A PET scan is done after injecting a radioactive drug into your vein. A PET scan shows the metabolic activity of various tissues and abnormalities throughout the body. Many abnormal areas in the body will be hotter on the scan. A PET scan is often done in conjunction with a CT scan so the anatomy is better defined.
A PET scan is most commonly done to stage cancers such as lymphoma. It is ordered to assess treatment response and to monitor for cancer recurrence. It is sometimes done to evaluate lung nodules and see if they are cancerous. It is uncommonly done to diagnose cancer in my experience.
Although PET is widely accepted and used for cancer care, some abnormalities which are not cancer will also show up as hot on the scan. This is particularly true for sites of inflammation, infection, healing, surgery and other conditions. While PET is good at picking up abnormalities, it is not specific for cancer. It is therefore not a great test to diagnose cancer.
PET scan also does not detect all cancerous conditions, like low grade cancers of the lung or kidney cancers. It is indicated for specific cancers which are best evaluated after the specific radioactive drug is injected. PET scans are commonly done to evaluate lymphoma, breast cancer, head and neck cancer, lung cancer, colon cancer in addition to some others.
PET scan is not commonly done to diagnose cancer. Since abnormalities other than cancer can be hot on the scan, PET will result in many patients having unnecessary worry and testing for non cancerous conditions. Additionally, some cancers will not show up on PET scan. PET scan is therefore most useful for specific cancers and only when staging or monitoring. PET is not commonly used to diagnose cancer. That is often done with other imaging and biopsy.