Digital Breast Tomosynthesis

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The Open University

A modern breast cancer screening and diagnosis tool: DIGITAL BREAST TOMOSYNTHESIS


Imaging in Medicine S809

Eleni Pitri

Table of Contents

1. Introduction 3

1.1 Scope and Topic Area 3

1.2 Project Aim and Objectives 3

1.2 Structure of the Report 4

2. Literature Review 5

2.1 Progress of Breast screening and diagnosis 5

2.2 Conventional X-ray Mammography 6 2.2.1 Conventional X-ray Mammography Principles 6

2.2.2 Disadvantages of Conventional Mammography 7

2.3 Digital Breast Tomosynthesis (DBT) 8 2.3.1 The Development of Tomosynthesis92.3.2 Basic Technologic Principles of DBT 10 2.3.3 DBT Technical Considerations12 2.3.4 Detector Requirements 14 2.3.5 Radiation Dose 16

3. DBT Clinical Trials – Comparison with Conventional Mammography 16

3.1 The Potential Advantages of DBT in clinical Practice 16

3.2 DBT clinical Trials 17

4. Discussion 23

5. Conclusion 25

References 26

1. Introduction:

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1.1 Scope and Topic Area

Breast cancer is the most frequent type of cancer and the second leading cancer type for

estimated deaths among the women in U.S.A.1. It is proved that screening and early detection of

breast cancer may contribute to the successful treatment of the disease. At the moment, X-ray

mammography, Analog or Full Field Digital Mammography (FFDM) are used in routine clinical

practice for screening and have contributed to the reduction of breast cancer mortality.

Since the implementation of the flat-panel digital detectors, a new valuable tool is available for

screening and diagnosing breast cancer, that is, Digital Breast Tomosynthesis (DBT). Despite the

other competing technologies such as Ultrasonography, contrast enhanced MRI and Breast CT,

Digital Breast Tomosynthesis is more likely to replace conventional mammography as it may be

considered as an improvement of the current FFDM.

1.2 Project Aim and Objectives

The aim of this project is to understand DBT technology and critically analyze the advantages

and limitations of the technique. In particular the effort will be focused on comparing

conventional X-ray Mammography with the literature results of DBT clinical case studies.

The project objectives can be summed up as follows:

Review of the progress in breast screening and diagnosis

Familiarization with Tomosynthesis imaging technique

Review of the existing literature on DBT

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Analysis of the results of clinical case studies

Comparison of DBT with conventional X-ray Mammography

Interpretation of the results obtained

1.3 Structure of the report

Chapter 2: Literature Review

This chapter presents the major aspects of theoretical background related to X-ray

Mammography and Breast Tomosynthesis

Chapter 3: DBT Clinical Trials – A comparison with Conventional Mammography

At this point the clinical case studies reported in literature are analyzed and compared.

Chapter 4: Discussion

The results obtained by the literature review are summarized and issues that arise from the

potential use of DBT into clinical practice are discussed.

Chapter 5: Conclusions

2 Literature Review

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2.1 Progress of Breast Screening and Diagnosis

Ever since early attempts to use X-rays took place, for the diagnosis of breast cancer, a great

progress has been made in the area. In 1913, the first mammograms were taken by Albert

Salomon at mastectomies and few years later Mammography was used in-vivo2. In 2000 the first

FFDM system gained the first approval by the U.S Food and Drug Administration (FDA) and,

since then, Analog Mammography has been replacing. In the last decade, a new X-ray tool for

breast screening and diagnosis has been developed, that of DBT. In addition to X-ray

investigating techniques, other modalities such as MRI and Ultrasound have been added to the

breast cancer diagnosis methods. Nowadays, apart from the techniques showing anatomical

changes, investigating tools based on functional changes have been introduced.

