Do physicians have an extraordinary lower incidence of cancer, as compared general population, since they are experts in maintaining a healthy body?

Yes and no: We are actually much healthier than other folks; anyone telling you otherwise is a doctor-basher who compares older physicians to all the population including children and teens. Cancers due to smoking, alcohol, and lack of sunscreen can generally be prevented; most of the others can't but we tend to find them earlier and get treated effectively. Best wishes.