I did go through this awhile ago. I had a cyst that showed up when I was 15. The doctor did an ultrasound and drew fluid from it and said it was up to me to have it taken out. I had it taken out/biopsy 10 years later when I was 25 (which was 10 yrs ago now). I just wanted to get rid of it at that point as it was painful at times. Another one turned up a couple years later, and I was given a surgeon's name to contact again. I chose not to do anything about that one and it hasn't caused me any trouble (except showing up as assymetrical tissue on my mammo & ultrasound in which I have to go back in 6 mos. for another check up like you). I know how you feel as the whole thing makes me nervous, too, but I think this is pretty common and it's good news that they want you to come back in 6mos. If the radiologist felt that this is serious, they would do a biopsy. I read this monitoring over 6mos is the route they are going now instead of surgery like I had/almost had years ago. Take care and best wishes to you!
Usually if things look at all suspicious, the radiologist will indicate that a biospy should be considered. Otherwsie early followup is typically recommended.
However, if you are not comfortable with that, you could: 1. Have another US sooner, perhaps in 3 mths; 2. Ask to have a radiologist in another practice review the images,as a second opinion; or 3. Request a biopsy.
Best wishes...