If discontinuing treatment is in the best interests of the patient then it is not an omission so does not form the actus reus.
Creating a dangerous situation
Miller (1983) - where D failed to take reasonable steps to deal with the fire he had started. He had created a dangeroussituation and owed a duty to call the fire brigade. He was therefore liable when he failed to do so.
Duty undertaken voluntarily - based on reliance
Stone and Dobinson (1977) - where D's took in an elderly relative and failed to look after her. D's were liable for her death.
Special relationship
Gibbins and Proctor (1918) - where a father starved his 7 year old daughter to death. He had a duty to feed her and the omission formed the AR of murder.
Officialposition - usually related to public office
Dytham (1979) - where a police officer stood by while V was beaten up. D was guilty for failing to perform his duty while in a public position.
Contractual duties - arise through contracts of employment
Pittwood (1902) - a railway crossing keeper had a duty to close the gates. His omission formed the AR of manslaughter.