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 Table of Contents  
ORIGINAL ARTICLE
Year : 2021  |  Volume : 28  |  Issue : 4  |  Page : 278-284

Thyroid dose and cancer risk from head and neck computed tomography at two selected centres in Nigeria


1 Department of Basic Sciences, Physics Electronics Unit, Babcock University, Ilishan-Remo; Department of Physics, University of Ibadan, Ibadan, Oyo State, Nigeria
2 Department of Physics, University of Ibadan, Ibadan, Oyo State, Nigeria
3 Department of Radiology, University College Teaching Hospital, Ibadan, Oyo State, Nigeria
4 Department of Basic Medical Sciences, College of Health Sciences and Technology, Ijero-Ekiti, Nigeria

Date of Submission17-Jul-2021
Date of Decision13-Oct-2021
Date of Acceptance02-Nov-2021
Date of Web Publication29-Nov-2021

Correspondence Address:
Dr. Ademola Joseph Adekanmi
Department of Radiology, University College Teaching Hospital, Ibadan, Oyo State
Nigeria
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/npmj.npmj_611_21

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  Abstract 


Objective: The objective of this study was to evaluate the thyroid glands' radiation dose and the risk of thyroid cancer induction from head or neck computed tomography (CT) examinations. Methods: In a prospective study, we evaluated all participants of all ages and sex referred for Head or Neck CT Scan at the University College Hospital, Ibadan and Me Cure Healthcare Limited, Ibadan, Oyo State, Nigeria. Thyroid radiation dose was estimated with impact scan calculator, and real-time dose measurement with thermoluminescent badge dosimeters (TLDs). Data were analysed and P < 0.05 was considered statistically significant. Results: One hundred and sixty-three participants (128 adults and 35 children) participated in the study. In most participants (74%), the tube voltage was 120 kVp. The estimated median thyroid gland dose by the imPACT scan calculator was 4.95 mGy (range = 1.20–30.0 mGy) and 4.40 mGy (range = 3.0–5.10 mGy), while the real-time dose measured by the TLD was 4.79 mGy (range = 1.73–96.7 mGy) and 2.33 mGy (range = 1.20–3.73 mGy) at Centre A and B, respectively. The estimated median thyroid cancer risk was 2.88 × 10−6 (maximum range of 52 × 10−6) at centre A and a median value of 3.20 × 10−6 with a cancer risk estimate that may reach 17.9 × 10−6 recorded at centre B, compared to a cumulative thyroid cancer risk of 0.12 × 10−5 among the general Nigerian population. Conclusions: Scanner specifications and technique may significantly contribute to variations seen in thyroid radiation doses. There may be a need to optimise centre protocols and apply dose reference levels for head and neck CT examinations to reduce thyroid cancer risk in Nigeria.

Keywords: Computed tomography, imPACT scan calculator, ionising radiation, Monte Carlo, paediatric, thermoluminescent dosimeters, thyroid dose


How to cite this article:
Awe OO, Obed RI, Adekanmi AJ, Ogbole GI, Agbele AT. Thyroid dose and cancer risk from head and neck computed tomography at two selected centres in Nigeria. Niger Postgrad Med J 2021;28:278-84

How to cite this URL:
Awe OO, Obed RI, Adekanmi AJ, Ogbole GI, Agbele AT. Thyroid dose and cancer risk from head and neck computed tomography at two selected centres in Nigeria. Niger Postgrad Med J [serial online] 2021 [cited 2022 Dec 2];28:278-84. Available from: https://www.npmj.org/text.asp?2021/28/4/278/331531




  Introduction Top


The use of computed tomography (CT) for clinical diagnosis has brought a tremendous revolution to X-ray imaging by providing high-quality cross-sectional images. Over the years, the number of examinations performed with CT have substantially increased with a great impact on patient care although with a corresponding increase in the risk and exposure to X-rays during medical examinations.[1] Nowadays, CT scan for medical evaluation accounts for up to 40% of the resultant collective dose from diagnostic radiology in some European Union.[2]

Although CT scans are clinically important, associated ionising radiation may pose a potential cancer risks, especially in children who are more radiosensitive than adults.[3],[4]

The significant rise in the global effective radiation dose annually from diagnostic medical and dental radiation exposure of 0.62 mSv (20%) in 1997–2007 compared with the 0.4 mSv in 1991–1996,[5] and the possible radiation cancer induction to irradiated organs among patients is a matter of concern that needs to be studied.

