There is no baseline. You go to the doctor because you’re concerned about a lump, probably because you had never felt it before. A scan needs to be done. It doesn’t always need to be MRI. But in my non-clinical knowledge of BC survivors, MRI are a must for young/dense breast tissue. It should be required.

I’ll give you specifics on a patient I know (I actually went to all doctors appointments).

32F. Feels something but hard to find next day, I felt it, she couldn’t. I urged her to see doctor. (I actually worked for a pathologist as college job, I know what pretty much every type of tissue looks/feels/smells like... and I knew it was cancer, but maybe benign). Anyways, she goes to doctor. He can’t feel it in exam. But, she has family history so he finds a way to get the MRI done. He explains why it should be standard on young women (denser tissue). Next day, MRI. Confirms lump, doctor is blown away as it was only ~0.5mm in diameter. He’s about 70 and it’s the smallest lump he’s ever identified, surprised it even showed up on MRI. Long story short, she had a full mastectomy ~5 days later and the pathology showed the lump was over 3cm in diameter, super aggressive (triple negative). Started chemo shortly after because the milk duct had opened and cells could have reached her lymph nodes and spread the cancer anywhere in her body.

I know many other people that have similar stories, so this is why you get thrown into full on crisis mode when you get a positive imaging. So I would ask, if she waited a month or more to come back and see if this thing was growing, how do you think that would have played out?