That is a hard one as most of our cancers were picked up on a mammogram or a CT scan. Mammogram, Ultrasound, CT, MRI and PET scan all show tumors. Personally, I think PET scans are the better of all scans, but insurance companies won't always pay for them. Another point to make is the eyes of the radiologist who read the scans. My sternal Mets was present in a CT scan I had 4 months before surgery but wasn't 'seen' by the radiologist. When I had another CT scan 2 weeks after surgery it was found, and they went back to look at the previous scan to compare and then they noted it was there. It wouldn't have made a difference in my outcome but for some women this kind of mishap would matter if a Breast mass wasn't seen .

There’s definitely such a thing as medical harassment!

So,we have to wonder what good are mammograms? I've said this here before, after a mammogram detects something, they send you to get an ultrasound and then they say they need to get better pictures so they order a CT scan but then they need a closer look so they send us to get an MRI. So why not skip the ultrasound & CT and get an MRI ? Because we are money making machines and that really pisses me off to no end. So, I am refusing the mammogram she wants me to have which is 3 months late because even though she says its just to see if Anastrozole is still working and removal wouldn't happen, I just know she would suggest a biopsy. I am not going down this endless road of scans and being cut open again.

And mine didn’t show on Mammogram. It showed on ultrasound, MRI and Pet however. So it depends on the cancer.

Pet scan is the most powerful one, but also, it picks up cancers that are bigger than some small size (info probably can be found online).
But micrometastases may not be detectable until surgery via pathology. There are indeed cases where women had all clear scans just before surgery but cancer was still found.