It really can’t as it has to be sent to pathology to determine if the cancer has spread to those nodes. That takes time! I know this because I asked my surgeon! I wish they could do it all during one surgery but apparently not possible!

I had the radioactive dye injection ( two actually as I had two known tumors) day before my unilateral mastectomy. My surgeons then used a pen sized Geiger counter ( so to speak) to trace the flow from tumor to nodal system. One sentinel node was known positive through a previous biopsy, a second looked suspicious at surgery ( and was positive) and then the next layer ( in the nodal flow pattern) of 5 axillary nodes were removed … all of those were negative!

The value of the injection is that it allows the surgeon to trace the path and remove any suspicious nodes along that path up to a clean layer ( so to speak). So they do need to remove more than what is immediately obvious.

The injections were brutal but very quick but have given me an abundance of relief to know that my next layer above the positive nodes was clean! I think of it as clean nodal margin.

My understanding is that the black spots would show in any areas that the dye had reached and detected cancer. If the dye had reached nodes beyond tne closest ( sentinel) nodes; they might do more.
I signed my release for surgery, after the surgeon reviewed the nuclear medicine resultsie- at 10:40
She was waiting to see if the dye drained to the armpit, neck or leg- the melanoma was in a place where it could have gone to any site.