We thought to estimate the AGD in 247 women who underwent mammographic X-ray examinations at eight clinics in Sudan. Women underwent symptomatic mammography but also screening mammography. The different types of investigated equipment included direct digital radiography (DR), computed radiography, and screen film (SF) radiography. Table 1 shows the studied mammography equipment information. For dose assessment, patient exposure parameters were retrospectively extracted from DICOM (Digital Imaging and Communications in Medicine) header. Individual patient consent was waived for this study.
Table 1
Mammography equipment information
Code
|
Make/model
|
Country of origin
|
Modality
|
Target/filter combination
|
Year of installation
|
Institution type
|
M1
|
Giotto
|
Italy
|
CR
|
Mo/Mo
|
2010
|
Government
|
M2
|
GE
|
USA
|
DR
|
Mo/Rh
|
2012
|
Government
|
M3
|
Philips
|
Netherlands
|
SF
|
Mo/Mo
|
2007
|
Private
|
M4
|
Lilyum/
Metaltronica
|
Italy
|
SF
|
Mo/Mo
|
2010
|
Private
|
M5
|
MOMMAT C/SIEMENS
|
Germany
|
CR
|
Rh/Al
|
2012
|
Private
|
M6
|
Lilyum/
Metaltronica
|
Italy
|
DR
|
Mo/Mo
|
2014
|
Private
|
M7
|
Philips
|
Netherlands
|
Dr
|
Mo/Mo
|
2014
|
Government
|
M8
|
OSH
|
|
Dr
|
Mo/Mo
|
2016
|
Government
|
In mammography, the radiation dose is determined in terms of the AGD, the recommended dosimetric quantitative value of interest for radiation risk assessments in mammography (NCRP 1987, 1996) [23]. AGD is estimated from the measurements of the entrance surface air kerma (ESAK) and conversion coefficients that depend on beam quality (half-value layer) [24, 25]. For each breast, the AGD is estimated for two views: the craniocaudal (CC; head to foot) and mediolateral oblique (MLO; from the middle of the chest out to the side of the body with the X-ray tube placed at an angle) views. In each of the CC and MLO views of the breast, the following parameters were recorded: compressed breast thickness (CBT), target and filtration material, tube peak kilovoltage (kVp), and exposure current-time product (mAs). Other machine parameters, such as beam half-value layer (HVL) and radiation output, were measured.
Determination of the Average Glandular Dose (AGD)
Average Glandular Dose (AGD) is the mean absorbed dose in the glandular tissue of the breast. Glandular tissue is the radiosensitive tissue of the breast, and therefore the AGD is recommended as a dose quantity of interest for radiation risk estimates in X-ray mammography (NCRP 1987, 1996) [23]. AGD is derived from measurements of the incident air kerma ( \({\text{K}}_{\text{i}}\) ) and applying conversion coefficients that depend on the radiation beam quality (HVL) determined by the anode /filter materials, breast thickness, and composition [24, 25]. AGD is estimated from the \({\text{K}}_{\text{i}}\) in a three-step process:
First, the normalized X-ray tube output, \(Y\left(d,kV\right)\), is obtained of the incident air kerma ( \({\text{K}}_{\text{i}}\)) measured at a 60 cm focus-to-detector distance (d) with the breast compression plate in position using calibrated dose rate meter type Piranha (RTI; Ballad, Sweden). \({\text{K}}_{\text{i}}\) is defined as the air kerma measured at a point at the radiation field the center at patient's entrance or phantom, excluding the backscattered radiation. The normalized X-ray tube output, \(Y\left(d,kV\right)\), is thereafter determined according to Eq. 1.
$$Y\left(d,kV\right)={K}_{a}(d,kV)/{P}_{It}$$
1
Where \({K}_{a}(d,kV)\) is the air kerma measured using a range of the tube voltage and exposure-time current product (mAs) value conditions encountered in mammography examinations for a particular mammography unit, a calibration curve of Y (d,kV) versus kV values was obtained and fitted using a power function [26, 27].
Next, patient incident air kerma was determined from the X-ray tube output, Y (d), corresponding to the specific kV value used during mammography corrected for the focal spot-to-surface distance, dFSD, and mAs, according to Eq. 2:
$${\text{K}}_{\text{i}}=\text{Y}\left(d,kV\right){\text{P}}_{\text{I}\text{t}}{\left(\frac{\text{d}}{{\text{d}}_{\text{F}\text{S}\text{D}}}\right)}^{2}$$
2
AGD is then estimated from the measured \({\text{K}}_{\text{i}}\) values using conversion coefficients according to equations 3
$$AGD ={C}_{{D}_{G50}{,K}_{i}}.{C}_{{D}_{Gg, }{D}_{G50}}.S.{K}_{i} \left(3\right)$$
Where, \({C}_{{D}_{G50}{,K}_{i}}\) is the coefficient to converts measured \({\text{K}}_{\text{i}}\) to AGD for a breast of 50% granularity; \({C}_{{D}_{Gg, }{D}_{G50}}\) converts AGD for a breast of 50% granularity to that for breast granularity, g, of the same thickness. The S correction factor represents the selected target/filter combination [26, 27].. Values of these conversion coefficients are tabulated as a function of the beam quality (HVL) for compressed breast thicknesses, composition but also for a reference phantom in the relevant International Atomic Energy Agency (IAEA) and the International Commission on Radiation Units and Measurements (ICRU) publications [26, 27].