Ethical considerations
This was a hospital-based, dual-modality, prospective and cross-sectional work undertaken between November and December 2019 at the radiology department of a foremost regional teaching hospital. It involved affixing portable, radioluscent thermoluminescent dosimeters (TLD) on the breasts of adult female patients during x-ray and CT examinations of the head to quantify the amount of scattered radiation reaching the breasts. Ethical approval for the CT component was obtained from the institutional Research Ethics Committee, while approval for the x-ray component was obtained from a neighbouring university ethical subcommittee. Hospital patients were enlisted for the work and they gave informed, signed consent. To guarantee confidentiality, only initials of their name was written in data collection sheet. The TLDs were also numbered and did not indicate name of subjects.
Equipment
The radiology department had two similar static x-ray machines which were engaged in the work. The machine was General Electrics (GE), silhouette VR, high frequency, 3-phase, static x-ray machines with maximum rating of 140 kVp (tube potential), 600 mA (tube current) and 2.7 mm Al (total filtration). It was manufactured in 2003 and installed in 2012. Within that time frame, it underwent several preventive and restorative maintenance, as well as re-calibrations. The machine had under-couch and erect potter-bucky detector trays incorporated with it [1]. It had full functionality as at the time of the work and had quarterly quality control checks from two dedicated hospital-based engineers. Other equipment and accessories were a computed radiography digitizer (model CR 12-x) (by Agfa healthcare Belgium) produced in December 2013. Detectors were a 25 cm × 30 cm (10″ × 12″) and 35 cm × 43 cm (14″ × 17″) standard photostimulable phosphor imaging plates (model CR MD4.0T General) also manufactured by Agfa Healthcare Germany.
The CT scanner used was also a GE product. It was GE Brightspeed, 4 slice scanner manufactured in 2007 and installed in the centre in 2012. Maximum technical imaging parameters were 140 kVp (tube potential), 350 (tube current), 4 s (gantry rotation time) and 85 cm (gantry aperture diameter). It had capabilities for axial and helical scan modes while cine mode was deactivated for the sake of minimizing dose. Calibration was done daily by radiographers using installed calibration software, while engineers carried out quarterly preventive maintenance and quality control.
Scattered radiation was quantified with thermoluminescent dosimeter chips (TLD-100 LiF: Mg, Ti) which had multidirectional energy response. They were acquired, calibrated, annealed and transported from one of the two regional dosimetric centres in the country. Due to cost, only two hundred of those chips were used. To shield them from background radiation, they were enclosed in small, black radiolucent polythene sachets. They were used with that additional enclosure during the procedure.
Subject selection
Subjects qualified for inclusion if they were ambulant, not on drip infusion or oxygen mask, not gravid as read from request cards and confirmed by them, no evidence of mastectomy, were aged ≥ 18 years, fell within a weight range of 60–90 kg in order to filter off outliers like asthenic and hypersthenic body shapes, had a visible neck that separated head from thoracic region, non-drooping breasts and signed a consent form. Although adult gynaecomastic cases were qualified for inclusion, none was encountered within the study period. Furthermore, patients were either for x-ray or CT and not both. So, investigations were carried out in separate diagnostic suites within the department and within the same period. For each modality, only fifty patients were enlisted.
Procedure
Anthropometric parameters were first obtained. Age was adopted from request cards that came from referring physicians. Gender that was not indicated on request cards was assumed from appearance since culturally, there were gender-specific appearances in the locality. Weight (kg) and height (m) were measured with balanced beam scale with an incorporated adjustable height rule. Both weight and height were read to the nearest 0.5 kg and 0.01 m (1 cm), respectively. Body mass index (kg/m2) was calculated as weight divided by height. Information about x-ray machine and CT scanners was obtained from labels on the machines themselves, while imaging protocols were obtained from control consoles during procedures.
For CT scan of the head, some aspects of the method of Sidi et al. [6] were adopted. Each breast had a TLD chip affixed at its mid-craniocaudal point and held in place by transparent adhesive tapes. In addition, a 30 × 30 cm gonad shield with 0.35 mm lead equivalent was used to shield the left TLD cum breast all through the investigations. This was with a view to compare scatters to both breasts. Computed tomography procedure was carried out by radiographers at the centre according to standard protocols. Subjects were positioned for the CT examination supine, with canthomeatal line (CML) at 90° to headrest and with an imputed azimuth of 90° and 180° for lateral and postero-anterior (PA) scout images. Axial (x-axis) centring beam on CT gantry intersected both meatuses, diverged slightly and emerged at infra-orbital margin. The TLDs needed stationary tubes for maximum quantum detection efficiency. Therefore, only scout images in PA and lateral projections with stationary tubes were involved. Popular CT protocol is one in which scout images are generated with the least exposure parameters. In keeping with that ideal, subjects were scanned with 80 kVp, 10 mA, but with tissue range of 140–250 mm depending on clinical indication.
For computed radiography of the head, subjects were examined either erect or supine according to standard radiographic procedure. This involved using a potter-bucky detector tray or a detector with a stationary grid manually attached. Subjects were positioned for postero-anterior (PA) and lateral views, with canthomeatal line (CML) being perpendicular (PA projection) or parallel (lateral projection) to the horizontal line of the detector tray, respectively. Upper limbs were extended from field of view (FOV) of radiation and employed for stability as well. Exposure parameters were 90–100 cm focus-detector-distance (FDD), variable tube current (mA/mAs) and tube potential (kVp) depending on body habitus. The TLD chips were affixed similar to the method used in CT. After scout projections in CT, and PA and lateral projections in CR, the TLD chips were carefully retrieved, packed and then sent for reading at the centre where they were initially acquired.
Data analysis
Data were analysed with statistical packages for social sciences, version 20.0 (SPSS Incorporated, Chicago, Illinois, USA). Descriptive statistical tools of frequency, mean and mode were employed to give a summary of machine and subjects’ parameters and dose outputs. Inferential statistical tools were also employed for analysis. A paired-sample t test was used to test for statistically significant difference in mean absorbed dose by both breasts while Pearson correlation analysis was used to test the strength of relationship between dose and biometric parameters. In the T test, difference found justified the necessity for torso shielding during radiographic examinations of contiguous anatomical regions. Level of significance was set at p ≤ 0.05.