Hello and welcome to the forum. Sorry you are going through this as it is always a bit frightening. But it sounds like you have a very proactive doctor that you are working with which is excellent. Better to rule out cancer than to have any doubt. I will say that normally a dilated duct which is a fluid filled cyst in the duct often times hurts and causes nipple discharge. However, for some women there are no symptoms. Treatment for these usually entails antibiotics. Were you prescribed these? Surgery does occur for it but is not very common. Here is some information on dilated ducts in breasts. https://www.mayoclinic.org/diseases-conditions/mammary-duct-ectasia/symptoms-causes/syc-20374801
Ductal carcinoma in situ or DCIS is what they are wanting to rule out. This is a non invasive cancer inside the duct. In situ means it's stayed in place where the abnormal cells started growing. This, however, can change and it can become invasive. A lump is usually the sign of this which you can see has some overlap with dilated ducts. https://www.mayoclinic.org/diseases-conditions/dcis/symptoms-causes/syc-20371889 The biopsy will tell you which one. But it appears they feel fairly sure it is a dilated duct rather than DCIS and fingers crossed this is the case. Better to know as early treatment is always the best course of action with breast cancer.
When is your biopsy?