We are looking for proof of concept here. It either works as...

We are looking for proof of concept here. It either works as intended or it doesn't. A placebo would add no value here. If we injected saline into breast tissue it would not show up a tumour let alone distinguish between a benign or malignant one.
If we injected nanos without the cancer seeking antigen then perhaps we might get a better overall picture because they would not bind to a tumour but that is not really what we are trying to prove.