MAMMOGRAPHY RADIATION DOSE AND BREAST ULTRASOUND

By DR JACKIE BLUE Bsc, MBChB, FASBP

Radiation and Mammograms

Modern mammography has only existed since 1969 when the first x-ray machines used just for breast imaging were available. Since then, the technology has advanced a great deal, so that today's mammogram is very different even from those of the mid-1980s.

The modern technique uses studies that are high quality but have a low radiation dose (usually about 0.1 to 0.2 rad dose per picture). In the past there were concerns about radiation risks; today if there is a risk, it is very, very small.

To put the radiation dose into perspective, a woman who receives radiation as a treatment for breast cancer will receive approximately 5000 rads. If a woman had yearly mammograms beginning at age 40 years and continuing until 90, she will have received a cumulative dose of 20-40 rads. In 1999, BreastScreen Victoria in Australia compared one mammogram equivalent to 3 months exposure to naturally occurring environmental radiation.      

It is difficult to make a direct comparison of mammography with other imaging techniques because only the breast tissue is irradiated and a mean breast/glandular dose can be calculated. Conversely, in other imaging techniques such as chest x-rays or CT scans the radiation directed to the organ in question is converted to a whole body dose or effective dose.

However, if the breast dose is theoretically converted to an effective dose it is possible to allow a risk comparison to be made with other imaging. For example, compared with mammography one year of natural background radiation has the same risk, while a CT of the abdomen has a 50 times higher risk of causing harm and interventional cardiology has a 75 times higher risk (1).

There is a theoretical risk that irradiating the breast could induce breast cancer. Estimations vary greatly as real evidence available is meagre. Women must be informed of the benefit versus risk of not having a mammogram.

Breast cancer has been steadily rising in New Zealand over the last 30 years and this has been evident well before Breastscreen Aotearoa was established at the end of 1998. Similarly, private mammographic screening has been ad hoc over the last 20 years. Radiation cannot be considered to be the cause of this increasing trend.

Clearly, there are a number of factors involved in breast cancer causation and while we cannot prevent breast cancer from developing the only means available to reduce the breast cancer death rate is currently through screening mammography and effective treatment.

1.  New Zealand National Radiation Laboratory September 2001 "The Source"

Breast Ultrasound role in breast cancer detection

At this point in time, breast ultrasound is not considered a screening tool for breast cancer detection. There are a number of reasons for this. There has never been a randomised controlled trial to support that screening breast ultrasound can reduce breast cancer death, breast ultrasound is time-intensive and operator dependent and there is the added concern of high false positive and negative rates. Most importantly screening breast ultrasound it is not sensitive enough to be able to detect microcalcification. 

Microcalcification on mammography can be the first sign of ductal carcinoma in situ or DCIS. DCIS is a modern disease.  It did not 'exist' 20 years ago, as it has only been possible through high quality, modern mammography to detect microcalcifications. DCIS represents 20% of all cancers detected in a high quality mammography screening programme

DCIS is a pre-malignant or pre-cancerous condition that, over time, has the potential to progress to invasive cancer if left in the breast. The main difference between invasive cancer and DCIS is the potential for invasive cancer to invade into blood or lymph vessels and spread through out the body. DCIS is not able to do this as the cells are contained within the duct walls and it therefore remains localised in the breast and is usually curable by surgery alone.

Depending on the cells present DCIS can be differentiated by the pathologist into high, intermediate or low grade. It appears that high grade DCIS has the ability to become invasive over a matter of years while low grade DCIS probably takes 10-20 years.

However, over the last few years there has been an increasing number of articles published on the important role that it has in the work-up of symptomatic women, particularly those women under the age of 45 years or those women with moderately dense or dense mammographic breast tissue (1,2,3). In these situations the addition of screening US as well as mammography significantly increases detection of small cancers and depicts significantly more cancers

1.      AJR 2003; 180:935-940

2.      AJR 2003; 180: 1675-1679

3.      Radiology 2002; 225:165-175. 

 

Dr Jackie Blue BSc, MBChB, FASBP

Breast Physician

St Marks Woman's Health

PO Box 100083

NorthShore Mail Centre

Auckland, New Zealand

Ph 64-9-441 9690

Fax 64-9-441 9697

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