Digital Breast Tomosynthesis

Digital breast tomosynthesis is a three dimensional form of mammography. The X-ray tube moves in an arc over the compressed breast, and many low dose projection images are taken. These are rebuilt into thin slices, usually 1 mm thick, that are scrolled through on the monitor.

Overlapping tissue is separated, so a cancer is less likely to be hidden and a normal overlap is less likely to look like a lesion.

The arc is 15 to 50 degrees, with 9 to 25 projections, in 4 to 10 seconds. The usual mammography views are taken (craniocaudal and mediolateral oblique).

Many systems also produce a synthetic two dimensional image from the data, which removes the need for a separate mammogram. The mean glandular dose is about 1 to 2 mGy for each view, and similar to digital mammography when synthetic images are used.

When a full mammogram is also taken, the dose is nearly doubled.

In screening studies, tomosynthesis finds 20 to 40% more invasive cancers, and lowers recalls by 15 to 30%. The gain is largest in women with dense breasts and in women under 60.

Sensitivity is about 85 to 90% and specificity about 90 to 95%.

Indications.

• Screening of asymptomatic women, in place of or alongside digital mammography.
• Recall after an abnormal screening mammogram, to confirm or dismiss an overlap of tissue.
• Women with dense breasts.
• Symptomatic women aged 40 and over, as a diagnostic test.
• Characterisation of a mass, asymmetry or distortion.
• Localisation of a lesion for biopsy (tomosynthesis directed biopsy) and for surgery.
• Women with breast implants (additional views with the implant displaced).
• Follow up after breast cancer treatment.

Contraindications and precautions.

• Pregnancy: avoided for screening, with ultrasound used first.
• Women under 30: rarely used.
• The same cautions for compression, implants, recent surgery, and deodorant or powder apply, as they do for digital mammography.
• Metal clips, wires and markers produce streak artefacts in the slices. Their position is noted.
• Longer compression time (up to 10 seconds) may be uncomfortable.
• Total radiation dose should be kept low by using synthetic images instead of a separate mammogram.
• Reading takes longer, and trained readers are needed.
• The data storage and display equipment are costly.

Preparation and technique.

Preparation is identical to that for mammography. The breast is positioned and compressed, and the tube moves while projections are taken.

The reconstructed slices are displayed in a stack that is scrolled, along with the synthetic two dimensional image. A lesion is checked in at least two views, and its size and position are noted for biopsy.

Quality checks cover positioning, motion, and the completeness of coverage.

Comparison is made with earlier two dimensional and three dimensional studies. Reading is by one or two radiologists, with software support available in some units.

Normal appearances.

Fibroglandular tissue is separated into thin layers, and fat is seen as dark areas. Ducts, vessels and Cooper ligaments run in smooth lines. Overlap of normal tissue, which can look like a mass on a mammogram, disappears as the slices are scrolled.

Benign calcifications are coarse, round or rim shaped, and are seen well in the synthetic image.

Interpretation.

• Mass: tomosynthesis shows the margin more clearly, and a spiculated margin is more easily seen. An irregular, spiculated mass is suspicious, and a circumscribed oval mass is usually benign.
• Architectural distortion: shown better than on mammography, and it is suspicious unless a scar explains it. A radial scar and a cancer can look alike.
• Asymmetry: a true mass is distinguished from overlapping tissue, which often removes the need for further tests.
• Calcifications: seen best on the synthetic image or the conventional mammogram. Clustered fine pleomorphic or linear calcifications are suspicious.
• Size: tomosynthesis gives a better estimate of tumour size and of multifocal disease.
• Lobular cancer and low grade cancers in dense tissue are more often found.
• Findings are placed in the categories used for digital mammography.
• The slice number locates a lesion in depth, which helps with biopsy planning and surgery.

Critical values and findings needing immediate action.

• Findings highly suggestive of cancer: tissue sampling and referral to a breast service within days.
• Inflammatory changes or a rapidly growing mass: urgent assessment.
• New mass in pregnancy or lactation: ultrasound at once.
• Bloody nipple discharge with a mass: urgent assessment.

Limitations and artefacts.

• Microcalcifications can be harder to detect on slices, which is why the synthetic image or a mammogram is used.
• Metal objects and implants produce streaks and blur in the slices.
• Motion during the longer exposure blurs the images.
• Out-of-plane blur and ripple artefacts from large dense objects can mimic a lesion.
• Reading time is up to twice as long as mammography reading, and fatigue can cause errors.
• Interval cancers and cancers in very dense breasts are still missed.
• Overdiagnosis of low grade lesions can rise with higher detection.
• The increase in detection of small cancers does not yet prove a larger fall in deaths.
• The dose is higher if a mammogram is added to each view.
• Cost, storage and training are barriers.

Special groups.

• Women with dense breasts: tomosynthesis gives the greatest gain, and ultrasound or MRI may still be added for those at high risk.
• Women with implants: additional views, and ultrasound or MRI for rupture.
• Pregnant women: ultrasound first.
• Women under 40: used for symptoms only.
• Women at high risk: MRI is the main test.
• Regions: most services use digital mammography, and tomosynthesis is limited to large centres. The alternatives are those used for mammography.

Follow up tests.

• Digital mammography with magnification.
• Breast ultrasound.
• Breast MRI.
• Contrast enhanced mammography.
• Tomosynthesis directed, stereotactic or ultrasound directed biopsy.
• Short interval repeat imaging for probably benign findings.
• Referral to a breast surgeon, oncologist or breast clinic.