Among the body's tissue exposed to radiation during head and neck CT examinations, the thyroid gland is a sensitive and highly exposed organ, particularly in the childhood period.[6] In general, the increased prevalence of thyroid cancer has been reported and attributed to increased thyroid cancer detection by medical imaging. Moreover, exposure to ionising radiation is now a major thyroid cancer risk factor, particularly papillary thyroid carcinoma.[6],[7] While thyroid radiation and thyroid cancer induction risk have been published in the developed world,[6] there is a paucity of similar data, despite increasing CT availability in developing countries such as Nigeria.

Some authors in Nigeria have reported selected organ doses from CT studies among the adult population.[8],[9],[10],[11],[12] However, many of the studies did not focus on the thyroid gland. Furthermore, there is a lacuna as regards the thyroid gland cancer risk from CT examinations in our environment. The Nigerian National Cancer Observatory Report of 2019[13] documented the 5-year prevalence of thyroid cancer to be 1.6 × 10−5, and the cumulative thyroid cancer risk in people living up to 75 years reported as 0.12 × 10−5, from all causes.

Therefore, to further explore the potential cancer induction risk to the thyroid gland, we directly investigated the thyroid dose and thyroid cancer risks among paediatric and adult participants referred for head or neck CT scans at two centres in Ibadan, South-west Nigeria. This study will not only provide data in Nigerians, it will also sensitise physicians to the risk of thyroid cancer that may result during head or neck CT studies.


  Methods Top


This was a prospective study carried out in the city of Ibadan, South-west Nigeria. Ibadan is the third-most populated city in Nigeria, with an estimated population of about 3 million people, according to the 2015 estimate.[14] This research was conducted following the principles of the Helsinki declaration (Helsinki, 1978). The study data were obtained between February and September 2016. Further work was done on it in 2019 at two centres, namely University College Hospital, Ibadan (Centre A), and Me Cure health care limited, a private diagnostic centre in Ibadan (Centre B).

A convenient sampling method was employed to include all eligible patients referred for head and neck CT examinations during the study duration at both centres. Patients with follow-up head and neck examinations were excluded from this study. A total of 163 participants of all ages referred for head and neck CT scan from centre A (126) and centre B (37) were included in this study.

All requests were reviewed and justified by an experienced radiologist and informed consent was taken from participants or caregivers before enrolment into the study.

The scanners used were; Toshiba Aquilion 64 slice, manufactured in 2009 at CENTER A and commissioned for use in 2011; Toshiba Astion CT, manufactured in 1995 and commissioned for use in 2012 at centre B. The two scanners passed all quality assurance tests before commissioning. Lithium fluoride thermoluminescent dosimeters (TLD–100) and Harshaw 4500 TLD readers were used.

An imPACT scan datasheet software was also used to review all obtained data in patients.

After obtaining consent, enrolled participants' biodata and anthropometric information were documented into a prepared datasheet. All participants changed into a gown and were positioned supine on the gantry table with their head in the gantry using the standard positioning protocol for the head or neck CT examination, in line with the request on the request cards. CT contrast, tube voltage, collimation pitch, tube current (mA), exposure time, scan range, the field of view, volume computed tomography dose index (CTDIvol) and the dose length product (DLP) were adjusted as necessary.

