Fig. 30.1
(a) Right CC view shows an asymmetrical density in the outer half. (b) Representative slice from a tomosynthesis series shows normal glandular tissue with no underlying abnormality
Techniques
The basic principle of DBT is the acquisition of a three-dimensional block of data by taking a number of images of the breast at slightly different angles. This is achieved by moving the X-ray tube and detector in an arc whilst making a series of exposures. The dose for each exposure is relatively small, such that the overall dose is comparable to that from a conventional mammogram.
Each manufacturer has a slightly different approach to the actual method of acquisition. The number of exposures taken per view ranges from 9 to 25, taken over an arc of between 11° and 50°. The acquisitions may be either continuous or “step and shoot”. The continuous method, as the name implies, involves a smooth continuous movement of the tube and detector during image acquisition. This is faster than the “step and shoot” method but results in slightly lower image resolution due to an element of motion blurring. The “step and shoot” method involves multiple pauses during image acquisition and is more time-consuming but results in sharper images. However, as with conventional mammography, the longer the acquisition time, the greater the opportunity for blurring due to patient or equipment movement. The image acquisition time varies from 3 to 25 s per view. This will have to be taken into account when considering the throughput of patients, especially if DBT is to be used in large population screening programmes.
Once acquired, the images are processed prior to display. The commonly used reconstruction algorithms are filtered back projection and iterative reconstruction. Filtered back projection is normally used in CT scanning and is faster as it calculates the image in a single reconstruction step but is more susceptible to noise. Iterative reconstruction is more complex, but the additional processing time is of little significance with modern computer technology. It has the advantage of being less sensitive to noise and streak artefact.
Indications
As a Primary Screening Tool
A number of population-based screening studies have demonstrated excellent results with the use of tomosynthesis in combination with digital mammography. These studies have shown an increase in cancer detection rate of 9.5–40 % [2–4] with a significant reduction in false positive rates. Other studies [5, 6] have also shown that DBT plus digital mammography has higher diagnostic accuracy than digital mammography alone. Figure 30.2a, b show an example of cancer seen on digital mammography and DBT. It is worth reading the next Chap. 31, as two practitioners and a radiologist reflect on their experiences of using DBT in screening.
Fig. 30.2
(a) Right MLO view demonstrates a large area of architectural distortion in the retroareolar region which is confirmed to be an invasive ductal carcinoma on core biopsy. (b) Representative slice from a tomosynthesis series demonstrating spiculated masses in the retroareolar region and overlying the pectoralis muscle in keeping with multifocal carcinoma
Although these results [2–6] support the use of tomosynthesis as a primary screening tool, there are a number of other factors to consider:
1.
Cost. In countries with a national screening programme, such as the United Kingdom, replacement or conversion of all the mammography machines to systems with tomosynthesis capability would require considerable investment. Although competition between manufacturers may result in some price reduction, cost is likely to remain a significant factor for the foreseeable future. Furthermore, the large amount of digital data generated by DBT will take up a considerable volume of storage space on PACS, in many cases necessitating the purchase of additional capacity.
2.
Image interpretation time. Studies have demonstrated a significant increase in image interpretation time with the addition of tomosynthesis to digital mammography compared to conventional digital mammography alone (91 s vs. 45 s per case in the Oslo Tomosynthesis Screening Trial [2]). Another study reported an average increase of 47 % in the image interpretation time with the addition of tomosynthesis, although the additional reading time was less for those with greater reading experience [7]. However, there is a limit to how much the image interpretation time can be reduced owing to the number of images that must be viewed, even for those readers with considerable experience. This has to be taken into account when planning workload and staffing requirements.
3.
Dose. Most of the published research has focused on the advantages of using DBT in addition to conventional digital mammography. However, this approximately doubles the radiation dose compared to digital mammography alone. The dose varies slightly between the different manufacturers [8, 9] but the total mean glandular dose remains within the UK diagnostic reference level of 3.5 mSv per view. This is regarded as acceptable, given the benefits of increased cancer detection rates and reduction in recall rates, although a lower dose would be preferable. Software is now available which will synthetically reconstruct the projection images to create a 2D image from the tomosynthesis dataset, thereby avoiding the need for a separate 2D exposure. The overall dose is therefore comparable to conventional mammograms. Although earlier studies have shown a slight reduction in sensitivity, a recent study in a large population group has shown that DBT plus synthetic 2D images are comparable to DBT plus conventional digital mammograms [10]. Another large study is currently in progress to evaluate another strategy to reduce dose. The Malmo Breast Tomosynthesis Screening Trial aims to compare DBT with digital mammography but women in the trial will undergo two-view mammography and single-view DBT in the MLO projection. A similar study comparing one-view DBT plus one-view digital mammography to two-view digital mammography has shown better lesion characterisation with one-view DBT in combination with one-view digital mammography [11]. One-view DBT has also been shown to have better sensitivity and negative predictive value than digital mammography in women recalled from screening [12]. Based on current evidence, the UK National Health Service Breast Screening Programme (NHSBSP) guidelines recommend the use of two-view DBT in screening assessment women [13].
For Further Assessment of Mammographic Abnormalities
Assessment of Asymmetry, Distortions or Masses
The current recommendation from the UK NHSBSP is that DBT can be used for further assessment of women with screen-detected asymmetry, distortions or masses. The Hologic Dimensions is the only DBT system currently approved for this within the United Kingdom [13]. Other manufacturers’ systems are currently being evaluated. A study by Michell et al. has shown that the addition of DBT increases the diagnostic accuracy in the assessment of screen-detected soft tissue abnormalities [14]. Other studies have demonstrated that DBT is at least as accurate as spot compression view in the assessment of non-calcified abnormalities [15, 16]. Zuley et al. demonstrated that DBT significantly improves diagnostic accuracy by better characterisation of the lesions in comparison to supplemental mammographic views [17]. DBT has also been shown to be superior to digital mammography in estimating tumour size [18, 19]. Although there are no clear guidelines for its use in symptomatic patients, it is expected that further assessment of indeterminate or suspicious mammographic abnormalities with DBT is likely to have the same benefits as in screening patients.
Assessment of Microcalcification
Most published studies have shown that DBT has an equivalent performance to digital mammography in the assessment of microcalcification, although overall it appears to offer no particular advantages. Furthermore, in the population-based Oslo Tomosynthesis Screening Trial by Skaane et al. [2], there was no increase in the detection rate of ductal carcinoma in situ. In view of its higher dose, therefore, the UK NHSBSP [13] has advised that DBT should not be routinely used for the assessment of calcification. This advice may change with further improvements in technology.
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
