My sister has this cactus with those brown spots in it. It does not seem to be spreading, or if it is, is very slowly. Does anyone know what it is so we can treat it?
Assuming the brown patches are dry and hard, not mushy, they look like "scar tissue" that is basically a bit of the plant that has died after it has been damaged.
They won't spread or cause any other problems, but there is nothing you can do to get rid of them.
The original damage could be caused by insect pests, mechanical damage (e.g. the plant was knocked over when it was smaller and damaged the lower part of the stem) or irregular watering which can cause cracks in the stem if it takes up too much water quickly and "bursts". Sometimes the damage appears spontaneously when a flower bud develops.
From your picture there is no way to tell what the cause might have been.