My cancer did not show up on mammo or u/s either, but it did show up on an MRI. If you have dense breast tissue tumors are often missed on mammo and u/s. Breast MRI is fairly new technology and not all radiology facilities are able to provide them. MRI is also "too" sensitive, and results in many false positives. There are certain criteria that determine the indications for MRI use in breast imaging; high risk and dense breasts are among them. They are also costly so your insurance may make it difficult to get one. But as the others have already said, a biopsy is the only way to know what is there for sure! Good luck! We will be hoping for benign results!Frayda