Some data show that it could take 10-15 years for an observable tumor to form. It is clear to cancer researchers that the original tumor could persist for years without metastasis. However, once the tumor spreaded, they grew really fast. The risk of needle biopsies is that it may break the possible body shield around the original tumor, and seed tumor cells along the track of biopsy needle, if you wait for 7 days, there will be 7 days’ chance of blood seeding of these tumor cells either from the needle track or from the “shield-breaked” original tumor.

Breast Cancer Risk Factors

http://www.cancer.org/Cancer/BreastCancer/OverviewGuide/breast-cancer-overview-what-causes