Case 53 Answer
I would describe this as asymmetric breast tissue. This brings us to some of the least understood terms in the BI-RADS lexicon:
What exactly is the difference between asymmetric breast tissue and a focal asymmetry?
Asymmetric breast tissue is a density that, after whatever work-up you need as a mammographer to make the assessment, you are CERTAIN is just normal breast tissue. I think this density is normal, just look at that lateral!
A focal asymmetry is seen on one or two views, and despite your best attempts to press it out, you are not 100% sure it is just breast tissue. For this YOU MUST perform an ultrasound to complete the work-up. If the ultrasound is negative, as they usually are in this situation, then this qualifies as "Probably Benign."
So, if you are not as convinced that the density in this case is normal, then it is fine to do the work-up, and follow the Probably Benign route. As you get more experienced, you will have more confidence in leaving things alone. Usually. Sometimes. OK no one is perfect and everyone is different, so just do your best, OK? For boards purposes, just follow the rules and you'll do fine!