I, a non-native English speaker, am writing an academic summary in medicine and I am trying to find a word for describing that cancer patients prognosis is bad. However, "bad" isn't an appropriate word for an academic context. The best I can think about is the first example below but is "serious" a suitable word in this case?
Cancer prognosis is serious.
Cancer prognosis is not always good.
Cancer prognosis is ________
Could you please help me with a suitable word?