ReviewReview of radiation dose estimates in digital breast tomosynthesis relative to those in two-view full-field digital mammography
Introduction
Digital breast tomosynthesis (DBT) has been shown to improve mammographic accuracy [1], [2], [3], [4], [5] and has emerged as a feasible replacement or adjunct technology to full-field digital mammography (FFDM). DBT reconstruction results in pseudo-tomographic images with partial blurring of features outside the selected plane, resulting in a significant reduction of the overlapping tissue effect present in conventional mammography. DBT is increasingly being used as a diagnostic imaging device, is used for screening in some settings in North America and is also being evaluated for population-based screening programs in many countries. Initial results from screening trials have been promising. Increase in breast cancer detection rates of 10%–53% has been achieved often at recall rates reduced by 20%–59% relative to FFDM [1], [6], [7], [8], [9], [10]. The additional breast cancers have been found in patients of different ages and breast density types, implying a potentially broad role for DBT. A high proportion of the DBT-detected cancers have been reported to be invasive carcinomas, which also indicates a potential impact for DBT in mammography screening.
In DBT, the X-ray tube rotates over a limited angular range and a low dose exposure of the compressed breast is acquired every few degrees. The average absorbed dose to the glandular tissues (AGD) is the summation of absorbed doses in the fibro-glandular tissue of the breast from all the multiple low-dose projection images. The concept of low-dose imaging in tomosynthesis has been made feasible due to the development of digital detectors with rapid read-out capabilities, high dose efficiency (high detector quantum efficiency; DQE) and low noise. The projection images become clinically useful as the reconstructed image information is additive. Tomosynthesis imaging includes multiple parameters that may influence the resulting breast dose. The angular range and number of exposures acquired during a scan are specific to the design of a system and thus these parameters are the same across acquisitions for a particular unit. Different manufacturers of DBT units have adopted quite different settings for these parameters, which are also associated with the detector type used and its design, and whether it is stationary or movable. Typically, the number of images acquired ranges from approximately 10 to 25, whereas the angle ranges from about 10 to 50° [11]. The tube loading, voltage and, in some cases, the anode/filter combination are, as in mammography, parameters, which are specific for the individual breast. In clinical units, these parameters are determined by the automatic exposure control (AEC) according to the characteristics of the imaged breast (e.g. breast thickness, glandular composition) so they will vary between acquisitions. In early clinical tomosynthesis studies, before AEC was implemented, the radiographer set these parameters manually using a technique chart. In DBT, the dosimetric effects of using different combinations of acquisition parameters are relatively well known [12], [13], [14], [15]. As the female breast is a radiosensitive organ and because tomosynthesis has been introduced into the screening setting, the radiation absorbed dose to the breast is of special concern. Diagnostic Reference levels (DRLs) were introduced by the International Commission on Radiological Protection (ICRP) as a practical guidance in the management of patient doses in radiology [16], [17]. In North America, FDA standards are outlined in the Mammography Quality Standard Act (MQSA), which set a breast dose restriction of 3 mGy per acquisition of the American College of Radiology (ACR) phantom [18]. To ensure that patient doses in tomosynthesis are within established recommendations or limits, similar absorbed dose levels should be pursued as is currently used in FFDM, although this should not compromise any benefit in clinical performance.
The purpose of this paper is to review and summarize absorbed doses reported in clinical studies using DBT and FFDM and describe the dose contribution from DBT relative that from FFDM.
Section snippets
Review of dose settings and dose estimates
A literature search was performed in reports of clinical studies on breast cancer detection comparing tomosynthesis and full-field digital mammography (FFDM), and which included absorbed dose estimates at FFDM and DBT using equipment developed by different manufacturers and thus of various designs (PubMed search: April 2008 to August 2014; literature search was performed by TS). Information was extracted on how patient-specific acquisition parameters were set and how dose was estimated, if
DBT systems
There were 17 papers found that matched the literature search criteria. These included the use of five different types of DBT units (from GE HealthCare, Siemens, Xcounter, Sectra and Hologic; see appendix for a description of their design). The studies were almost exclusively performed in an experimental or early clinical application setting. All DBT systems were of investigational design (i.e. prototype units) except one that was a clinical unit, the Hologic Selenia Dimensions. The DBT systems
Discussion and conclusion
Evidence on the clinical performance of DBT is rapidly growing, as is the clinical application of this new technology for imaging the breast. This necessitates careful consideration of potential radiation safety issues. Absorbed dose levels for DBT and FFDM in clinical studies (2008–2014) were therefore reviewed and summarized in terms of the relative dose contribution from DBT to that of FFDM. The dose estimates indicate that when tomosynthesis was used as a stand-alone technique, in one or in
Conflict of interest statement
None declared.
Acknowledgement
One of the authors (N.H.) was supported by a National Breast Cancer Foundation (NBCF Australia) Practitioner Fellowship (PRAC-13-01), while another author (I.S.) was supported by the National Cancer Institute (R01CA163746) and the Susan G. Komen Foundation for the Cure (IIR13262248).
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