What percentage of women who have lumps in their breasts are diagnosed with breast cancer after being scanned?

Smallbutsignificant: A lump in the breast is the most common presenting sign of breast cancer, so one must never assume it's benign without working it up. That said, often they are benign. Btw while scans and radiological procedures are very useful in working up breast cancers, a lump which can be felt needs to be biopsied regardless of a normal mammogram.

Not very high: Majority of lumps ( not masses ) are lumpy breast with areas of thickness are not malignant, but biopsy is needed not to miss a cancer, about 20% of time they will be malignant.

Routine: X-ray mammograms are falsely postive 5-15% of the time, which means that a a "suspicious mammo is quiet likely to be cancer, but ultrasounds are commonly used to help define characteristis and biopsy. Mri is used in woman with brca 1&2, and "dense breasts". When used indiscriminately, the false positive may be greater than 30%.