How can a ultrasound miss breast cancer?

It can: Breast ultrasound is used to distinguish cystic vs solid mass and then decide need for biopsy.Mammogram is used for diagnosing breast cancer and yes it may sometime miss cancer.

Dense Breast Tissue: The breast can be difficult to image due to dense glandular tissue. In addition, some tumors are very hard to see. No imaging test is perfect, and often we will use a combination of mammogram, ultrasound, and mri, depending on the density of the breast tissue and other risk factors. It is important that if you feel a lump even if your imaging is "negative", to see a specialist.

With standard U/S: It is directed by the mammo to an area to define size shape, solid/cystic. Emerging technology: breast u/s tomo uses an array of detectors similar to ct...Very sensitive even in dense breast, but not available most places.

SCREENING BREAST CA: Mammogram is the gold standard for breast cancer screening, ultrasonds are used to differenciate solid/fluid filled cysts and serve that purpose.Ultrasond can miss cancers more often.Mammogams can also miss some early cancers, and has well recognised limitations. So ultrasond and magnetic resonance imaging is used along with mammograms in high risk patients.