Cancer patients often receive a large number of scans for these typical reasons:
- For initial assessment of cancer status/disease burden
- To observe if treatment is working
- As follow up to assure that the cancer has not relapsed
- To monitor any complications of cancer (such as blood clots) or therapy
Types of radiology scans and when they are used:
PET/CT scan (see example below)
This amazing newer technology fuses images from a PET and a CT (or “cat”) scan. In a PET scan, radioactive glucose is given intravenously. This glucose will localize to tumors which are more metabolically active than normal cells. When looking at a PET scan for areas of tumor, the scan will light up like a “Christmas tree” in areas of tumor. Because PET scans cannot accurately look at anatomical locations, a “low resolution” CT scan is performed at the same time as a PET scan to localize exactly where the PET is lighting up.
PET/CT scans are used widely in many different tumor types (but not all). The major advantage over a CT scan alone is that a PET/CT will give us information on how big a tumor is and how metabolically active a tumor is. Treatment (especially with our newer targeted agents) can sometimes affect the metabolic activity of a tumor without affecting the size. Alternatively, the metabolic activity may be affected by treatment before tumor shrinkage occurs. However, note a downside to this wonderful imaging technique is that PET/CT is “oversensitive” and can give false positive metabolic uptakes leading to unneeded biopsies of sites of uptake. For examples, areas of infection or radiation fibrosis can mimic tumors in looking the same on a PET/CT. Additionally, a “pure” high resolution CT without the PET component can give better anatomical size information.
CT scan (see example below)
A CT scan gives a great anatomical picture of the body, but cannot give metabolic activity information. Another unique use of CT scan is to evaluate for blood clots in the lung.
MRI (see example below)
MRIs are predominantly used in our field to look at the central nervous system (brain/spine), bone, and liver, and the anatomical definitions provided by MRI in these areas is usually superior to CT scan. The major problem for patients in MRIs is claustrophobia. Open MRIs are available for these patients but the image quality can be worse than closed MRI.
This is a useful quick test to scan the legs/arms/neck for blood clots, to look for fluid in the abdomen, to get an approximate look at the liver or spleen, or to assess the function and anatomy of the heart (echocardiogram)
A nice video and website on echocardiograms: http://www.cardiosmart.org/HeartDisease/CTTVideo.aspx?id=878
A brief video on ultrasound of the abdomen:
This is a quick test that will give a black and white picture of the chest, abdomen, or bones.
Nuclear medicine scans
- MUGA- this is a test on your heart to check the ejection fraction. it is done when we use drugs (ie adriamycin, herceptin) that can affect your heart function. it can also be used when we are worried about heart failure. The MUGA will give a more accurate estimation of the ejection fraction than an echocardiogram. However an echocardiogram will be able to see heart anatomy while a MUGA will not.
- Bone scan– this test is used to see if prostate or breast cancer has spread to the bones
When is contrast used?
Iodinated contrast is used in PET/CT and CT scans. This contrast is given iv to better define lymph nodes and sites of tumors. It is also given by mouth when better definition of the intestines is needed. Patients can have allergies to iv iodinated contrast. There is also a small but real risk of kidney damage (sometimes reversible, sometimes not) with iodinated contrast. Gadolinium contrast is given with MRIs and does not cross react with iodine allergies. Gadolinium also has a small risk of kidney damage.
Comparing radiology scans from different institutions or companies
Generally speaking it is best to keep all your imaging under one institution or company. it is much easier to compare scans that way. Sometimes if you are constantly switching back and forth between institutions or companies it is difficult to compare scans. Also note that if you take 2 different radiologists they can read the same scan slightly differently. This especially can apply to PET/CT scans when reading the metabolic uptake of tumors.
Keeping a copy of all your scan reports and CDs of the actual scans is very important! if you switch doctors or move we may want to look at your old scans. or if you get a scan that there are questions about it can be very helpful to look at your scan with your doctor on their laptop.
What about overdoing it with radiation from all these scans?
Ionizing radiation is a concern to patients who have received multiple PET/CT or CT scans. We don’t really understand the long term side effects of ionizing radiation, but increased cancer risk is definitely a concern. The scans that give ionizing radiation are PET/CT and CT scans. The radiation given by PET/CT vs CT scan is approximately equivalent. Xrays also give ionizing radiation, but for example, a chest xray will give 1/500th the radiation of a CT scan. MRIs and ultrasounds do not give ionizing radiation. The amount of ionizing radiation given by nuclear medicine scans is usually small (i.e. the amount you would experience in a plane flight) and not of concern.