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Regina J. Hooley, MD
Assistant Professor of Diagnostic Radiology
Yale School of Medicine
New Haven, CT
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Date of Last Review:May 18, 2016
Expiration Date: May 31, 2018
Screening ultrasound can detect mammographically occult breast cancer and is becoming a widely accepted supplemental screening tool in women with dense breast tissue on mammography. Multiple studies have shown that screening ultrasound, in addition to mammography, can detect an additional 2-5 cancers per 1000 women screened (1-9) similar to the 2-8 per 1000 cancers detected on screening mammography (10). Screening ultrasound is particularly useful in women with dense breast tissue on mammography because dense breast tissue can obscure an underlying cancer and decrease the sensitivity of the mammogram. The sensitivity of mammography in women with dense breast tissue is 30-48% compared to a sensitivity of 80-90% in women without fatty breast tissue (1,11).
Screening breast ultrasound is widely available, well tolerated by women, and is a relatively inexpensive medical test. Despite these advantages and the ability to detect mammographically occult breast cancers, screening ultrasound has its limitations. Unlike mammography, ultrasound has limited ability to detect small pure ductal carcinoma in situ (DCIS) and the positive predictive value is low. A recent American College of Radiology Imaging Network (ACRIN) study by Berg, et al, showed that supplemental ultrasound performed in women with dense breast tissue and elevated breast cancer risk resulted in biopsy of 5% of women in addition to the 2% recommended for biopsy on the basis of mammography findings alone. Furthermore, in this study only 7.4% of the biopsies of suspicious findings seen only on ultrasound proved to be malignant (3). This is in contrast to mammography, where approximately 20-40% of biopsies recommended prove to be benign (10, 12).
Ultrasound is highly operator dependent and there can be significant intra- and inter-observer variability. In most studies, expert trained physicians performed the screening breast ultrasound. This practice may not be easily replicated in the general community because there is a shortage of manpower. Automated whole breast ultrasound is a potential new tool that may decrease operator dependence, but it is currently not widely used and its accuracy in detecting small cancers is not yet determined.
A recent study suggests that sonographer performed breast ultrasound can detect cancer at similar rates seen in physician performed exams (9). As the demand for screening breast ultrasound grows, sonographers will need to be highly proficient with this exam by applying careful scanning technique and image interpretation to accurately characterize benign and malignant findings.
Unlike targeted breast ultrasound where only a limited region of the breast is evaluated in order to assess an abnormal mammographic or clinical finding, screening breast ultrasound requires the entire (or both) breasts to be scanned in order to find suspicious lesions that are neither palpable nor visible on mammography. Published reports of bilateral whole breast screening ultrasound scan times are variable, with some investigators reporting a scan time of 4 minutes, with some investigators reporting a scan tine of 4 minutes (1) and others reporting an average exam time of 31 minutes (13). The variable scan time is mostly dependent on differences in technique, the operator's experience, and patient characteristics. In general, small breasted women and those women without extensive fibrocystic changes and cysts will require less scanning time, compared to large breasted women and/or those women with extensive fibrocystic changes and cysts.
The breasts should be scanned using a high-resolution linear transducer, of at least 12.5 MHz. Multiple overlapping transverse and sagittal, as well as radial and anti-radial scan planes should be performed along the anterior, middle, and posterior aspect of the breast in a clockwise fashion, beginning at 12:00. Anti-radial scan planes from the periphery to the areola are most efficient and may eliminate artifact from skin folds (14). Meticulous scanning of the entire breast is important to avoid missing a lesion. This can be challenging in women with large, pendulous breasts.
Lesion documentation is identical to documentation used in targeted ultrasound. Lesions, if present, should be documented in the radial and anti-radial planes. Measurements and the estimated distance from the nipple, in centimenters, should also be obtained. The retroareolar region and axilla should also be scanned. It is important to correlate any finding with the patient's recent mammogram. Occasionally, a suspicious lesion may be identified which is subsequently proven to correlate with a stable benign finding on mammography.
If no lesions are visualized, then documentation of the normal exam should be performed by obtaining radial images at 12, 3, 6, and 9:00. Images of the normal retroareolar region and axilla are optional, but encouraged.
Cysts are the most common type of breast mass and the most frequent finding on screening breast ultrasound. In the ACRIN screening ultrasound study, cysts were seen nearly 50% of women screened (15). Cysts are hormone sensitive and can fluctuate in number and size, depending on the menstrual cycle or the use of hormone replacement therapy. Cysts are most frequent in women between the ages of 35 and 50 years, but may also be seen in nearly one third of postmenopausal women (15).
