Digital Mammography

Digital mammography is a low dose X-ray examination of the breast. A detector converts the X-ray signal into a digital image, which is read on a monitor and can be adjusted for contrast and magnification.

It is the main test for screening and for the diagnosis of breast disease in women aged 40 and over. It shows masses, calcifications, distortion of the breast architecture and asymmetry.

Two views of each breast are taken in screening: the craniocaudal view and the mediolateral oblique view. Diagnostic examinations add spot compression, magnification, and lateral or exaggerated views as needed.

The breast is compressed firmly for a few seconds to spread the tissue, lower the dose and reduce movement. The mean glandular dose is about 1 to 2 mGy for each view, and the effective dose for a two view study of both breasts is about 0.4 mSv.

Breast cancer is the commonest cancer in women worldwide, with a lifetime risk of about 1 in 8 in high income countries. Screening mammography lowers breast cancer mortality by 20 to 40% in women aged 50 to 69.

Overall sensitivity is about 75 to 85%, falling to 30 to 65% in extremely dense breasts. Specificity is about 90 to 95%.

In screening, 5 to 10% of women are recalled for more tests, and cancer is found in 4 to 7 of each 1000 women at the first round.

Indications.

• Population screening in women of the age range set by the local programme, usually 40 or 50 to 69 or 74 years, every 1 to 3 years.
• Symptoms: breast lump, nipple discharge, nipple retraction, skin change, and focal pain that does not settle.
• Recall after an abnormal screening mammogram.
• Follow up after breast cancer treatment or breast conserving surgery.
• Women with a high risk of breast cancer, alongside MRI.
• Assessment of an axillary node or a metastasis from an unknown primary tumour.
• Localisation of a lesion before surgery, and imaging of the surgical specimen.
• Men with a suspicious breast mass (limited use).

Contraindications and precautions.

• Pregnancy: avoided for screening. If a mass needs assessment, ultrasound is used first and the abdomen is shielded if mammography is essential.
• Women under 30: rarely used because dense tissue limits the test and the radiation risk to young breast tissue is higher. Ultrasound comes first.
• Breast implants: special views that displace the implant are needed, and the examination takes longer. Compression is gentler, and a rupture is a risk.
• Recent breast surgery, biopsy or infection: compression can be painful, and timing is adjusted.
• Pain on compression is the main reason for refusal, and the technologist should explain the steps beforehand.
• Deodorant, powder and cream can mimic calcifications, so they are removed.
• Hormone replacement therapy and some drugs increase breast density.
• Pacemakers and chest ports do not prevent the test, but positioning is adjusted.

Preparation and technique.

No fasting is needed. The patient removes clothing and jewellery from the waist up. In women who still menstruate, the best time is the week after a period, when the breasts are least tender.

The history is recorded: symptoms, previous surgery, implants, hormone use, family history, and prior imaging for comparison. Marks are placed on scars, palpable lumps and moles.

The breast is positioned on the detector, the pectoral muscle is included in the oblique view, and the nipple is in profile. Compression is applied until the tissue is firm, and an exposure is made with automatic selection of exposure settings.

Images are checked for positioning, motion and coverage. Reading is done with comparison to earlier examinations, and in screening by two readers or by one reader with software assistance.

Normal appearances.

The breast has fat, which is dark, and fibroglandular tissue, which is white. Density is graded in four steps: almost entirely fatty, scattered areas of density, heterogeneously dense and extremely dense. Dense tissue lowers sensitivity and is a risk factor by itself.

The nipple is seen in profile, and the skin is thin and smooth. Ducts converge to the nipple. Lymph nodes in the axilla are oval, with a fatty hilum. Vascular calcification in a vessel wall is seen as parallel tracks.

Interpretation.

• Mass: described by shape (round, oval, irregular), margin (circumscribed, obscured, microlobulated, indistinct, spiculated) and density. An irregular, spiculated, high density mass is suspicious for cancer.
• Calcifications: coarse and large calcifications are usually benign. Fine pleomorphic, fine linear and branching calcifications, and clusters of amorphous calcifications, are suspicious.
• Architectural distortion: radiating lines or tethering without a mass. It can mean cancer or a radial scar, and it needs biopsy.
• Asymmetry: a new or enlarging area of density is suspicious, whereas a stable area seen on both views is usually normal tissue.
• Skin and nipple changes: skin thickening, nipple retraction and trabecular thickening are seen in cancer and in inflammatory conditions.
• Axillary nodes: round shape, loss of the fatty hilum and cortical thickening are suspicious.
• Benign findings: fibroadenoma (circumscribed mass, sometimes with popcorn calcification), cysts, fat necrosis (oil cyst with an eggshell rim), lipoma and intramammary lymph node.
• The result is placed in one of these categories: incomplete (more imaging needed), negative, benign, probably benign (short interval follow up), suspicious (biopsy), highly suggestive of malignancy, or known cancer.

Critical values and findings needing immediate action.

• Findings highly suggestive of cancer: tissue sampling and referral to a breast service within days.
• Inflammatory breast changes with skin thickening: urgent assessment for inflammatory cancer and for abscess.
• New mass during pregnancy or lactation: ultrasound at once.
• Implant rupture with a lump or infection: surgical review.
• Bloody nipple discharge with a mass: urgent assessment.

Limitations and artefacts.

• Dense breasts hide cancers. Supplementary ultrasound, tomosynthesis or MRI may be added.
• Interval cancers appear between screening rounds, and some cancers are not seen even in retrospect.
• Lobular cancer and cancers close to the chest wall or in the axillary tail are easy to miss. Poor positioning leaves tissue out.
• Overdiagnosis of low grade ductal carcinoma in situ and slow growing cancers is an accepted cost of screening.
• False positive results cause anxiety, extra tests and unnecessary biopsy.
• Motion blur, skin folds, deodorant, talc and detector artefacts can mimic calcifications.
• Implants obscure tissue, and silicone gel outside the capsule shows as dense material.
• Radiation risk is small and is higher for young women and carriers of some gene mutations.
• Pain and embarrassment reduce attendance.

Special groups.

• Women at high risk (a mutation in BRCA1 or BRCA2, chest irradiation before age 30, strong family history): MRI is the main test, with mammography added from about age 30 to 40.
• Women with implants: additional displaced views, and ultrasound or MRI for rupture.
• Pregnant and breast feeding women: ultrasound first.
• Men: mammography for a suspicious mass, with gynaecomastia the usual finding.
• Older women: screening continues while life expectancy is over 5 to 10 years.
• Regions: breast cancer is diagnosed late and at younger ages, and mammography is available in few places. Clinical breast examination, ultrasound and core needle biopsy are used, with mammography as availability allows.

Follow up tests.

• Digital breast tomosynthesis.
• Breast ultrasound.
• Breast MRI.
• Contrast enhanced mammography.
• Core needle biopsy or vacuum assisted biopsy (stereotactic or ultrasound directed).
• Fine needle aspiration of a node or a cyst.
• Short interval repeat imaging at 6 months for probably benign findings.
• Genetic counselling and testing.
• Referral to a breast surgeon, oncologist or breast clinic.