Figure 1: A schematic diagram showing the progress of breast screening and diagnosis. Where CR = Computed Radiography, MRI = Magnetic Resonance Imaging, CE-DSM = Dual-Energy Contrast-enhanced Digital Subtraction Mammography and CE-DBT = Dual-Energy Contrast-enhanced Digital Breast Tomosynthesis

2.2 Conventional X-ray Mammography

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Analog Mammography

CR- Mammmography

FFDM

Anatomic Changes

DBT Ultrasound Elastography

Physio-pathological Changes

Contrast enhanced MRI

CE-DSM

CE-DBT

Cellular Metabolic Changes



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2.2.1 Conventional X-ray Mammography Principles:

X-ray mammography is proved to be an efficient technique for screening and diagnosing breast

cancer at an early stage. Currently both Analog and FFDM Mammography systems are used in

clinical practice. However, in the last decade most of the analog systems with screen-film

cassettes have been replaced.

Image creation in conventional mammography is based on X-ray attenuation. The mammograms

must image the contrast between similar soft tissues (e.g. fat, blood vessel and soft tissues with

different density), thus, low kV is required to maximize photoelectric effect and make tissues

differences distinguishable.

Figure 2: In conventional Mammography the X-ray tube, the compressed breast and the detector are stationary, while one image is obtained from one projection3.

During the scan, the breast is compressed between the compression paddle and the stationary

breast support. The compression evens out the breast thickness and achieves better visualization

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Detector

BreastCompression paddle

X-rays



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of tissues and abnormalities, less radiation dose to the patient and reduction of scattering.

Furthermore, compression is used to prevent patient motion that causes image blurring. The

views most frequently used in conventional screening mammography are craniocaudal (CC) and

mediolateral oblique (MLO), whereas in diagnostic cases other views are also used4.

2.2.2 Disadvantages of Conventional Mammography:

Although conventional Mammography is an efficient modality which provides reasonably high

sensitivity, high resolution and low patient dose, a disadvantage may be traced in terms of

receiving a 2D imaging of a 3D object. The attenuated beam which arrives on the flat-panel

detector (or the screen film cassette in case of analog system) is proportional to the total

attenuation of all the structures between the X-ray tube and the detector (Figure 3). This causes

an overlap between structures leading to potential loss of information, sensitivity and specificity.

This limitation of conventional Mammography complicates the interpretation of mammograms;

it further deteriorates the detection of breast cancer and renders the localization of abnormalities

difficult. The consequence is that, in cases of questionable findings, even where findings prove

benign, further examinations are needed in order to investigate them and this may cause

additional anxiety to patients let alone additional costs.

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Figure 3: In conventional mammography overlapping structures may lead in loss of information (hidden objects of interest), this effect is eliminated in the 3D Breast Tomosynthesis. (Image captured from Smith et al3)

2.3 Digital Breast Tomosynthesis

Digital Breast Tomosynthesis (DBT) is a new tool which is expected to address these key

limitations of mammography and has the potential efficacy to replace conventional

mammography. DBT is similar to X-ray mammography. It is also based on X-ray attenuation,

but, instead of 2D images, it reconstructs a number of planar projections providing 3D

information. The DBT device is, in most cases, based on the existing digital mammography

system; patient positioning during image acquisition is very similar to conventional digital

mammography, but the required image reconstruction differs.

DBT is a type of limited angle CT with low radiation dose to the patient, similar to that of

mammography. Comparably to 2D mammograms, the reconstructed breast images produced by

DBT reduce potential false diagnosis due to structure overlapping and increases cancer

localization and delineation of borders.

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2.3.1 The development of Tomosynthesis:

Tomosynthesis is not a newly invented technique. The conception of the technique goes back to

1930s when Ziedes des Plantes introduced the theory of limited angle tomography only its

implementation was not possible5. Digital detectors made the implementation of Tomosynthesis

feasible and in 1972 Grant made up the term “Tomosynthesis”. At this early stage the limitations

of digital detectors obstruct trials from taking place for the clinical use of the technique. From

late 1980s to late 1990s Tomosynthesis was almost abandoned due to the development of spiral

CT which seemed to dominate over all 3D X-ray imaging.

Until flat-panel detectors were introduced in late 1990s, clinical applications were not feasible.

Full-filled flat panel detectors are characterized by low noise, high frame rate and high detective

quantum efficiency (DQE) which are required for a reasonable acquisition time of several (low

dose) projections used in Tomosynthesis systems.

Over the last years Tomosynthesis clinical applications have increased and improved. The

applications which gather most interest are Breast Tomosynthesis and pulmonary nodule

imaging. The interest in Breast Tomosynthesis has been further growing after the U.S. FDA

approved the first Breast Tomosynthesis System in February 2011.

