Maria, it's good that you are getting the perspective of the breast surgeon.

Breast cysts are very common, and most cysts - well over 99%, I suspect - are benign. But complex cysts can be more concerning because they contain both fluid (as all cysts do) but also solid masses or thick septations. The question is whether your cyst is really a complex cyst or a complicated cyst. Complicated cysts are common, and they fall in-between simple cysts (all fluid) and complex cysts (significant solid component); complicated cysts generally contain just small particles of solid matter. The problem is that the term "complex cyst" is often used interchangeably with the term "complicated cyst". Do you have a copy of the imaging report, describing the cyst? I also suspect that it's sometimes a bit of a judgement call on whether a cyst qualifies as a complicated cyst, which is very low risk and is usually monitored, or a complex cyst, which is higher risk and warrants a biopsy. So getting another opinion from the breast surgeon is important.

Cysts with masses and masses with cysts: An imaging review of cystic breast masses https://www.appliedradiology.com/articles/cysts-wi...

"Simple, complicated and clustered cysts are typically benign; however, complex cystic masses containing mixed cystic and solid components are indeterminate. Complex breast cysts have thick septations, thick walls, intracystic masses or other solid components. Between 23% and 31% are associated with malignancy; therefore, biopsyis required."

It's interesting that the cyst is in a duct - I've been hanging around here a long time, and have myself had lots of simple and complicated cysts over the years, and I can't recall hearing of that before. Usually cysts are in the breast tissue. What often forms in ducts are calcifications, which can be a sign of DCIS, which is a non-invasive Stage 0 diagnosis.

Good luck with the appointment and let us know what happens.