Dr. Govindarajan MJ writes on Digital Breast Tomosynthesis (DBT) and its effectiveness as an imaging tool that reduces the masking effect of overlapping fibroglandular tissue, and improving breast cancer detection.

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Screening mammography is a well-accepted method of screening for breast cancer across the globe. Multiple randomised controlled trials and observational studies have shown that screening mammography can reduce breast cancer mortality by 30% or more despite the limited sensitivity and specificity due to the fibroglandular tissues masking or mimicking cancers. The accuracy of mammography has improved since early 2000s, with the conversion from analog film-screen mammography to full-field digital mammography (FFDM) and the results more evident in women with dense breasts.

DBT is yet another advancement of digital mammography (DM) technology where in multiple tomographic images can be obtained in any conventional mammographic view, reconstructing a semi 3-dimensional mammogram which is also called 3D tomosynthesis. This allows visualization of a sequential stack of thin images (also called slices) of the breast tissues, significantly reducing the ‘masking effect limitation’ of the 2D images.

Two dimensional (2D) radiographic image is actually a summation image which is displayed on a single plane as a visible representation of any structure which is traversed by the X-ray beam between input and output surfaces; this leads to poor visualisation of a few structures or lesions due to superimposed denser structures (masking effect) resulting in false negative (FN) interpretation.

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Alessandro Vallebona, an Italian radiologist, in 1930 implemented a technique called the “stratigraphy”, a complementary radiodiagnostic technique aimed at circumventing this‘masking effect limitation’ of the 2D image; it is also called “tomography” and helped accurate visualisation of analytical images, namely representative just of the structures in the pre-selected layers of the concerned region.

The limitations of this technique were:

1. Limited contrast resolution due to intrinsic shading of the image
2. Parasitic shadowing which is nothing but the background noise
3. The higher radiation dose due to multiple sequential acquisitions

Orthopantomography (for visualising entire mandible and the teeth) is one of the most popular tomographic tools used in the field of medical imaging. The flat-panel technology with tomography, which is a reinterpretation in digital key of Vallebona’s tomography, has been proposed as a new tool for early detection of breast cancers and is also called the Digital Breast Tomosynthesis (DBT).

DBT has rapidly emerged as an important new imaging tool that reduces the masking effect of overlapping fibroglandular tissue, thereby improving breast cancer detection. This increases sensitivity and specificity by reducing false negative (FN) and increasing false positive (FP) results.

DBT was approved for the first time by the United States Food and Drug Administration (USFDA) in the year 2011 for diagnostic use and since then, multiple studies have proven its superior results as compared to digital 2D mammography in screening set up as well.

According to early trials data, DBT is designed to offer the conspicuity of a higher percentage of breast cancers than conventional mammography, reducing false negative (FN) percentage at an estimated value around 15%. More recent studies indicate about 30% increased DBT sensitivity and specificity compared to FFDM with a recalls reduction in screening by approximately 40%.

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Technical aspects

Basic technique of DBT involves acquisition of multiple images of the breast tissue as the X-ray tube moves across a limited arc above the breast with multiple low-dose x-ray exposures. The motion of the tube, the length of the arc, and the time it takes to obtain a complete set of projection images, that are reconstructed into thin image slices spaced at 0.5–1.0 mm, are variable across different manufacturers.

As on 2017, there are four DBT unit vendors that are approved by FDA, viz. HologicSelenia Dimensions, GE SenoClaire, Siemens Mammomat Inspiration, and the Fujifilm ASPIRE Cristalle, though it is expected to be more in the coming years due to increased awareness about the utility of the modality and its increasing applications in diagnostic and screening settings.

Features common to every DBT systems are the execution mode (MLO projection), acquisition time (10-20 sec) and reconstruction time (between 40 and 180 sec), slices thickness (0.5 to1mm), display mode (single slice, or slab cine loop), chance to perform standard mammograms and FFDM/DBT real time selection with breast compression in place.

