Dr Peter Schoub explains the difference between 2D and 3D mammography, and why all women should choose 3D if it’s available.
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Mammograms are X-rays of the breasts. The role of a mammogram is to detect breast cancer. This can be as a screening or a diagnostic study. Screening mammograms are performed on a regular basis to pick up cancers in asymptomatic women. The earlier a cancer is detected, the more effective treatment is likely to be and consequently, the lower the chance of the cancer returning. Treatment of smaller cancers also allows for less aggressive treatment with fewer complications and better aesthetic outcomes.
Diagnostic mammograms are used to determine which lumps require further investigation e.g. ultrasound and biopsy. A diagnostic test is necessary if a patient is symptomatic (a woman who has a palpable lump, nipple discharge or notices a change in the appearance of the breast or nipple). A diagnostic mammogram is also done if an abnormality is found at screening mammography. The mammogram appearance of a lump, or calcifications can indicate whether the lesion is harmless, or suspicious for cancer. Important things that we assess, on the mammogram, are shape and outline of a lump or the pattern of calcifications.
The biggest factor that affects the accuracy of a mammogram study is the breast density. Breast density refers to the amount of fibroglandular tissue (glands, ducts and fibrous tissue) compared to background fat. Dense breast tissue means a high ratio of fibroglandular tissue to fat. The fibroglandular tissue appears as white areas on a mammogram picture. Dense tissue means lots of white on the mammogram. Breast cancers are also white on mammogram and may blend into the background white fibroglandular tissue. Fatty breasts on the other hand, contain very little fibroglandular tissue, and the breasts appear black on mammogram. As a result, the white cancers are more visible against the dark background. Therefore, mammograms find cancers more easily in fatty breasts than in dense breasts.
2D mammogram studies (the original mammograms) involve two X-rays of each breast, from the side and from on top. A cancer hiding in the middle of the fibroglandular tissue may be hidden from view. This is similar to a regular photograph where an object in the front can obscure an object behind. Think of a person standing behind a tree.
A 3D or tomosynthesis mammogram is obtained by an X-ray tube that takes numerous X-rays while moving in an arc around the breast. The result is a stack of very thin images taken from many angles. This allows the radiologist to scroll through the pictures as if they were looking at a CT scan. The overall masking effect of dense fibroglandular tissue is significantly reduced as we can now see through the breast tissue. Cancers are unable to hide. Also, 3D imaging enhances those features that indicate whether a lump is benign or suspicious.
This means that 3D mammograms miss fewer cancers and are more accurate at identifying abnormalities than 2D mammograms. There have been many large comparative trials that show that 3D is superior to 2D in both a screening and a diagnostic role.
The radiation dose for 3D mammograms is similar to that of a 2D mammogram. Less breast compression is generally required, making 3D mammograms more comfortable than the older mammograms. The medical aid tariff is the same for 2D and 3D mammograms.
One of the newer developments is 3D-guided biopsy. This means that an abnormality found only on 3D mammogram can be targeted using the same 3D machine. More accurate biopsies result in better cancer detection and less overtreatment.
In South Africa, most private practices and the larger provincial mammography departments offer 3D mammography. Therefore, it’s strongly recommended that all women choose a 3D mammogram if it’s available.
MEET THE EXPERT – Dr Peter Schoub
Dr Peter Schoub is a radiologist at Parklane Women’s Imaging Centre. He obtained the European Diploma of Breast Imaging in 2018 and is an honorary lecturer in the Department of Radiology at the University of the Witwatersrand.
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