What is it?
Screening breast ultrasound, which looks at the whole breast, is the most commonly used supplemental (additional) screening tool used after a screening mammogram. Ultrasound uses high frequency sound waves that cannot be heard by humans. It requires no ionizing radiation. Screening ultrasound examinations can either be performed entirely by hand by a trained technologist or radiologist where the operator moves the probe (transducer), or by using an “automated” or semi-automated device (operated by a motor with positioning assistance from a technologist).
Screening breast ultrasound is designed to scan the whole breast in order to find cancers that might not be felt or visible on a mammogram. Screening ultrasound may benefit women of average risk for breast cancer and who have dense breasts on mammography. For women of high risk, ultrasound may be an option in women who cannot tolerate MRI. Targeted ultrasound is usually directed to an area of symptoms (such as a lump), or an abnormality seen on mammography or other breast imaging.
Types of ultrasounds
Handheld screening ultrasound (Fig. US-1) requires skill on the part of the technologist or radiologist performing the study since she/he must decide whether an abnormality is present and take images while scanning. It may be necessary to perform adjustments of the ultrasound machine settings while performing hand-held breast ultrasound to optimize imaging technique. On average, handheld screening ultrasound takes about 15 minutes to perform, though it can take longer if there are multiple findings requiring documentation. The axilla (area under the arm) can be included as part of the exam. Following the examination, the images are interpreted and reported by the radiologist.
Automated breast ultrasound (Figs. US-2, -3) imaging is acquired with the patient lying on her back using a wide (usually 15-cm) transducer to scan the entire breast, usually performing three sweeps across each breast. Automated breast ultrasound facilitates 3D volumetric imaging creating transverse, sagittal and coronal (unique to automated breast ultrasound) planes available to the radiologist for 3D interpretation of the breast tissue. The hundreds of images acquired are reconstructed on a special workstation for the radiologist to review. Several other approaches to whole breast ultrasound include a semi-automated approach that couples the use of a motorized arm to the transducer which is moved across the breast in overlapping segments. In yet other automated ultrasound methods (e.g., ultrasound tomography), the woman is lying face down with the breast hanging through an opening in the table. Automated screening ultrasound is less dependent on the technologist performing the exam. On average, the exam takes approximately 15 minutes to perform. Cancer detection rates for automated ultrasound are similar to handheld ultrasound . Results with semi-automated ultrasound showed cancer detection rates at least as high as with handheld ultrasound, with fewer benign biopsies . When a potential abnormality is seen on automated ultrasound, the patient will usually return for additional testing with targeted handheld ultrasound in order to determine appropriate care.
How it works
Ultrasound uses high-frequency sound waves to form an image (sonogram). The sound waves pass through the breast and bounce back or “echo” from various tissues to form a picture of the internal structures of the breast. Only gentle pressure is applied to the breasts and ultrasound rarely causes any significant discomfort. There is no ionizing radiation and no injection of contrast. A water-soluble gel or lotion is placed on the skin of the breast. A handheld or automated device (transducer) directs the sound waves to the breast tissue. The transducer is moved over the skin of the breast to create a picture that can be seen on a computer screen. Cancers are usually seen as masses that are slightly darker (“hypoechoic”) relative to the lighter gray fat or white (fibrous) breast tissue (Figs. US-4, 5). Cysts are a benign (non-cancerous) finding often seen with ultrasound and are round or oval, black (“anechoic”), fluid-filled sacs (Fig. US-6). Some ultrasound equipment also allows assessment of tissue stiffness through use of elastography; this can be used to help determine need for biopsy of low suspicion findings with soft masses more likely benign and stiff masses more likely malignant [3, 4].
Among 1000 women screened by mammography and not found to have cancer, ultrasound will identify cancer in 2 to 3 women . More than 85% of cancers seen only on ultrasound are invasive, early stage, and node negative [1, 5]. In a prospective randomized study from Japan , women who had screening ultrasound in addition to mammography were half as likely to have cancer detected because of a lump or other symptoms before the next screen (known as an “interval cancer”). Ultrasound is readily available and cost effective, though not all centers offer screening ultrasound due to a shortage of trained personnel.
Screening ultrasound is most beneficial to women with dense breast tissue. Screening ultrasound does not replace mammography screening. The benefits come from adding supplemental ultrasound screening to mammography to improve cancer detection. In two multicenter prospective trials [6, 7], 20-30% of cancers were seen only on mammography and 29-33% of cancers were seen only on ultrasound. Both the individual performing the scanning (technologist and/or radiologist) and the radiologist who interprets the images should have experience and skill in screening breast ultrasound. On average, ultrasound will show more areas which need follow-up additional imaging than does mammography. Some of those “finds” will be cancer, but the majority will not (known as “false positives”).
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