However, there is great variability in:

1. Acquisitions number take-over (between 13 and 25)
2. Aacquisition angle (between 15° and 50°)
3. Features in image quality that in DBT depends on the dose and the number of projections and acquisition angle as well the number of exposures: for example, if a narrow angle with little exposure allows a fast but low-resolution 3D acquisition, a wide angle with more number of exposures provide a good resolution 3D image but at a low-speed acquisition with consequent motion artifacts and quality deterioration of reconstructed images

An interesting alternative is represented by variable geometry (V-DBT), which offers the highest 3D resolution at maximum speed acquisition due to a non-uniform sampling. In V-DBT 13 images are taken through the 40° tube oscillation movement, the central one with 50% of the total dose delivered (the same as that required for a single mammography projection), and the remaining 50% unevenly split among the twelve remaining acquisitions.

Reconstruction algorithm in V-DBT system takes full advantage of any information provided in the 0° projection, which is basically a standard mammogram characterized by high contrast, which also provides valuable information for the microcalcifications’ visualization and their identification by 3D CAD (which is not yet available, but certainly among the future developments related to DBT).

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Applications of DBT

Retrospective studies in the US and prospective studies in Europe find reduction in recall rates and significantly improved breast cancer detection rates with DBT when used in conjunction with 2D mammography. An abundance of new research focused on DBT continues to improve the knowledge about this modality’s potential in facilitating breast cancer detection. An increasing number of clinicians in the U.S. and around the world are adopting DBT into practice as a supplemental breast cancer-screening tool.

DBT as a screening modality in many studies has demonstrated significantly improved sensitivity and specificity of cancer detection across all age and breast density groups. The invasive cancers detected on DBT are smaller and node negative compared to cancers detected on digital mammography alone, and is also more expressed in the 40 – 49 years age group with these cancers being more often being Her2 negative; in essence the DBT has the potential to detect smaller cancers with better prognostic characterisations.

We believe clinical usage of DBT has the undoubted potential to detect cancers in breast earlier than the 2D digital mammography, can detect smaller tumors and reduce the false negative effect of superimposed shadowing (major disadvantage of 2D digital mammography); in addition, there are many non-tangible benefits to the women due to reduced costs of being away from work, lesser inconvenience (due to reduced recall rates) and better prognosis (due to smaller size of cancer detected at earlier stages).

There is a learning curve to 3D mammography (DBT), and the radiologists may experience difficulties in the beginning, resulting in increased turn-around time, though these issues can be circumvented with appropriately organised training. It is only a matter of time that DBT is going to become the gold standard as for as the breast cancer screening/diagnosis is concerned.

Additionally, DBT can also provide information regarding the vascularity and angiogenicity of previously identified tumours, when used along with intra-venously injected iodinated contrast media, similar to those obtained with Contrast-enhanced spectral mammography (CESM), with a sensitivity similar to if not more than breast MRI.

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However, there are a few demerits of DBT as well...

Increased radiation dose to the woman due to additional sequential exposure- This can be circumvented by using DBT as the single screening tool rather than an adjuvant tool for standard two projections of the FFDM; there is possibility of reconstructing 2D mammographic like images from the acquired volume of data, also called synthetic 2D mammography, which is already approved by FDI since 2013.

Additional cost of the equipment resulting in less number of installations- Increase in the number of installations eventually reduces the costs involved with each installation, so that the cost to the women reduces.

DBT despite being more efficient in detecting smaller breast cancers is less likely to replace supplemental ultrasound examinations in women with dense breasts and MRI screening of breasts in women at high risk for breast cancers. Hence, it cannot be a one stop shop replacement for comprehensive breast imaging.

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In conclusion, DBT has been proven beyond doubts to be able to improve accuracy and efficiency of breast cancer screening when used as an adjuvant modality with DM (FFDM) and probably has a role as an independent screening tool as well and is expected to be a gold standard in this setting in the near future. A small learning curve associated with this new technology can be circumvented by appropriate training.