To include CT scanners that were not captured in the Monte Carlo data sets. The imPACT CTDosimetry spreadsheet data with National Radiological Protection Board (NRPB) SR250 dose data sets CTDosimetry (version 1.0.4) was used to allow for newer CT matching and modeling various conditions of exposure. The CT tube current, rotation time and spiral pitch were manually inputted into the CTDosimetry spreadsheet.

The thyroid radiation dose was calculated in milligrays and recorded for each participant.

In this study, 50 randomly selected participants also have real-time thyroid gland absorbed doses calculated from the reading obtained from the TLDs. An annealled TLD was placed anteriorly on the participant's neck at the level of the thyroid gland (6th cervical vertebral level) and held in place by transparent adhesive tape. The background radiation was checked, which is negligible due to the necessary shielding in the examination room but noted in each case. The TLDs were carefully removed immediately after each patient's CT examination and enclosed in a transparent cellophane bag labelled accordingly with a code and number for each participant for confidentiality purposes. The TLDs were enclosed in a black bag to exclude spurious background exposure. Furthermore, the calibration and reading of the TLDs were done in line with the method described by Jansen et al.,[15] at the Nigerian Nuclear Regulatory Authority office in Ibadan, Oyo State, Nigeria. The deep organ dose expressed in milligray was recorded as the thyroid dose the participants.

Thyroid cancer induction risk calculation

The risk of thyroid cancer induction risk from the head and neck CT was calculated from the individual organ dose multiplied by the age and sex-appropriate radiation risk coefficient.[16],[17] For this study, we used the radiation risk coefficient of the thyroid as presented by Balonov and Shrimpton [Table 1].[18]
Table 1: Radiation risk coefficients (10-3/Gy) (r) of the thyroid for males and females by age at exposure by Balonov and Shrimpton[19]

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Data were entered and analysed with the Statistical Package for the Social Sciences (SPSS) software version 20.0 (IBM SPSS, Chicago, IL, USA). Descriptive statistics were reported using frequencies, proportions, median and range of values in tables and charts. Mann–Whitney test was used to test for differences in the organ dose of the thyroid, TLD absorbed dose to the thyroid and cancer risk between participants' characteristics. Scatter plots were used to show the relationship between organ dose to the thyroid, TLD absorbed thyroid dose, cancer risk and participants' age. P < 0.05 was considered statistically significant for this study.


  Results Top


There were more male participants (55.2%), and more than three-quarters of the participants were adults, of which about 19% were 65 years or older. Head CT examination was more common among the participants (87.3%), and the majority of the examinations (72.4%) utilised a voltage of 120 kVp and a median current of 200 mAs (range-24.0 mAs to 277 mAs [Table 2].
Table 2: Demographics and computed tomography scanning parameters and radiation dose in the study population

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The median organ dose of the thyroid among adult subjects by iMPACT scan and TLD measurements were higher at centre A than at centre B, as presented in [Table 2].

However, the estimated cancer risk from the head and neck CT although slightly higher in centre A, showed no statistically significant difference compared with centre B (P = 0.203) [Table 3].
Table 3: Comparison of the thyroid absorbed dose measured by IMPACT scan calculator and thermoluminescent dosimeter among adults at the two centers

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There was no significant difference between the iMPACT scan and TLD thyroid absorbed dose in male and female participants. P value was 0.837 and 0.689, respectively. Furthermore, the median estimated cancer risk was 7.60 × 10−6 (Range: 0.25 × 10−6; 52.0 × 10−6) in females and in males 1.65 × 10−6 (Range: 0.20 × 10−6; 30 × 10−6) P < 0.001 in males [Table 4].
Table 4: Sex comparison of the organ dose of the thyroid by the iMPACT scan and thermoluminescent dosimeter among the study participants

