4.3 Mammography Dosimetry
4.3.3 Mammographic Effective Dose and Effective Risk
The internationally accepted method for estimating the risk from an X-ray procedure is to use effective dose. Effective dose has enabled doses to be summed from whole and partial body exposure from external radiation of various types to estimate the risk of cancer development (ICRP, 2007). As recommended by the ICRP, effective dose is not suitable for epidemiological evaluations or the assessment of individual exposure and risk, but can be used as a radiation protection quantity by comparing it with reference values. The calculation of the effective dose depends on tissue weighting factors which are regularly updated by the ICRP based on the available evidence from epidemiological data (Nuclear Energy Agency [NEA], 2011). Since the ICRP considered that it is more suitable for radiation protection calculations to utilise averaged gender and age tissue weighting factors, effective dose does not take into account an individual‘s age and gender. Accordingly, Brenner (2008) recommended the replacement of effective dose by effective risk.
Effective risk is a useful quantity which was originally proposed by Brenner (2008). It is a more suitable quantity for epidemiological assessment of radiation risk than effective dose (Brenner, 2012). Effective risk is a good indicator of the radiation dose that the patient received (Brenner & Huda, 2008). In contrast to effective dose, which averages cancer incidence, cancer mortality, life shortening, and heredity risks, the only focus of effective risk is cancer incidence arising from the exposure to ionising radiation. The effective risk of developing cancer is less for people who have 20 years to live compared to those who have 60 years, because it is related to tissue specific, age specific, and gender specific lifetime attributable risk (LAR). Therefore, the calculation of effective risk involves summing the products of age, and gender lifetime-attributable risk of cancer incidence, per unit equivalent dose for each type of tissue and the dose received by that tissue. The effective risk calculation is not more complicated or difficult than the calculation of the effective dose.
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However, the data from the effective risk calculation are more understandable to the general public than that produced by effective dose calculation (Brenner, 2012).
Brenner‘s recommendation (Brenner, 2008) to replace effective dose by effective risk has been criticised by Dietze, Harrison, and Menzel (2009) for a number of reasons. Firstly, they consider that the continuous change in tissue weighting factor is not a reasonable criticism for the ICRP because the continuous update of tissue weighting factors increases their reliability. Secondly, with regard to age, they stated that the ICRP discussion in their recommendations (2007) suggests the need to find another alternative quantity to consider the individual‘s age, but not as a replacement for effective dose. Finally, Dietze et al. (2009) suggest that the effective risk is not suitable for all radiation protection applications such as the assessment of radiation dose received by astronauts. Also, Huda was not enthusiastic about the introduction of effective risk because he considered the main advantage of the effective risk calculation is to compare different types of non-uniform exposures qualitatively. However, this can be achieved by comparing the effective dose of a specific procedure with that of annual background radiation (Brenner & Huda, 2008). In fact, effective risk is a more suitable quantity for the evaluation of radiation-induced cancer from screening mammography than effective dose because this examination is continuously repeated at different ages during women‘s lifetime. Accordingly, effective risk is adopted into this thesis as the main tool for assessing radiation risk from screening mammography when comparing different mammography screening programmes.
Before the suggestion of effective risk by Brenner (2008), some investigators used radiation- induced cancer as a dosimetric measure for radiation risk assessment of each organ separately. For example, Sulieman et al. (2007) calculated the risk of radiation-induced cancer in thyroid, testes and ovaries from paediatric micturating cystography using the direct surface dose measurement by TLDs placed on the child‘s skin. The radiation-induced fatal cancer for each organ (thyroid, ovaries, and testis) was then calculated using ICRP 60 LAR factors. However, Perisinakis et al. (2001) calculated the risk of radiation-induced cancer in all body organs from radiofrequency catheter ablation procedures by multiplying the effective dose by the total LAR reported in BEIR V report by National Academy of Siences. In the work of Perisinakis et al. (2001), organ radiation doses were measured using a Rando
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phantom loaded with dosimeters. Although many researchers used the effective risk concept, after Brenner‘s proposal for radiation risk assessment from different radiographic examination, they did not use the term ―effective risk‖, see Table (4-2). It can be seen that all the studies in Table (4-2) used the term lifetime attributable risk (LAR), and this may result in misunderstanding as to whether it refers to cancer incidence in specific tissue or in all body tissues. However, only Li et al. (2011) used the term risk index to indicate the effective risk.
The lifetime attributable risk of radiation-induced breast cancer from mammography has been calculated by Hendrick (2010) and more recently by Yaffe and Mainprize (2011). Hendrick (2010) calculated the incidence and mortality of radiation-induced cancer from mammographic imaging procedures. Yaffe and Mainprize (2011) assessed the LAR of radiation-induced breast cancer following mammography at different client ages. Beemsterboer, Warmerdam, Boer, and de Koning (1998) and Freitas-Junior, Correa, Peixoto, Ferreira, and Tanaka (2012) justified screening mammography with regard to breast cancer mortality reduction and risk of radiation-induced breast cancer from screening mammography in the Netherlands and Brazil, respectively. In general, both studies found that radiation risk from mammography was small and that the benefits outweigh the risks. The number of lives saved by early screening mammography in BRCA mutation carriers was compared to the number of radiation-induced breast cancer by Berrington de Gonzalez, Berg, Visvanathan, and Robson (2009) and Jansen-van der Weide et al. (2010). Berrington de Gonzalez et al. (2009) concluded that there was no net benefit of early screening before 35 years of age for BRCA mutation carriers. However, there was a 1.3 fold additional breast cancer risk due to the exposure of high breast cancer risk women to low radiation doses reported by Jansen-van der Weide et al. (2010).
Nevertheless, the above authors did not progress their work to include effective lifetime risk of radiation induced-cancer from screening mammography; they only considered the examined breast radiation risk. Therefore, in this thesis the effective risk of radiation-induced cancer from screening mammography will be assessed, for different female ages and country-based screening programmes, considering the radiation dose received by all body tissues in addition to the examined breast.
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Table (4-2) Lists studies that utilised effective risk for radiation-induced cancer assessments from different radiographic examinations.
Study Author Examination Study Details
Huang, Law, and Khong (2009)
Whole-body PET/CT
Rando phantom and TLDs used for organ dose measurement. LAR calculated using BEIR VII method utilised for total LAR calculation.
Griffey and Sodickson (2009)
Emergency multiple or repeated CT
Cumulative effective dose and BEIR VII report total LAR were used.
Huang et al. (2010)
ECG-gated coronary CT angiography
ImPACT Monte Carlo software used for organ dose estimation then LAR calculated using BEIR VII method utilised for total LAR calculation.
Li et al. (2011) Paediatric chest CT
Monte Carlo simulation for organ dose estimation. The risk index calculated using Brenner‘s equation.
Johnson et al. (2014)
Children with heart disease imaging
Child ATOM phantom used for effective dose calculation. Then total LAR of all cancers from BEIR VII multiplied by effective dose. Seo et al. (2015) Neck X-ray
radiography
PCXMC and BEIR VII LAR factors were used to calculate total LAR.
Law et al. (2016) Spine radiography for scoliosis patient
PCXMC and BEIR VII LAR were used to calculate total LAR during patient lifetime.