CT follow up for "circular shadow" in left lung

Recently my Dad caught H1N1 (Swine Flu). He recieved an x ray to see if he had fluid in his lungs, but the x ray also showed a very circular shadow in the left portion of his lung. Doctors wanted him to get a follow up CT because they didn't know what it could be.What could this circular shadow be? Cancer usually isn't symmetrical like that I don't believe. Also, my Dad says he thinks it could be a scar from when he had Pleurisy when he was 18. Could this be true? Any insights would be appreciated.