Golly gosh, @pjs05 - that would be SO frustrating ..... so 6 'call backs' but always clear?
What age are you? Are you on HRT?
I had a major concern 20 years ago when I was on HRT for severe menopause symptoms (and had one big scare in my late 20s - where I was told I had 'lumpy breasts' - which I assume now, means dense breast tissue.)
The testosterone component of the HRT caused changes in my breast & I was sent to John Hunter Hospital for more indepth testing (I live in the Mid North Coast.) I then stopped the HRT & my next 15 breastscreens were 'clear' ..... until 2017, when my GP found my lump (after a clear Mammogram) , surgery in 2018.
Have they done a full body MRI/CT on you or just your chest area? If only the chest area, maybe agitate for a full body one (as a point of reference for any scans afterwards ....)
take care - they are just making sure you are clear, which is a good thing, tho VERY frustrating. :(