How accurate is an ultrasound for breast 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.