The cells are already exposed to oxygen via your own red blood cells coming into contact with them.
They do not exist in a vacuum, but rather they are mixed into the stroma of the gland. It’s only when a large number of them grow together in a mass that it becomes a distinct tumor.
A lot of the ‘focal’ therapies, by necessity, have to ablate or destroy healthy cells at the margins (like radiation) and cannot simply destroy the tumor only.