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Evaluating the radiation dose and cancer risk to the thyroid from head CT versus neck CT, the median organ dose of the thyroid from head and neck CT examinations by iMAPCT scan were 4.60 mGy (Range: 1.20 mGy; 24.0 mGy) and 9.90 mGy (Range: 2.50 mGy; 30.0 mGy), respectively, P < 0.001. Similarly, the median thyroid absorbed dose by TLD from head and neck CT examinations was 3.84 mGy (Range: 1.53 mGy; 90.0 mGy) and 45.5 mGy (Range: 2.98 mGy; 96.7 mGy), respectively, P = 0.036 [Table 5].
Table 5: Comparison of organ dose of the thyroid and cancer risk from head computed tomography and neck computed tomography

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The median absorbed dose to the thyroid was 2.80 mGy (Range: 1.20 mGy; 14.0 mGy) in children from age 0 to 9 years and 5.05 mGy (Range: 2.50 mGy; 19.0 mGy) in adults ≥65 years while the median estimated cancer risk was 5.10 × 10−6 (Range: 1.44 × 10−6; 18.7 × 10−6) in children from age 0 to 9 years and 0.34 × 10−6 (Range: 0.20 × 10−6; 4.40 × 10−6) in adults ≥65 years [Table 6].
Table 6: Estimated lifetime thyroid cancer risk versus age group of the study population

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In general, there was no significant difference between the median organ dose of the thyroid (imPACT) (4.45 mGy, IQR: 3.20 mGy; 4.90 mGy) and the median absorbed dose to the thyroid by TLD (3.93 mGy, IQR: 2.40 mGy; 5.30 mGy), P = 0.474.

The scatter plot of iMPACT scan absorb dose and age of participants showed a tendency for the thyroid absorbed dose to increase with the participants' age R2 = 0.046 P = 0.006. However, the absorbed dose to the thyroid by TLD badge showed no significant linear relationship between age and absorbed dose to the thyroid gland, P = 0.530, as shown in [Figure 1] and [Figure 2], respectively Furthermore, there was a tendency for the estimated cancer risk to decrease with the participants' age R2 = 0.192 P < 0.001 [Figure 3].
Figure 1: Scatter graph showing a positive linear relationship between organ dose of the thyroid and participants age

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Figure 2: Scatter graph showing no relationship between TLD thyroid dose and participants' age

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Figure 3: Scatter graph showing a negative correlation between lifetime cancer risk and participant's age

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  Discussion Top


The thyroid gland absorbed doses from head and neck CT examinations were estimated with TLD badges and imPACT scan calculator using the CT machine dose output summary.

We observed that the voltage used for all CT examinations was either 100 kVp or 120 kVp. The tube current was between 75 mAs and 225 mAs in the two study centres. The higher tube current would translate to better image quality, however higher patient dose.[20] We observed that 120 kVp was used at the two centres for most examinations, presumably since more adults were in this study implying that technologists still adhere to the age-long recommendation of 120 kVP. Recent studies have advocated for low kVp (80–100 kVp) in CT examinations to reduce the radiation dose since radiation dose reduces with the square of the kVp used.[21]

DLP-based approximation of absorbed dose obtained from CT dosimetry software using the ICRP 60 was less than the TLD-based thyroid absorbed dose result.

Expectedly, centre A results of thyroid absorbed dose were higher than the result obtained in centre B. The reasons for the observed difference might have resulted from the fact that cases in centre B were mostly due to head injury, suspected brain infarction, or haemorrhagic cerebrovascular disease that required no post-contrast scan. On the other hand, cases referred for head or neck CT scan in centre A were for numerous clinical reasons and required post-contrast scans and volumetric scans, to better delineate lesions for precise diagnosis, took longer time and increased radiation dose. Furthermore, there was no significant difference in the result obtained from the imPACT organ dose compared to the TLD, indicating that the patients' radiation dose is within the X-ray dose output expected from the CT scanners in this study. There is a need for further elaborate study on using TLD chips to investigate absorbed dose during CT examination.