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2.3.2 Basic technologic principles of Digital Breast Tomosynthesis

As mentioned before, the DBT device in almost all commercial systems is combined with the

existing digital mammography system. Similarly to conventional mammography the breast is

compressed between the compression paddle and the stationary breast support which contains the

digital detector. The integrated systems can take a conventional mammogram and a

Tomosynthesis set of slices at the same compression.

Figure 4: Schematic representation of DBT operation principles6.

There are more than one motion geometries of the tube and the detector for Tomosynthesis

image acquisition5. In current DBT systems, the partial isocentric motion is used (Shown in

Figure 5). The X-ray tube moves in an arc about the compressed breast during the exposures

while the detector is fixed, and a number of projections are obtained. This type of geometry is

preferable because it may more easily be integrated into the current Digital Mammography

Systems.

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Figure 5: Schematic representation of partial isocentric motion (captured from Dobbins et al5)

During image acquisition there are two methods of moving the tube. The first one is the step-

and-shoot method that the tube completely stops and ‘shoots’ at each position. The second is the

continuous exposure method where the tube moves continuously and the exposures are low dose

short pulses. Both methods are not without advantages and disadvantages; yet currently the

second method is preferred due to the faster scan times it provides.

The projection data obtained by the scan are reconstructed into a series of high resolution images

aligned in parallel to the breast platform, perpendicular to the X-ray beam. Because of the

limited scan angle there is less resolution in the axis of X-ray beam and higher resolution in the

perpendicular plane. The number of images is proportional to the breast thickness and the

distance between them. The most frequent spacing is 1mm. The set of images are transferred and

read at a common workstation where the radiologist can interpret the reconstructed images.

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2.3.3 DBT Technical Considerations

As mentioned above DBT reconstructs images which provide 3D information but, as a result of

the limited scan angle, the depth or out-of-plane resolution is low. This limitation of DBT

produces out-of-plane artifacts such as the streak artifact of Figure 67. This type of artifact is

prominent in visualization of calcifications with DBT. As shown in Figure 6, it does not only

degrade the image quality of the focus plane but reduces the visualization in neighboring slices

as well. This artifact can be reduced by increasing the scan angle and therefore increasing the

depth resolution.

Figure 6: A calcification is seen in four different slices due to an out-of-plane artifact and therefore

reduces the visualization in these slices: (a) in focus plane; showing the in-plane black artifact below and

above the high contrast calcification, (b) 6 mm from the focus plane, (c) 10 mm from the focus plane and

(d) 17 mm from the focus plane. The tube motion is parallel to the streak artifact from top to bottom.

(Image captured from Tingberg et al7)

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The optimal scan angle of the tube must be estimated according to a number of factors; the

appropriate depth resolution is only one of them. Another factor to be taken into consideration is

the scan time. Wider scan angle demands longer scan time, which increases the probability of

motion artifacts. Given that the detector is fixed, the field of view is also influenced by wide scan

angles and may lead to missing tissues. Furthermore, wide scan angles produce thicker effective

breast that induces noise and scattering. Other issues negatively affected by wide angle range are

the in-plane and detector resolution.

The number of projection is another technical issue for DBT. The optimal number of projection

is also depended on several parameters such as:

The scan angle: serves to avoid reconstruction artifacts. A number of projections per

degree-scan-angle is required –referred as projection sampling.

Scan time must be kept as short as possible thus can become a limiting factor for the

number of projections considering the limited frame rate of detectors.

Large numbers of projections may induce electronic noise.

In current systems the number of projections varies from 11 to 49 and the scan angle of the tube

from 150 to 5008. Each manufacturer has used different methods and reconstruction algorithms to

develop a DBT system, thus different optimal scan angle and number of projections are received.

Table 1 gives the scan angle of the DBT systems provided by four manufacturers; each of them

uses a different method.

