The doctor(s) or practice which spotted the mass 5 months ago and never brought it to your attention could indeed be liable if you suffer any ill consequences, bad medical outcomes, disability or life impairment, or increased medical costs (over what they would have been had the mass been addressed earlier) which result from the delay, since it would seem to almost be malpractice (professional negligence, or carelessness) to not point a large mass out to a patient.