What are the differences between mammograms and ultrasound?

What are the differences between mammograms and ultrasound?
Thursday, Dec 06, 2012
By Dr See Hui Ti, Parkway Cancer Centre

1) Can you explain the differences between mammograms and ultrasound in terms of what they entail, the cost, efficiency at picking up tumours or abnormalities in breast tissue, etc? Are there any side effects for both?

Mammography and ultrasound are in principle different modalities so they show different things.

In ultrasound, high frequency sound waves are bounced off the breast tissue and collected as an echo to produce an image. So the pattern that is produced will allow the ultrasonographer to determine if the image is a suspicious mass or normal breast tissue. It is good at distinguishing solid masses (nodules) from fluid filled cysts (simple or complex cyst). It is also used as an adjunctive imaging test to a mammogram or physician's clinical examination of the breast. It is useful in young women whose breast tissues are very dense therefore reducing the accuracy of the mammograms.

Mammograms uses X-rays to produce an image that is a shadow of dense structures. Suspicious areas need to be dense enough to be seen. It picks up cancer or suspicious areas by picking up suspicious and abnormal calcium deposits in the X-ray (microcalcifications). Not all calcium deposits are suspicious. Some are benign (no cancer); and others are suspicious. When there is calcium deposits they need to be followed up to look for interval changes; and if the interval change or differences are too great, biopsies will be recommended.

Ultrasounds usually pick up lumps that can be felt; while mammograms can pick up abnormalities that cannot be felt physically. Mammograms are better for older ladies where the breast tissue is less dense (density determined genetically and by hormonal influence so if a lady is pre-menopausal breast tends to be more dense); while ultrasounds are better for younger ladies where breast tissue is more dense. However, if by physical examination a doctor can find a lump, then both mammograms and ultrasounds are ordered, regardless of the age of the patient. This is because neither one is diagnostic, so both examinations help a doctor determine the suspiciousness of the lump.

On the other hand, when we talk about screening x-rays, only mammograms should be used. Screening means there is no lump, and in nationwide screenings, women do not see any doctors. They just go for mammograms which would pick up suspicious abnormalities in about 10 per cent of the time; which requires further action.

The problem is that when a woman experiences a lump, she may not see a doctor; and just go for a screening mammogram and if the mammogram is normal; she may take that to be gospel truth and be assured. Unfortunately, this is when cancers are potentially missed. Screening mammograms are reported by two radiologists and the accuracy is about 75-80 per cent; meaning that in about 20-25 per cent of the time, it can still hide an early cancer but it is not obvious enough to be picked up. This is also the reason why women have to have it repeated on a one-two year basis to look for changes. Many women who have had one done in their lifetime think they do not need another one for 5 years. This is wrong thinking.

Ultrasounds are used as an adjunct to mammograms. Younger women (less than 40 years old) often question the need for a mammograms and like to come for screening ultrasounds. Screening ultrasounds have not been validated as a screening tool because the lumps and cysts that they pick up can be considered physiologically normal and therefore ultrasound patterns can change rapidly depending on the menstrual cycle. Only if the lumps are large (and therefore can be felt) will an ultrasound be useful. So ultrasounds cannot be used for screening.

Both ultrasounds and mammograms are of mid range price: between SGD $150-300. The only cost effective modality that has been tested is mammogram in older women of more than 50 years old in Scandinavian countries. Ultrasounds have not been tested for cost effectiveness for screening.

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