That's why King Charles' family doesn't reveal what cancer he has. That's why he left out one important detail – for good reason.
The public does not know what type of tumor it is. All that is known is that the new illness was discovered as part of Charles' recent treatment and the cancer has nothing to do with the prostate. The royals obviously chose carefully not to make certain details of the diagnosis public. When Buckingham Palace announced Charles' prostate treatment, it was to raise awareness, as the subject is taboo for many men.