confusing diagnosis

vivo

Hello everybody..Last November my mom - 57 years old- was diagnosed with metastatic cancer in the left supraclavicullar lymph nodes with no axillary lympth nodes. The bad news came out of the blue because at that time she was feeling so healthy! The doctors decided that it was caused by the parotid gland but after the surgery, that took 8 hours, they found out that the problem was not there. As a consequence  the cancer was spread at her skin at the place of the surgery! After the prompt of a doctor who we found in a private hospital my mom went through  Pet scan and there was the first time the doctors found cancer in her breast with a little bone metastasis. The problem- if so- is that the cancer is so short that they cannot found it with simple or digital mammographies or needle. One toumor of the gland was HER2 positive and now my mom is on chemotherapy with hercepin since February and is doing well. However we are very confoused because the diagnosis is confusing. How can one so shot tumour do so harm- in fact it is not a tumour but some cells? The doctors say that it is very strange and that the cancer is stage IV. What are the chances the tumour to disappear since it is so short and if so is there a chance my mom to get through this bad dream?