Wednesday, April 10, 2013

CT and PET scans

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….