To use your example of cancer, if you were diagnosed by the very best professionals with (instead of cancer) something, literally "some.... thing", would you consider that you had enough information with which to make an informed choice?
I'm going to expand on my illustration because it was for me a real experience.
Option 1 is effective, but has side-effects. Option 2 doesn't have the side-effects, but all the information pertaining to its effectiveness is not yet available. Option 3 isn't quite as effective, but doesn't have the side-effects. Option 4 is to wait for all the information is available for option 2. Option 5 is not to choose, which is itself a decision. What do YOU do?
Athiesm (option 5) says "none of the above"; agnosticism (option 4) says "I can't choose because I don't have enough information to make a completely informed choice." Because time is of the essence, both are effectively making the same decision.
Theism says one must have eyes trained to the task; atheism says there is nothing to see; agnosticism says since it is unsure, there's no sense in going to the trouble of training the eyes. No amount of sophistry (nuance) will relieve one of this dilemma.