Interesting questions because, as you may know from some of our other articles about medical malpractice, a plaintiff is typically required to show that the defendant's negligent conduct caused his injuries.  That requirement raises difficult issues when the plaintiff, would likely have died of his disease even if he had never seen the doctor in question.  However, in a case from 2008 (Matsuyama v. Birnbaum), the Massachusetts Supreme Judicial Court recognized the Loss of Chance Doctrine in medical malpractice claims.  The Court held that, if the doctor's negligence eliminates the patient's chances of attaining a more favorable outcome, that is a recognizable harm and the doctor may be liable for damages.  This approach makes sense to me because, for example, if the patient might have lived five years with proper treatment but only survived one year after the missed diagnosis, that is a huge loss.   Follow this link for another post about the Loss of Chance Doctrine.

With respect to damages, the Court instructed lower courts to use a somewhat complicated formula known as the "proportional damages method":

Applying the proportional damages method, the court must first measure the monetary value of the patient's full life expectancy and, if relevant, work life expectancy as it would in any wrongful death case. But the defendant must then be held liable only for the portion of that value that the defendant's negligence destroyed.

I hope this answer helps and, if you are asking your question because of a situation with a loved one, I encourage you to contact a medical malpractice attorney, someone who will be able able to explain this doctrine in much greater detail and offer an opinion about whether it may apply to your situation.