There was a lot of press in the last year about the dangers
of excess scans. Several key articles in
the New York Times brought the topic to the forefront of people’s minds. Rarely does a day go by where at least one
patient articulates concern about how many scans they have had. Other patients ask about the differences
between PET and CAT scans. Well, here is
my attempt to distinguish between them and characterize their risk.
CT scan (which is short for CAT scan) stands for
“computerized axial tomography.” It uses
an x-ray source to take pictures of the insides of the body. In the “olden days” (1980’s) tomography was
utilized for a variety of purposes. You
needed to move the x-ray source in one direction and the detector in the
opposite direction at the same speed.
This created blurriness to everything except for the “axis” or pivot
point between the two. I am sure it was
good for something but frankly, I cannot remember what.
The big advance came when you could begin to use computers
to get a digital interpretation of the images and create an entire “plane” (two
dimensions) within the body instead of a line (one dimension). This allowed you to take “cuts” of the body,
nowadays it is typically 5mm in thickness.
When your doc flips through the pictures in the office, each image is
“one cut.” Using software that allows
you to flip between images pretty quickly allows you to get a pretty good
impression of what is going on in 3D space and compare from one scan to the
next.
PET scans stand for “positron emission tomography.” Very different technology. There are a lot of different things you can
measure with PET using different reagents but the one we are most accustomed to
is metabolism. FDG (flurodeoxyglucose)
is just a sugar with a small bit of radiation on it. For reasons we are only just now starting to
understand, cancer cells have a different type of metabolism than normal
tissues. They soak up the sugar and you
can measure that with a positron detector (not an argument for low sugar diets –
see my post on nutritional supplements).
In short – CT scans are just pictures of the inside and PET
scans detect differences in metabolism.
I think there is a bias to believe that PET scans are more “sensitive”
than CAT scans because they are newer and cost more. Not necessarily true. Some diseases like CLL/SLL, and some cases of
mantle cell are virtually invisible to PET scan because the metabolism isn’t all
that different (though it can be helpful if Richter's Transformation is suspected). Other diseases like
DLBCL can be very “hot” on the scan and sometimes you find disease in places
you didn’t see on CAT scan.
There are sometimes that one scan is better than
another. Most of the time when you get a
PET scan, they are also doing a CT scan at the same time so that they can
overlap or “fuse” the images. One of the
problems though is that PET can find a lot of things that are not even
there. Sometimes you get non-specific
uptake in the colon. The kidneys, heart,
and brain are always “hot” so reading PET scans there doesn’t always work well.
One other question I get all the time is about, “how many
CAT scans can I get before I get another cancer from all the pictures?” In my mind this has gotten blown way out of
proportion. I understand the concern,
but studies have been done and show that even though there is a clear
attributable risk – it is quite small.
Since CAT scans use X-rays they can cause DNA
mutations. Keep in mind, that happens
when you are bombarded by cosmic x-rays all the time – even worse if you fly in
planes a lot. First of all, you need a
lot of scans before you are in the risk zone (probably on the order of 15-25
scans of a particular body part). That
is a number many cancer patients may actually accumulate – particularly in
patients with a long natural history of disease. Once that number has been accumulated, there
is approximately a 1-3% risk of developing a cancer at a time interval of about
20 years. Yes it is real – but it is not
a major risk.
When I worked in ER’s I was always surprised to see an
occasional young patient who had been in the ER monthly for two years
complaining of abdominal pain. Perhaps
they had some emotional issues or legitimately had an undiagnosed medical
condition. Unfortunately, you cannot go
into an ER with belly pain without getting a CT scan. It was not uncommon to see a young patient
who had gotten 10 scans or more and never had anything going on. That is the patient I worry about most.
While I think you want to be judicious about scans – if you
have a known cancer and there is an appropriate medical question being asked, I
think it completely justifies the risk in most cases.
Hope that helps “shed some light” on the subject….
