Yes, there are definitely cancers that can not be detected on a PET scan for various reasons. A negative PET scan does not mean you don’t have cancer. It does not mean that there is no recurrence of cancer if you already have cancer. It is a test that is complementary to other tests done in the workup and evaluation of cancer.
How does a PET scan find cancer?
PET scan uses radioactive sugar to give us a map of how much energy the cells and organs of the body are using. The idea is that cancers will use more energy and therefore be picked up on a PET scan. PET scans are very good at staging many cancers. This means that the test will tell us if the cancer has spread to other parts of the body. It is also good at telling us where the primary cancer is.
Why does PET scan not detect all cancers?
There are areas of the body which normally look hot and can obscure a cancer. The bladder and kidneys because the PET agent is excreted in urine. The bowel often takes up PET agent and can obscure tumors. There is often patchy uptake in the liver which is normal and can sometimes not allow liver tumors to be diagnosed.
Cancer may be present and not show up on a PET scan because the tumor has low radioactive active sugar uptake. Some examples of this include adenocarcinomas of lung, especially the precursors and early ones. Carcinoid tumors and low grade lymphomas are some other examples.
Small tumors under a centimeter can be falsely negative. This is because they are below the resolution of the PET scanner. Therefore abnormal areas in the body that are small may be cancer but are too small for accurate PET scan evaluation.
A negative PET scan can still mean there is cancer. There are some tumors which do not show up on the PET scan. Additionally, some tumors are too small. While others are located in areas of the body which obscure the tumors. These are areas which are normally hot in the body.
Therefore, any suspicion for cancer does not end with a PET scan. Some abnormalities need to be followed closely with imaging while others may need a biopsy. A PET scan therefore is a test used in combination with all other information and testing.