How accurate is an ultrasound for breast cancer?

It depends. on what specifically you are asking. Ultrasound can be used for screening asymptomatic women, it can be used in a diagnostic setting to evaluate lumps or abnormalities detected on other imaging, or for biopsy guidance. In the screening setting, it is 60-80 percent sensitive for detecting cancer. .
Very. When ultrasounds are used to evaluate a palpable lump or a mammographic abnormality, a directed ultrasound can very accurately differentiate a benign tumor from a malignant one. Nevertheless, it is best to not rely exclusively on an ultrasound and to proceed to tissue diagnosis via biopsy, especially for women over age 40.
Very accurate. Breast cancer usually makes or presents as a mass or tumor or a lump. Ultrasound is very accurate in such cases and may be used to do a biopsy.