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Figure 7: DBT systems by four different manufacturers. (A) Siemens – Mammomat Inspiration9 (B) GE Breast Tomosynthesis10 (C) Hologic- Selenia Dimensions11 (D) Giotto - IMS12

Table 1: Differences among commercially available DBT systems

Manufacturer Name Total scan angle

Projections Detector

Siemens MAMMOMAT Inspiration

500 25 a-Se

GE GE Breast Tomosynthesis

250 9 CsI

Hologic* Selenia Dimensions 150 15 a-Se

Giotto IMS 400

(non uniform distribution)

11 a-Se

*Hologic: Selenia Dimensions is the first commercially available system that gained the FDA approval in February 2011.

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2.3.4 Detector Requirements:

One of the most essential parts of the DBT system is the full-field flat-paned detector. Due to the

low dose of the exposures, high detective quantum efficiency (DQE) and low noise are required.

Because of the numerous projections high frame rate is also essential. The detectors used in most

of the current DBT systems are selenium-based but cesium iodide (CsI) crystals based on

amorphous silicon are also suitable. Selenium has the highest DQE at the low energies of

mammography, more than 95% X-ray absorption. This allows imaging at radiation doses as low

as those used in DBT.

2.3.5 Radiation Dose

The radiation dose to the patient is a critical consideration for Mammography system

manufacturers and regulators. Radiation dose to the patient and image quality are related and

must be used in an optimum way.

The current DBT systems are within the Mammography Quality Standards Act (MQSA)13 limits

which are set at 3 mGy per exposure. The limits concern the Mean Glandular Dose (MGD) at a

phantom of a standard breast for a single CC view. The MGD of a single view DBT

examination, as they are calculated in early clinical studies, is equal or slightly higher than that

of a single view FFDM. As Teerstra et al14 and Good et al15 have reported in their studies, the

MGD of DBT system is almost equal to that of FFDM; the first has reported 1.74 mGy and the

latter 2 mGy.

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3 DBT Clinical Trials – Comparison with Conventional Mammography

3.1 The potential advantages of DBT in clinical practice

DBT is a new tool developed to contribute to the detection and diagnosis of breast lesions,

especially in women with dense breast tissue. The set of reconstructed images from different

angle projections allows visualization of thin breast sections and provides more detailed

information for breast structures (normal tissues and abnormalities). Theoretically, DBT has

superior sensitivity compared to 2D X-ray Mammography. DBT is expected to provide improved

detection of lesions, even to allow detection of small cancers not visualized by conventional

mammography. DBT has the capability to provide more information on the margins of a mass

and characterize the abnormalities more accurately. Thus, the specificity of the technique is

increased while the false-positive recalls are reduced.

The numerous potential benefits of DBT clinical use must be proved and evaluated in large

randomized clinical trials. The sensitivity and specificity of the technique, in the different types

of breast abnormalities, should also be evaluated during clinical trials.

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3.2 DBT clinical trials

Recent DBT clinical trials aim to give an early evaluation of the DBT technique. The findings

from these studies have generally shown a superiority of DBT in the detection and border

determination of masses. In the detection of calcification there are several opinions and the

physicians’ preferred technique is still questionable. The main problem with calcifications

detection is that calcifications are dispersed in space; therefore 1mm thick DBT slice may

include only a small number of calcifications, making the calcification cluster less perceptible.

These early studies have concluded that DBT cannot provide 100% sensitivity and specificity.

However, most of them have agreed that it can provide improved sensitivity and specificity.

Finally, all of the studies have shown that DBT demands considerably longer reading times by

the physician. This major limitation of DBT over conventional mammography is expected to be

balanced. First because of the reduction of diagnostic examinations resulting out of an

improvement in recall rates. Second because of the familiarization of physicians with the new

technique that is expected to reduce the interpretation times.

One of the first clinical studies was that of Poplack et al16; the aim of whose project was the

comparison between diagnostic DBT and film-screen mammography in a group of 98 women

recalled to be examined after a digital screening mammogram. The results have shown that the

DBT image quality was equal (51 subjects) or superior (37 subjects) to screen-film

mammography in 89% of the cases. A percentage of 11% was assessed to be inferior to screen-

film mammography, calcifications were reported at 73% of these cases. Masses showed better

image quality on DBT in a considerable higher amount of cases (68%) or equal quality in 26% of

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them. Among the conclusions of the project was that DBT with FFDM in screening has the

potential to reduce the recall rate and the reduction was estimated to 40%.

