What are the issues related to medical treatment that prolongs life of questionable quality? While reading an answer to How to overcome the suffering of attachement to the body's continued existence (in Western modernity)?, I began to wonder about a related matter.

In the answer:


  In preparation for death you should:
  1) accept the fact everyone must die and let go of the fear of death


OK, but how about this situation:  

Let's say I have terminal cancer, and the doctor says I will only live another 6 months. However, if I get some difficult treatment (let's say chemotherapy that often causes severe pain), I may live up to 10 years longer.

So from a Buddhism point of view:
1) Would there be any concern if I didn't get any treatment? It isn't suicide, but I am not taking an action will hasten death.  

2) Would there be any concern if I did get the treatment? Does that imply a fear of death, by postponing the inevitable? And since a goal of Buddhism is to reduce suffering, while understanding pain can be managed, can voluntarily increasing ones suffering be justified?

In each case, is there any impact on karma looking to the next rebirth?

Finally, I am not referring to medical treatment in general.  Only when it is very invasive and severely impacts ones normal life (and lifespan).
Here are some quotes from commentaries on the monastic rules.

The Bhikkhus' Rules --
A Guide for Laypeople


  Properly considering medicinal requisites for curing the sick, I use them: simply to ward off any pains of illness that have arisen, and for the maximum freedom from disease.


Also, "praising the advantages of dying" is considered in the same category as "intentionally causing death":


  Should any bhikkhu intentionally deprive a human being of life, or search for an
  assassin for him, or praise the advantages of death, or incite him to die (saying): “My
  good man, what use is this evil, miserable life to you? Death would be better for you than
  life,” or with such an idea in mind, such a purpose in mind, should in various ways 
  praise the advantages of death or incite him to die, he also is defeated and no longer in
  affiliation.


The Khandhaka Rules


  “A sick person endowed with five qualities is hard to tend to: He does what is
  not amenable to his cure; he does not know the proper amount in things
  amenable to his cure; he does not take his medicine; he does not tell his
  symptoms, as they actually are present, to the nurse desiring his welfare ...


I think the above implies that the patient has some kind of duty of care towards their nurse (as well as vice versa).

The Patimokkha Rules


  The Commentary extrapolates from this case to apply the dukka˛a to all
  attempts at suicide, including even the decision not to take food when motivated
  by a desire to die. However, it then runs into the question of how far this penalty
  applies to a bhikkhu who is ill. Its verdict: As long as medicine and attendants are
  available to him, the penalty would still apply. But then it lists two cases where
  the penalty would not apply: (a) A bhikkhu is suffering from a long and serious
  illness, and the attendant bhikkhus are fed up with caring for him, thinking,
  “When will we be free of this sick one?” If the bhikkhu reflects that, even with
  medical care, his body won’t last and that the bhikkhus are being put to
  difficulties, he incurs no penalty in refusing food and medicine. (b) A bhikkhu—
  reflecting that his illness is harsh, the forces of life are running out, and yet the
  noble attainments appear to be within his reach—may refuse food and medicine
  without penalty.
  
  The Commentary’s deliberations here show how difficult it is to legislate in
  this area, and there are reasons to question the way it applies the Great
  Standards here. Case (b) is apparently derived from SN 4.23, where Ven.
  Godhika takes his life and gains arahantship just moments before death; and
  from SN 35.87, where the Buddha says that one who puts down this body
  without taking up another body dies blamelessly. However, in arriving at its
  verdict in this case, the Commentary has to add the factors of motivation and
  perception to the equation, factors that are absent from the rule on which the
  judgment is based. It also leaves unanswered the question of how harsh the 
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  disease has to be, and how near the anticipated attainments, to qualify for this
  exemption.
  
  This same holds true for case (a), which entails even more dubious reasoning.
  The Commentary’s judgment here has no clear precedent in the Canon; there is
  no clear line for deciding exactly how bad the illness and how fed up the
  attendants have to be for this case to apply; and why should the feelings of other
  people determine when it is or is not allowable to refuse food?
  
  It is worth noting that the origin story to the original rule here gave the
  Buddha the opportunity, had he wanted it, to formulate a general rule against
  attempted suicides, but he chose not to. He later formulated this subsidiary rule
  only when a bhikkhu attempted a suicide in a way that endangered the life and
  safety of another person. Thus a more appropriate way of applying the Great
  Standards to this subsidiary rule would be to extend it only to cases of that sort:
  where a bhikkhu’s attempts at suicide would bring danger to another person’s
  life and limb.
  
  As for ways of attempting suicide that do not endanger others, it seems
  better to follow the Buddha’s wisdom in not legislating about this issue at all, and
  to treat it as a matter of Dhamma rather than Vinaya. In other words, one
  should keep in mind his comment in SN 35.87 that the only blameless death is an
  arahant’s. If, lacking that attainment, one chooses to refuse food when ill to
  speed up one’s death, one should be heedful of the risks that death and rebirth
  can involve.