I've been thinking about this topic alot tonight and while I fully stand behind my previous comment about being of sound mind, the question maybe is when? The thought of dementia scares me far more than cancer. Honestly, if I got to the point of not even recognising my own husband and kids... well I would want out.
So for me, I would like to see it as a legal document which can be created ahead of time much like organ donation. A person could say they request euthanasia for the following medical issues, of course being of sound mind when signing. Then if those illnesses happen, then the document is there to guide med pros even if that person is no longer of sound mind.