Another Study
Points to Increase in Cancer Risk from Low-Dose Radiation Scans
Study looked at
senior heart attack patients; questions enthusiasm for imaging after
acute myocardial infarction; exposure should be tracked
Feb.
7, 2011 - Exposure to low-dose radiation from cardiac imaging and other
procedures after a heart attack is associated with an increased risk of
cancer, found a new study in the Canadian Medical Association Journal
(CMAJ). The study is another in a growing list raising concern about
the cancer risk of common medical scans.
The rapid
increase in the use of procedures with low-dose ionizing radiation, such
as computed tomography (CT) angiography and nuclear scans, has led to
mounting concern in the medical community that patients may be at
increased risk of cancer.
For patients
with known or suspected coronary artery disease, the trend towards
increased use of these procedures is particularly strong. In many
centers, these procedures are replacing those that do not use radiation,
such as stress tests on exercise treadmills and echocardiography.
However, many fear not enough is known about the effects of exposure to
radiation and the risk of cancer.
What is Computed Tomography?
By Food and Drug Administration
Conventional
X ray Images
All x-ray
imaging is based on the absorption of x rays as they pass
through the different parts of a patient's body. Depending on
the amount absorbed in a particular tissue such as muscle or
lung, a different amount of x rays will pass through and exit
the body.
The amount
of x rays absorbed contributes to the
radiation
dose to the patient. During conventional x-ray
imaging, the exiting x rays interact with a detection device
(x-ray film or other image receptor) and provide a 2-dimensional
projection image of the tissues within the patient's body - an
x-ray produced "photograph" called a "radiograph."
The chest
x ray (see at top) is the most common medical imaging
examination. During this examination, an image of the heart,
lungs, and other anatomy is recorded on the film.
Computed Tomography (CT), or (CAT) Scan
Although
also based on the variable absorption of x rays by different
tissues, computed tomography (CT) imaging, also known as "CAT
scanning" (Computerized Axial Tomography), provides a different
form of imaging known as cross-sectional imaging.
The origin
of the word "tomography" is from the Greek word "tomos" meaning
"slice" or "section" and "graphe" meaning "drawing."
A CT
imaging system produces cross-sectional images or "slices" of
anatomy, like the slices in a loaf of bread. The cross-sectional
images (Figure 2) are used for a
variety
of diagnostic and therapeutic purposes2.
The study,
conducted by researchers from the McGill University Health Centre (MUHC)
and the Jewish General Hospital in Montrιal, Quebec, looked at data on
82 861 patients who had a heart attack between April 1996 and March
2006, but with no history of cancer.
Of this number,
77% underwent at least one cardiac procedure with low-dose ionizing
radiation within one year of the attack.
"We found a
relation between the cumulative exposure to low-dose ionizing radiation
from cardiac imaging and therapeutic procedures after acute myocardial
infarction, and the risk of incident cancer," writes Dr. Louise Pilote,
researcher in epidemiology at the Research Institute of the MUHC and
director of the Division of Internal Medicine at the MUHC with
coauthors.
Two reports in radiology journal: Epigenetics may
determine risk of low-dose radiation... and explain mechanisms of aging,
human development, and the origins of cancer, heart disease, mental
illness, etc.
"Although most
patients were exposed to low or moderate levels of radiation, a
substantial group were exposed to high levels and in general tended to
be younger male patients with fewer comorbidities."
The median age
of patients was 63.2 years and 31.7% were women.
Patients whose
treating physician was a cardiologist had higher levels of exposure to
radiation compared with those whose treating physician was a general
practitioner.
There were 12
020 incident cancers detected during follow up, with two-thirds of the
cancers affecting the abdomen/pelvis and chest areas.
"These results
call into question whether our current enthusiasm for imaging and
therapeutic procedures after acute myocardial infarction should be
tempered," conclude the authors.
"We should at
least consider putting into place a system of prospectively documenting
the imaging tests and procedures that each patient undergoes and
estimating his or her cumulative exposure to low-dose ionizing
radiation."
In a related
commentary, Mathew Mercuri, McMaster University, and coauthors write
that, although the radiation exposure of many tests is often low, "the
frequency with which such tests are performed may pose a population
risk."
They say the
best solution may be prevention, which could mean using procedures with
lower or no radiation exposure, especially if there are multiple
procedures involved. They call for programs to track radiation doses to
help patients and physicians track the risk of cumulative exposure.
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