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"
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