It sounds like your tumor is quite small so they are going to put a small wire in the affected area so it can easily be identified during the surgery. I had a similar procedure done during a biopsy as one area of concern was small and the biopsy itself removed most of it. It is done with a needle under local anesthetic.
I had a wire localisation 3 years ago (rather than a magseed) because of the shape of the tumour & I think because it was non-papable. It was done the morning of my surgery. It was done in conjunction with a mammogram with some local anaesthetic. It was a bit uncomfortable & rather weird seeing a roll of coiled wires taped to my chest but basically fine. It was done at the breast clinic at 9.30. I then had to travel to the hospital for surgery, which didn't take place until 3.30 but I can't say I was in pain. I was told to be careful not to dislodge it.
