It's important. To tell your boss that you have a serious medical problem that is going to require a few months of treatment, but really is not their business to know the specifics, nor your obligation to talk about it with them or co-workers. This is why there is protected health care information. That means private.
Talk to your doctor. Why would you want to tell your boss that you "might" have anal cancer. Talk to your doctor to find out it you do and seek her/his advice.