Study design

This study was a prospective, cross-sectional study that involved 125 female adult patients that underwent CT examination of the head over a one year period in two CT centres in the Niger Delta region of Nigeria labelled as G1 and G2, respectively.

Study technique

The protocol of the CT examination of the head was unenhanced scans (done without administration of contrast agent) and enhanced scans (done following intravenous administration of contrast agent). The relevant information for the study was obtained from the records of the patients (age, weight, height), while parameters collated from the console of the CT scanners during the scan of each of the patients were age at exposure, volume computed tomography dose index (CTDIvol) and dose-length product (DLP) values.

Eligibility criteria

The eligibility criteria included all adult female patients who were referred for head CT examination in the two centres during the study period and whose records had the relevant information required for the study. Also eligible for inclusion into the study were only the female patients whose weight were within 70 ± 10 kg [14] and those who had information on the technical parameters displayed on the CT console.

Study size

The sample size for this study was based on the European commission recommendation of a minimum of sample size of 10 standard-sized patients [14]. However, data from more than 10 patients in each CT examination was used to broaden the base of the study and increase the statistical relevance of the data.

The technical characteristics of the CT scanners are described in Table 1. Scan protocols for the head examinations were set at a potential tube voltage of 120kvp for all the patients in both centres. The tube current, slice thickness, pitch, rotation time and scan length varied among the two centres as displayed in Table 2.

Table 1 Technical characteristics of the CT scanners
Table 2 Technical parameters of the CT Scanners for head examinations

Estimation of effective dose

The effective dose of all the patients was calculated using the expression in Eq. 1:

$${text{Effective}};{text{dose }} = {text{ DLP }} times , k$$

(1)

where k is the conversion coefficient based on the head (k = 0.0021 mSvm Gy−1 cm−1 for head) [15].

Estimation of lifetime attributable risk (LAR) for breast cancer incidence and mortality

Estimation of LAR for breast cancer incidence and mortality was based on the effective dose, patient’s sex and age at exposure using the BIER VII report. This was estimated from tables 12D-1 and 12D-2, respectively, documented in the BEIR VII report (displayed as Tables 3 and 4 in this study) for each patient based on the age of the patient at exposure and sex using an equivalent dose of 0.1 Gy [11]. Data for specific ages that were not available in the tables were linearly interpolated using the nearest two ages in the tables.

$${text{LAR}}_{{{text{at }};{text{age }};{text{of }};{text{exposure}}}} = frac{{Effective ;dose ;left( {Sv} right)}}{0.1} times frac{{LAR left( {cancer; incidence} right) ;at; age ;of; exposure}}{100,000}$$

(2)

$${text{LAR}}_{{{text{at}};{text{ age }};{text{of}};{text{ exposure}}}} = frac{{Effective ;dose; left( {Sv} right)}}{0.1} times frac{{LAR; left( {cancer ;mortality} right) ;at ;age; of; exposure}}{100,000}$$

(3)

The categorization of the risk level for LAR for breast cancer incidence and mortality displayed in Table 5 was determined as documented in the BIER VII report.

Table 3 Lifetime attributable risk of cancer incidence [11]
Table 4 Lifetime attributable risk of cancer mortality [11]
Table 5 Additional lifetime risk of fatal cancer from radiodiagnostic examination [11]

Statistical analysis

The mean, standard error of mean and standard deviation values were analysed for the effective dose, while, the minimum, maximum and mean values of the LAR incidence and mortality of breast cancer in the patients were analysed using statistical package for the social sciences (SPSS) version 22.0 (SPSS Inc., Chicago, IL, USA). Descriptive and independent sample t test was used at a 95% level of significance for the effective dose. P < 0.05 was considered statistically significant.

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