ERBB2 and neoplasm: This ultrasmall, particle-based design addresses current challenges in nanomedicine and immune targeting of HER2+ breast cancer, and achieves (1) highly specific HER2-targeted PET imaging with substantially reduced off-target accumulations in the RES or kidney over a 24 h period; (2) bulk renal clearance without excessive kidney irradiation; (3) very high tumor-targeting efficiency and high tumor-to-background ratios not attainable with scFv molecules or scFv-conjugated nanoparticles larger than 10 nm in diameter.