the importance of Ki-67 marker

Hello. A young woman of 26 diagnosed with infiltrating ductal carcinoma, at least one lymph node involved. Biopsy Est.75%, Prog. 25-50%, HER2 score1 range 0-3; Ki-67 ~50% (score 3-4 range 1-4). Undergoing chemo AC+Taxol. tumor reacts well to chemo, lumpectomy and radiation will follow  Chemo+surgery+radiation all targeted to wipe out this tumor;  why is there such danger of development of metastases ?? THIS  tumor, if it already sent some microscopic cells (no traces found elsewhere in PET/CT) , if the tumor reacts well to chemo ,  shouldn't we assume that its possible wandering cells also die from the chemotherapy ???
Thank you.

[Cleveland Clinic]

Dear overseas:  The concern is that this woman is extremely young to have developed a locally advanced (in the lymph nodes) cancer.  That there is lymph node involvement does increase the risk of recurrence.  The purpose of chemotherapy is to kill any and all wandering cells as you point out, however, this cannot be guaranteed.  We have no way of measuring this.  The comment on the genetic information is actually pertinent as carrying such a mutation would have potential implications for treatment and recurrence (new primary) rates.  

[SueYoung55]

The doctor will answer your questions but I wanted to suggest that this 26 yr old woman have BRCA 1 & 2 genetic mutation test done. Should she have this mutation she will need to speak with a genetic counselor who is savvy with BRCA mutations. The impact of BRCA positive mutations will need to be known by all of her physicians for treatment and follow up. Best wishes.

[overseas]

Thank you for your concern,  but I did not ask about genetics.  My question was VERY  specific  and for a reason.

[overseas]

Thank you, Doctor.  You reply was of great help  for my  "WHY poor prognosis"  question. As to all the necessary tests and therapy, everything  is being taken care of , as you may see in my brief summary. The oncologist is aware of everything and he takes very informed and responsible decisions as to the treatment of the patient.
I thanked Ms. SueYoung55  for her concern, but the genetic test had nothing to do with my  Ki-67 question; it simply was another matter of which we are aware and did not ask.
Again, I thank you so much for the explanation about Ki-67  and risk.
Sincerely,  overseas.