This is probably a question for an oncologist or breast surgeon as I simply guide the procedures and the decision to perform the biopsy is made by the medical practitioners and specialists. The fact that we are routinely asked to perform FNA and core biopsies suggests that the oncologists and breast surgeons have no concern about increasing spread. It is certainly an issue with other areas like a sarcoma in a muscle for example. In the breast there seems to be no concern about such spread. If you watch a 14 guage core biopsy it really explodes through the tissue and must move cells in all directions. Despite this it is still considered best practice.