Furthermore, we observed that the absorbed dose to the thyroid gland was higher from the neck CT scans than the head CT examination, as the thyroid gland is directly exposed to radiation in neck CTs, while the absorbed dose to the thyroid from head CT scans is from scattered radiation. However, the thyroid cancer induction risk was generally low in this study in both children and adults (<10−4), in agreement with the work of Balonov et al.[17] The thyroid cancer induction risk was much higher in children and relatively low in elderly participants. This observation is in accordance with the reports of previous researchers.[7],[18] In addition, the significantly higher thyroid cancer risk in females than males despite no significant difference in organ dose between males and females with similar age distribution, is due to higher thyroid cancer risk coefficients in children and females than in adults and males.

Although there is a significant difference in the thyroid absorbed dose in neck CT scan compared to head CT, there was no statistically significant difference in the thyroid cancer risk among the patients. This observation may result from the age and gender dependency of the risk coefficients in this study in agreement with Balonov and Shrimpton.[18] Although there were no significant differences in absorbed dose. In contrast, the cancer risk differs; we infer that age affected the relationship between thyroid cancer induction risk and the thyroid absorbed dose. None of the paediatric patients had neck CT. However, 23.9% of the patients who had head CT scans were children. The proportion of children who had head CT might have increased the estimated cancer risk, thereby responsible for the non-significant difference in cancer risk compared to those with neck CT. In addition, many previous studies have also validated thyroid radiation cancer risk estimations dependency on age and gender.[19],[22],[23],[24],[25] Furthermore, Tipnis et al. in a study to estimate absorbed thyroid dose and consequent cancer risks in adult patients undergoing neck CT examinations, reported that in estimating the thyroid radiation cancer risk, the most important factors are patient age and sex, with the absorbed dose to the thyroid being a secondary factor.[26]

Another major observation in this study was a wide range of absolute values for the CTDIvol, DLP, and radiation dose for the thyroid gland; this is probably due to patient factors and the lack of national dose reference values in Nigeria and varying values of tube parameters used by the technicians.

In this study, the correlation test showed that the iMPACT scan calculated absorbed dose to the thyroid from the CT parameters have a relationship with patients age with adults having higher doses than children. However, correlation of absorbed dose by the TLD and age was not done as no child was included in the TLD dose estimation.

It is noted from the ALARA principle that the population should only have radiation that is just adequate and where possible diagnostic procedures involve nonionising radiation should always be considered above ionising radiation.[27] There is a need to consider the total diagnostic CT procedure performed on an individual because this could get to a meaningful dose level over time. The risk of children undergoing CT scans in their lifetime should not be trivialised since they have a longer life span than an average adult. There is a need to properly calibrate patient exposure to CT procedure by proper settings per examination, and where possible, have appropriate CT dose parameters for different paediatric age groups.

There is a need to include various radiation reduction techniques in CT imaging and the formulation of standardised national dose reference levels to avoid unnecessary radiation exposure.

There are some limitations in this study. First, the study design employed was a non-probability sampling method (convenience sampling), hence caution should be taken in interpreting results in the general population. Furthermore, we used the Balonov and Shrimpton[18] radiation risk coefficient for the thyroid gland in diagnostic radiology (low radiation exposure) modelled from high radiation exposure, the best available model, to predict the absolute cancer risk, as there are no direct conclusive and ample data of stochastic effects at low radiation exposure levels.[28] Finally, only 50 participants (all adults) in this study had a TLD thyroid radiation dose estimated.

We recommend a multicentre study in Nigeria to validate the results from this current study.


  Conclusion Top


The thyroid dose values reported appear higher than those seen in western literature. Apart from age, gender and direct exposure of organs, the scanning parameters and CT specifications contribute to the ultimate radiation dose to the patient. There is a need to standardise imaging protocols across centres in Nigeria. Application of newer radiation reduction techniques should be advocated to reduce the risk of thyroid cancers from the head and neck CT examinations.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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  [Figure 1], [Figure 2], [Figure 3]
 
 
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