A CT scan is the right thing to do. You should try to locate previous chest x-rays and retrieve them for comparison with your mom’s present x-rays, or second best would be to obtain the reports of previous chest x-rays for comparison. It is way too early to start worrying about possible lung cancer, especially since we don't know anything about the size or shape. There is the possibility that it could just be a small fluid collection. If there is time prior to the lung scan, ask her doctor what the radiologist believes is the most likely diagnosis. Never having smoked, shifts the odds far in her favor.