In the catholic church?

From the Catechism of the Catholic Church, paragraph 2278

"Discontinuing medical procedures that are burdensome, dangerous, extraordinary, or disproportionate to the expected outcome can be legitimate; it is the refusal of "over-zealous" treatment.

Here one does not will to cause death; one's inability to impede it is merely accepted. The decisions should be made by the patient if he is competent and able or, if not, by those legally entitled to act for the patient, whose reasonable will and legitimate interests must always be respected."

In other words, the fact that medical treatment is possible doesn't put one under a moral obligation to accept it.

No sin to refuse cancer treatment, and it is not suicide. Even suicide isn't a grave sin anymore in the church is mu understanding. Cancer treatment is painful, even very painful and the treatment doesn't generally grantee remission.

It depends.

If your cancer is:

+ Curable then probably yes

+ Terminal then no

While the Catholic Church is the champion of the right to life in all circumstances, she never asks us to try to preserve life where God is clearly calling an person from this world to the next.

In other words, the Church does not place obligations on us to interfere in the dying process and in fact rejects attempts to do so as excessive and unjust.

The ultimate meaning of a person’s life is not found in this world but rather in the world to come, and the Church does not place obstacles to his or her entry into the fullness of Life.

Suicide is making a decision to take your own life

A person with cancer didn't choose to have cancer,

so its NOT choosing to end their own life

Some cancers are untreatable anyway, and some are too advanced to

have any treatment that would halt the progress of the disease

It depends on the diagnosis or the chance of a cure. If it were stage 1 or two, then there is a reasonable chance of success and maybe considered suicide, stage 3 or 4, the patient is better off preparing himself to meet his creator than prolonging a terminal condition.

That depends on the nature of the diagnosis.

IF the diagnosis is terminal cancer, refusing treatment isn't considered suicide.

IF the diagnosis is NOT terminal, then refusing treatment is considered suicide.