Can not exclude something is terminology that is sometimes used in reports ranging from X-rays to high level modalities like CT and Pet Scan. Often the radiologist who uses this terminology means that the abnormality mentioned is something that is a possibility but by no means a definite diagnosis. In other words, it’s in the realm of possibility amongst other diagnosis.
I see this terminology often used when the radiologist is not sure of the diagnosis but wants to remind the referrer that a serious diagnosis remains a consideration. Often, this can apply to illnesses like cancer, infection and other serious illnesses. Imaging studies are often not clear cut, and providing a range of diagnosis is appropriate.
An example of this can be with a spot on a chest x ray. Let’s say a 37 year old comes in with cough and fever to the emergency room. A chest x ray is done to look for the most likely diagnosis which is suspected respiratory infection and pneumonia. The 37 year old may have a spot that is likely to represent a pneumonia on chest x ray given the age and symptoms. However, a spot on a chest X-ray can also be a cancer or other cause. Therefore the radiologist may state that this likely represents a pneumonia but a cancer can not be excluded.
This will prompt the referring physician to note that cancer while less likely, remains a possibility at least based on the imaging. In this case, the referring doctor will offer follow up for the patient. This may consist of another chest X-ray to make sure the white spot clears after antibiotics. Perhaps the referring physician would do this anyways, but the radiologist provides guidance and tries to prevent a bad outcome for the patient.
Sometimes imaging findings really don’t look like anything typical or the symptoms of the patient strongly point to a diagnosis. In these cases, the radiologist will provide the best and most likely diagnosis and then may say that something more serious can’t be excluded. Perhaps infection or cancer. He may offer guidance on what to do to exclude this possibility.
Better to be safe then sorry. There are a range of presentations and possibilities for imaging findings. It may be easy to exclude more serious diagnosis with another imaging study rather then play the odds. Young patients get cancer and other serious illness. Some radiologists will use different terms like diagnosis x is most likely and diagnosis y and z are less likely. But they are both saying essentially the same thing.