Do you need to remove 100% of a lymph node to check for breast cancer, or can they be checked with a needle biopsy?

Yes: lymph nodes are only of concern if a malignant lesion of breast is present. Usually the breast tumors are small and the incidence of nodal involvement is low so that following excisional biopsy a sentinel node bx is performed removing only those nodes in the field of drainage. If a node is palpable, it is best to remove by excisional bx unless very large when aspiration can be tried.