Figure 8: Left breast of 84-year old woman16. (A)Focal compression view: irregular mass is shown by arrows (B)Single slice of DBT: Irregular shaped mass with its margins is shown by arrows and central fat by circle (C)Arrows show infiltrating lobular carcinoma with central fat deposition

Another early clinical study was that of Good et al15; 9 physicians evaluated the image quality of

30 cases for three different modes: FFDM, 11 projections acquired for DBT before the

reconstruction and the reconstructed DBT slices. The results are similar to those of Poplack et al;

they have shown that the reconstructed DBT images were significantly better in 23% of cases,

somewhat better in 44% of cases, comparable in 31% of cases and inferior in only 1.9% of cases.

Good et al also estimated the readout times of DBT (2.7 min per examination) and FFDM (1.6

min per examination) and verified that the read out times were longer in the case of DBT

examinations.

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Analog Mammography DBT Photomicrograph



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Gur et al17, using 8 experienced radiologists and 125 cases (35 with verified findings of cancer

and 90 with no findings) assessed the image quality of four different display conditions: FFDM

alone, 11 low dose projections, reconstructed DBT images and combined FFDM with DBT. The

investigators found that the sensitivity of DBT alone was similar to that of the combination of

DBT with FFDM (93%) and the sensitivity of FFDM alone was non-significantly reduced

(88%). The specificity of combined DBT with FFDM was the greatest (0.72) compared to DBT

alone (0.64) and FFDM alone (0.60). Gur et al have concluded that the combination of DBT with

FFDM would have led to 30% reduction of recall rate for cancer free examinations compared to

FFDM alone; slightly lower than that of Poplack et al. Using of DBT alone would have

diminished the recall rate by 10%. The time to view and rate images was also evaluated for each

mode; as it is seen in Table 2 the longest mean time was for the combined mode. The results for

FFDM and DBT are slightly lower than those estimated by Good et al, though the difference

between the two modes is still almost the same.

Table 2: Mean Time for all readers and examinations to view and rate images from each imaging display mode. Data taken by Gur et al17.

Mode Mean Time per Examination (min)

Standard Deviation

FFDM alone 1.22 1.15

Frame 1.38 0.99

DBT 2.05 1.46

Combined FFDM and DBT

2.39 1.65

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Rafferty et al18 were the first to produce a large multi-centre trial to compare and analyze the

results of FFDM alone and FFDM with combinations of CC and MLO views of DBT. They used

the Receiver Operating Curve (ROC) method to analyze data from 310 subjects and 15 readers.

As it is shown in Figure 9 there is significant improvement in ROC performance in both fatty

and dense breasts when the combination of FFDM with DBT views was used. It is also notable

that the ROC performance of combined FFDM with DBT in dense breasts is superior to FFDM

alone, not only in fatty dense breasts but also in fatty breasts. These results support the opinion

that DBT would improve the detection of breast lesions in women with dense breast tissue.

However, the best performance is still for fatty breasts. The investigators have shown a reduction

in recall rates for cancer free cases of 43% in fatty breasts and 34% in dense breasts when using

combined FFDM with DBT compared to FFDM alone. Estimations close to those stated by Gur

et al and Poplack et al.

Figure 9: ROC curves of FFDM alone and combination of FFDM and DBT for fatty and dense breasts. It is shown that the combination of the two modalities is better than FFDM alone both for dense and fatty breasts.

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Teerstra et al14 compared the diagnostic images from FFDM and DBT of 513 cases. As opposed,

to the majority of clinical trials, this study has concluded that DBT is as good as FFDM and is

not superior in detection of breast cancer. Estimations for sensitivity showed that the two modes

had almost the same sensitivity (92.9%). The specificity of FFDM (86.1%) was slightly better

than DBT (84.4%). It is also important to note that this study found DBT to be as good as FFDM

in detection of malignant calcification (all clusters were detected by using both techniques)

which is also to be contrasted with the results of the majority of studies.

Helvie et al, as it is presented in a recent paper6, assessed the DBT capabilities in delineation of

masses margins (all cases were scheduled for biopsy.) The comparison of DBT with

conventional Mammography showed that DBT was better in determination of the perimeter of

masses: 77% of the perimeter of a mass could be visualized by DBT and only 53% by FFDM.

