The short answer is that a PET scan helps in the workup of an abnormality in the lung, but can not definitely rule in or rule out a cancer. A PET scan is often ordered when there is uncertainty as to whether an abnormality found in the lungs is cancerous. The idea with PET scan is that cancerous tissues will be more metabolically active and be hotter on the scan then non cancerous abnormalities.
The abnormality that is often found on CT scan of the chest generally has to be larger then 1 cm or so for PET to be most accurate. Also, PET scan has overlap between the appearance of cancerous and non cancerous abnormalities, like infections. Also, some cancers are not hot on PET scan, and give what’s called a false negative result.
So what good are PET scans then if they give such uncertain results? In my experience, they are most useful when an abnormality lights up much more then the background of the body. We measure this with what is called an SUV. Basically, how much hotter is the abnormality then the background. When an abnormality lights up and has a higher SUV, it becomes much more concerning for cancer.
When you have a hot abnormality on PET scan in the lungs, then a biopsy will likely follow. A hot abnormality in the lungs on PET scan does not necessarily mean cancer, but it makes it a more likely possibility. Other things like infections can also look like cancer. In some cases, it is better to wait a month or so, to see if the abnormality clears. Idea being that a pneumonia will eventually clear up while cancer won’t.
An abnormality that doesn’t light up bright or only slightly doesn’t rule out cancer unfortunately. This may allow the biopsy to be delayed or cancelled however. In many cases, these are inflammatory abnormalities or non aggressive early cancers, which can be followed with serial CT scans. If there is a change in the appearance of the abnormality, like growth, then a biopsy may follow. Sometimes the abnormality will go away because it was inflamed lung which resolved.
In some cases, the PET scan will be more suspicious for cancer, like when the radiologist finds abnormal hot lymph nodes or hot spots in organs or bones. This may indicate metastasis. Biopsy will often follow, as confirmation of spread of the cancer far from its origin will mean it’s late stage and surgery is not an option.
Therefore, PET scan is just one piece of the workup of an abnormality in the chest. It can not determine whether something is cancerous or not, but it can help guide the next steps and provide a level of suspicion. As always, the results of a PET scan are best managed by an experienced specialist such as a pulmonologist in the context of your entire history and results of any other testing.