The email of the question that I will be answering was titled “stupid question”. For clarification, I mean it when I say there really are NO stupid questions. Doctors spend time around other doctors and we speak a common language that we learned in medical school. Understandably, this can sometimes result in not explaining things in ways that are comprehensible to our patients. I promise that we are not intentionally trying to confuse you; it just makes sense in our minds. Please forgive us! Always feel free to ask your doctor to clarify. Ask me to clarify my posts in the comments section. You are doing us a favor and helping us learn how to better communicate medical lingo into english!
Reader’s Question: My doctor says that some cancer is too microscopic to be picked up on PET&CT then how can they every give a patient an all clear or NED?
CT scans show shape, size, and location. CT scans can identify new masses or if taken serially over time can detect growth. Typically masses need to be larger then 1 cm to detect and distinguish from other normal surrounding structures such as lymph node. The minimum size for detection varies according to area in the body. In places such as the lung (where there is lots of air and less normal tissue mass) much smaller masses can be picked up. When I use the word “mass” it means there is aggregation of cells that can be seen by imaging. It can refer to a growth such as a benign or malignant tumor or an infectious/inflammatory process. Often the radiologist can make inferences based on the imagining to the most likely pathology of a mass, but not always. Biopsy of the mass is the best way to know for sure.
CT with contrast provides information about blood supply. Often CT scans are obtained with and without contrast. Contrast is a dye that is injected into the veins during scaning and turns blood vessels opaque on images. It is the injection of contrast that gives a weird sensation during the scan. Contrast CTs are often avoided in the setting of kidney dysfunction. The contrast enables the radiologist to pick out more details in terms of location because it clearly distinguishes blood vessels from other structures thereby providing landmarks. These scans also provide more information about the etiology of the mass and may help the radiologist predict whether the mass is benign or malignant. A mass that picks up contrast is said to be “enhancing” on CT scan, and this can happen at different time periods after one is injected. Infectious process such as abscesses can often clearly be diagnosed by imagining because of the distinctive appearance. Many malignancies are enhancing because they have a robust supply of blood vessels. But in most cases the only way to truly distinguish a benign mass from a malignant legion and to identify the origin of the mass is to have a biopsy taken and then have a pathologist analyze the tissue under a microscope.
PET scans pick up tumor activity and inflammation (to a lesser degree). Increased PET activity means increased sugar metabolism. During PET imaging your blood is injected with a “tagged” sugar about an hour before the images are taken. The scanner visualizes this tag. While you’re waiting to be scanned your body’s tissues and cancer (if there) have a chance to metabolize the tagged sugar. Tumors are PET avid if they metabolize the sugar and then “light up” on scan. It can pick up both tumors and inflammation. The degree of increase of this activity is sometimes more characteristic of a tumor (higher) or inflammation (lower) and sometimes it can be hard to tell. The radiologist can put a numerical value on the degree of activity, called SUV. By repeating serial scans over time, we can determine if an area is becoming more or less active. There is also a minimum size that a tumor needs to be before it’s big enough to metabolize enough sugar to be picked up on PET scan. There needs to be a large enough mass of cells metabolizing sugar to show increase activity above background. Furthermore, not all tumors show up on PET scans. Some tumors just do not metabolize this sugar fast enough to show up on images. Therefore PETs are best for following over time. If you already know that the cancer in your body “lights up” on PET scan then one can follow the activity over time to see if things are getting better or worse.
PET-CT scans are a merged image of both of the above. PET-CTs are merged images that give data on size, location and activity. They are helpful in locating the location of the PET activity. In these cases the CT scan is non contrast CT scan.
Cancers grow when present and untreated. Cancer grow at different rates, from slowly to rapidly. Triggers may cause a cancer to stop growing or signal re-growth. Growth rates are often described as the amount of time that it takes a tumor to double in size. If a cancer is microscopic in size and there are only a small number of cells that comprise the tumor and are doubling, it will take time before it will shows up on scans. For example let us take two tumors with the same growth rate over the same time interval. If one of the tumor is the size of a grain of rice and doubles in size it remains undetectable over that time interval. Compare this with a tumor that is the size of kernel of corn, in this case with the same doubling time the tumor may go from undetectable to detectable on scans at the same time interval. We can declare a number of cancers cured when they have serially been reevaluated for 5+ years and have never shown up because data from lots of patients with the same disease give us the window of time for which relapses occur. The number of years that it takes to declare a cure varies amongst cancer types, some take more then 5 years. Other cancers do not have reliable data (maybe because there rare, more difficult to cure, or are known to have later relapses). In these cases patients with no disease are perpetually labeled as being in remission. The good news is that the further in time that one gets from treatment and reevaluation shows no evidence of disease then the more likely it is that there is no tumor at the undetectable level as well.
NED=No evidence of disease. This does NOT mean there is no microscopic disease but means that there is no tumor that can detected with current medical imaging. We can celebrate that we are not seeing anything on scans, but we will need to repeat scans over time to assure that nothing is growing. When there is nothing that we can see it is also means that there is not anything that is going to harm us at this time.
When I was interviewing for fellowship programs in pediatric oncology, one of my interviewers told me that she was trying to prove someone did not have cancer. She remarked that it is much harder to prove that someone has “cancer” then to prove they “do not have cancer”. This remains true.