you have history being disrespectful with staff and i do believe joking about cancer could be offensive to some people.
Anything you say could be taken as offensive. You could get offended about this comment, but its a risk I am willing to take to change your mind in pursue of a fair action.
Instead of saying it could be offensive I would describe it as an action aimed to offend a staff member as that it was
here is an interesting piece regarding this from a person I respect very much. (its less than 2 minutes you have time to watch this)