Lesions that Produce Posterior Acoustic Shadowing. Acoustical shadowing is not always indicative of malignancy. Cooper's ligaments may shadow because of refraction from the steep angle of incidence. Fibrosis or a previous surgical scar tissue may produce an acoustic shadow. These fibrotic areas are particularly seen in the upper outer quadrant. Fibrocystic breast tissue with pericystic fibrosis may also cause shadowing.
Ultrasound Appearance Of Malignant Disease Of The Breast
* irregularly marginated (especially anterior margin)
* heterogeneous internal echoes
* acoustic shadowing
Taken from:GE Healthcare but no idea how reliable a source it is.