physician interviews, and available literature of the Canadian Fluoroscopy Cohort Study and, on occasion, from a similar study of tuberculosis patients from Massachusetts, United States, treated between 1925 and 1954. We used computational phantoms of the human anatomy and Monte Carlo radiation transport methods to compute dose coefficients that relate dose in air, at a point 20 cm away from the source, to absorbed dose in 58 organs. We selected five male and five female phantoms, based on the mean height and weight of Canadian tuberculosis patients in that era, for the 1-, 5-, 10-, 15-year old and adult ages. Using high-performance computers at the National Institutes of Health, we simulated 2,400 unique fluoroscopic and radiographic exposures by varying x-ray beam quality, field size, field shuttering, imaged anatomy, phantom orientation, and computational phantom. Compared with previous dose coefficients reported for this population, our dosimetry system uses improved anatomical phantoms constructed from computed tomography imaging datasets. The new set of dose coefficients includes tissues that were not previously assessed, in particular, for tissues outside the x-ray field or for pediatric patients. In addition, we provide dose coefficients for radiography and for fluoroscopic procedures not previously assessed in the dosimetry of this cohort (i.e. pneumoperitoneum and chest aspirations). These new dose coefficients would allow a comprehensive assessment of exposures in the cohort. In addition to providing newly derived dose coefficients, we believe the automation and methods developed to complete these dosimetry calculations are generalizable and can be applied to other epidemiological studies interested in an exposure assessment from medical x-ray imaging. These epidemiological studies provide important data for assessing health risks of radiation exposure to help inform the current system of radiological protection and efforts to optimize the use of radiation in medical studies.

Organ Doses from Chest Radiographs in Tuberculosis Patients in Canada and Their Uncertainties in Periods from 1930 to 1969

“…Chest thicknesses of the hybrid anthropomorphic phantoms in which organ DCCs in adults used in this analysis were calculated ( Borrego et al 2019 ) are about 27 cm in males and 28 cm in females. Chest thickness varies from patient to patient and, in any patient, is variable over the area of an incident x-ray beam.…”

“…Based on the chest thicknesses of the hybrid anthropomorphic phantoms at ages younger than adults ( Borrego et al 2019 ), the mean source-to-skin distance is assumed to be 154 cm in males and 153 cm in females at age 15 y, 157 cm at age 10 y (both sexes), 160 cm at age 5 y (both sexes), and 162 cm at age 1 y (both sexes). Uncertainties in mean source-to-skin distances at younger ages are assumed to be described by the triangular probability distribution for adults given above.…”

“…Organ DCCs (dose [Gy] to an organ per dose in air [Gy] at skin entrance) for chest radiographs in PA orientation used in this study ( Borrego et al 2019 ) were calculated using the most recent hybrid anthropomorphic computational phantoms ( Geyer et al 2014 ). 9 Organ DCCs were calculated in adults and children of ages 1, 5, 10, and 15 y using sex- and age-specific assumptions about body weights and heights intended to be representative of a mid-20th century Canadian population with tuberculosis ( Thiessen 2017 ).…”

“… 9 Calculated coefficients are organ doses (Gy) per kerma free-in-air (Gy) at skin entrance ( Borrego et al 2019 ). Absorbed dose free-in-air at skin entrance is assumed to be equal to air kerma at the source-to-skin distances and x-ray energies of interest.…”

“… 10 Organ DCCs used in this analysis ( Borrego et al 2019 ) were calculated without accounting for the thickness of a film cassette and cassette holder of 5 cm. However, when the incident beam is nearly plane-parallel, a discrepancy of about 3% in an assumed source-to-skin distance has a negligible effect on calculated DCCs compared with the effect of the assumed uncertainty in source-to-skin distances on estimates of the tube output at skin entrance.…”