Figure 10: Invasive ductal cancer 6 : The cancer of this case is only apparent in the CC view of conventional mammography. On the other hand DBT (1mm thick slice) provides information not only for the cancer but also for the margins at both views.

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Helvie et al assessed the Maximum Intensity Projection (MIP) images of DBT in the detection of

microcalcifications. MIP images are 1 or 2 cm thick slices specially reconstructed from DBT

examinations in order to overcome the problem of calcifications dispersion in several DBT slices

(Figure 11). The investigators have shown that calcification clusters were visible by DBT MIP

images in 100% of cases, based on conventional mammography results. It is worth noting that

the visualization of calcification was slightly less when 1 mm thick DBT slices were examined

(96% for benign and 97% for malignant cases).

Figure 11: Ductal carcinoma in situ is shown by arrows as a microcalcification cluster6. (A) CC view of FFDM (B) 11 mm thick MIP image of DBT. The MIP image of a DBT examination shows the calcifications as good as FFDM.

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4 Discussion

The positive early results from clinical trials justify the positive reactions for DBT by the

majority of radiologists. However, prior to further induction of DBT in clinical practice the

considerations and questions arising from DBT use must be investigated. Not only the

superiority but also the cost-effectiveness and the utility of the technique must be proved during

large randomized clinical trials.

What seems to be important is to first determine the best use of DBT into routine clinical

practice. It is expected to be very useful at both diagnostic and screening level, but it is still

questionable whether it is feasible to be used in both cases. The early studies have shown that

due to its improved visualization of overlying structures and improved delineation of masses

margins, DBT has an important role to play in diagnostic applications. Also DBT has the

capability to more accurately determine the type of abnormalities. The use of DBT in screening

is more complicated. DBT has a major disadvantage compared to conventional mammography; it

demands significantly longer interpretation times by the physician. This disadvantage must be

overcome by the benefits of DBT in screening. In order to conclude about the utility of DBT in

screening; it is important to accurately evaluate the recall rates – biopsy rates and the specificity

of DBT in large-scale clinical trials. The high specificity of DBT may be proved enough to

improve current diagnostic procedures and patients may go directly for biopsy after a DBT

screening.

Another important consideration arising from DBT clinical use is the number of views which

need to be acquired. So far it has not been clarified whether DBT should be performed in both

CC and MLO view or in only one of the two. It is not even clarified whether it should be used

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alone or as an adjunctive to FFDM. The majority of studies have shown a weakness of DBT in

the detection of calcifications in favor of the opinion that a static view should also be taken

(FFDM view or an analogous MIP image). The results from the large multi-centre study of

Rafferty et al, were also in consonance with this opinion. They have shown that one DBT view

adjunctive to FFDM can provide higher sensitivity and specificity than FFDM alone.

The efficient use of DBT will require substantial training, by the physician. The non-familiarity

of physicians with DBT images induces the risk of an initial increase in recall rates due to

normal tissues that look different and suspicious. In addition physicians familiarity with DBT is

essential not only for interpreting the DBT examinations but also for minimizing the

interpretation times.

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Conclusion

DBT is an outstanding new development for screening and diagnosing of breast cancer. The

initial results from early clinical studies are in favor of the new technique, especially in the case

of masses. The evaluation of calcifications is still questionable and needs further investigation.

So far it has not been clarified whether DBT should be used alone or as an adjunctive to FFDM

However, the early results reveal that DBT alone or as an adjunctive will significantly contribute

in the improvement of current breast cancer screening and early diagnosis by decreasing recall

rates and false positive biopsies. It is essential to confirm these early results with large

randomized clinical trials.

An important advantage of DBT, over other competing technologies, is that its basic technology

is X-ray mammography. It can be integrated in the existing installed systems; it uses the existing

familiar digital radiologic equipment and seems to be easily accepted by patients and health care

professionals.

All these factors create a positive prospective that DBT may overcome the barriers, which it

currently faces, and finally will become a cost-effective, low dose imaging modality in the field

of mammography.

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http://www.fda.gov/Radiation-EmittingProducts/MammographyQualityStandardsActandProgram/Regulations/ucm110906.htm#s90012

Digital Breast Tomosynthesis

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