What is correct grammar a patient died from cancer by cancer.?

The grammar is not quite correct. Here's why and how to fix it:

* "By cancer" is redundant. "Died from cancer" already implies that cancer was the cause of death.

* "Of cancer" is a more accurate way to express the cause of death.

Here are some correct ways to phrase the sentence:

* The patient died of cancer.

* The patient succumbed to cancer.

* Cancer claimed the patient's life.

Let me know if you have any other grammar questions!