ESR News December 2014
Mammographic screening and radiation risk
John Damilakis, Professor of Medical Physics
Several imaging techniques have been developed for breast cancer diagnosis. X–ray mammography is the most widely used modality for early detection and follow-up of lesions. Ultrasound examination, magnetic resonance imaging, magnetic resonance spectroscopy and positron emission tomography can provide additional information for the early diagnosis and characterisation of breast tumours. CT systems have been developed especially for the three-dimensional high-resolution imaging of the breast. However, X-ray mammography has been considered the ‘gold standard’ for screening of asymptomatic women. Screening mammograms are associated with low radiation dose and are capable of reducing breast cancer mortality considerably (1,2).
Yearly mammograms are recommended for asymptomatic women starting at age 40. Mammography screening usually involves two views of each breast. The mean glandular dose associated with the two-view examination is about 3 mGy. It is well known that the glandular tissue in the breast is very sensitive to radiation. Yaffe and Mainprize have found that “for a cohort of 100,000 women, each receiving a dose of 3.7 mGy to both breasts and who were screened annually from age 40 to 55 years and biennially thereafter to age 74 years, it is predicted that there will be 86 cancers induced and 11 deaths due to radiation-induced breast cancer” (3). When discussing the potential effects of radiation exposure, however, it is important to note that the true health impact of low-level radiation is unknown.
Although mammography is a low-dose technique, optimisation of protection, i.e. reduction of radiation dose without loss of diagnostic information, is of paramount importance. Important factors determining both radiation dose and image quality are the energy spectrum of the x-ray beam, breast composition and thickness, and the characteristics of the x-ray detector. Glandular dose increases with decreasing tube potential and increasing breast thickness. Scattered photons degrade image quality considerably. The use of an anti-scatter grid reduces scatter, but patient dose is increased. The use of automatic exposure control and proper breast compression are also important measures for reducing dose and improving image quality. Although patient radiation doses associated with most x-ray mammograms are low in comparison with those from other x-ray examinations, every facility should take action to avoid unnecessary patient exposure to radiation.
The new European Basic Safety Standards (4) advocate the establishment and use of diagnostic reference levels (DRLs). In mammography, DRLs are expressed in terms of Entrance Surface Air Kerma (ESAK) free-in-air or Entrance Skin Dose (ESD), or in terms of Mean Glandular Dose (MGD) estimated using a standard PMMA phantom. There is not much information on DRLs for mammography. National DRLs set by authoritative bodies in European countries were reviewed in 2010-11 in the Dose Datamed 2 (DDM2) project (5). There is a need to establish DRLs for mammography in all European member states, consolidate available information and provide guidance on what actions are needed in using DRLs to further enhance radiation protection of female patients.
1. Hellquist BN, Duffy SW, Abdsaleh S et al. Effectiveness of population-based service screening with mammography for women ages 40 to 49 years: evaluation of the Swedish Mammography Screening in Young Women (SCRY) cohort. Cancer 2011;117:714-722.
2. Tabar L, Vitak B, Chen TH, et al. Swedish two-country trial: impact of mammographic screening on breast cancer mortality during 3 decades. Radiology 2011;260:658-663.
3. Yaffe M and Mainprize J. Risk of radiation-induced breast cancer from mammographic screening. Radiology 2011;258:98-105
4. Council of the European Union. (2013). Council Directive 2013/59/Euratom laying down basic safety standards for protection against the dangers arising from exposure to ionising radiation, and repealing Directives 89/618/Euratom, 90/641/Euratom, 96/29/Euratom, 97/43/Euratom and 2003/122/Euratom. Official Journal L-13 of 17.01.2014.
5. European Commission (EC), 2014. Dose Datamed 2 (DDM2) Project Report Part 2: Diagnostic Reference Levels (DRLs) in Europe.