The presence of multiple cysts (ie, at least 3 cysts, with at least one in each breast) are usually a benign finding (Fig.1). A recent study demonstrated that complicated cysts in the presence of multiple bilateral cysts, as well as complicated cysts less than or equal to 5 mm in size, could be classified as benign, without a significant loss in sensitivity (9). Nevertheless, these cysts can be a distraction, increasing the likelihood that a small cancer will be overlooked. When multiple cysts are present, it is important to assess the breast tissue adjacent to the cysts and search for any small, hypoechoic solid mass. In addition, careful sonographic evaluation of complicated cysts is imperative because in rare cases a cancer can mimic the appearance of a complicated cyst (14) (Fig. 2).
When an oval or round, circumscribed, anechoic mass with good through transmission is identified, the diagnosis of a simple cyst can easily be made. If a cystic lesion has internal echoes or poor posterior acoustic enhancement, and no evidence of a solid component or thickened wall, a diagnosis of a complicated cyst should be made. On screening ultrasound, solitary cysts should be measured and documented appropriately. If multiple cysts are present, each cyst does not need to be documented and measured. Instead, it may be more appropriate and efficient to document and report only the largest cyst in each quadrant.
Ultrasound is complementary to mammography because it performs particularly well in dense breast tissue. Dense fibroglandular tissue is more echogenic than fat. Because most breast cancers are hypoechoic on ultrasound, these masses are usually more apparent in a background of echogenic and dense breast tissue (Fig.3).
Most cancers detected on supplemental screening ultrasound have been shown to be small (less than 1 cm in size), stage 1 node negative invasive tumors (16). Ultrasound does not detect DCIS as well as mammography. In addition to hypoechogenicity, suspicious masses include those lesions that are "taller-than-wide" with irregular borders, angular margins, ductal extension and posterior acoustic shadowing. If a suspicious mass is found, ultrasound guided core needle biopsy can be performed for tissue diagnosis.
The sonographic features of a benign solid mass include hypoechoic echotexture, smooth circumscribed margins and lack of posterior acoustic shadowing (17). Fibroadenomas and lymph nodes are the most common benign solid masses encountered on screening breast ultrasound. Fibroadenomas are typically hypoechoic, oval (wider-than-tall), smoothly marginated masses with or without posterior acoustic enhancement. Benign intramammary and axillary lymph nodes should be reniform in shape, with a thin (<3 mm) hypoechoic cortex and a fatty central hilum with associated feeding vessels identified on color Doppler interrogation (Fig 4).
If a benign appearing solid mass is identified, meticulous ultrasound evaluation is imperative to ensure the absence of any subtle suspicious ultrasound finding. The presence of a single suspicious ultrasound finding precludes a benign or probably benign diagnosis. Correlation with mammography is also essential. If no stable corresponding mass is found or if the mass does not have the typical sonographic appearance of a benign intramammary lymph node, then either ultrasound guided biopsy or short interval ultrasound follow-up (usually performed at 6 and 12 months) may be appropriate. Little data exists regarding the optimal management of benign appearing solid masses identified only by screening ultrasound. Therefore, the decision to perform short interval follow-up or ultrasound guided biopsy may depend on the patient's overall risk for breast cancer, as well as patient and physician preference.
Post-operative scars are a relatively common finding on screening breast ultrasound and it is important to be familiar with their variable sonographic appearance. Careful correlation with patient history and any skin scars is required to make the proper diagnosis. On ultrasound, post-operative and post-biopsy scars appear irregular and hypoechoic, usually mimicking the appearance of a malignancy. Visualization of the scar tracking towards the skin scar can help distinguish an irregular hypoechoic scar from a malignancy.
Scars may be seen following core needle biopsy, benign excisional biopsy or breast conservation surgery. Scars are most prominent in women who have had breast conservation surgery and radiation treatment. Fluid filled seromas may also be seen in these patients. Post–operative scars should not have flow on color Doppler evaluation.
Screening breast ultrasound can detect additional small breast cancers not seen on mammography. Ultrasound is an optional and supplemental, secondary screening tool in women with dense breast tissue (with or without elevated personal breast cancer risk) and should not be used to replace the screening mammogram. Meticulous attention to scanning technique and image interpretation is essential for optimal diagnosis of benign and malignant findings.
There are numerous controversies regarding screening breast ultrasound (18). For example, no outcome study has been performed that demonstrates a direct survival benefit secondary to screening breast ultrasound. Still, it is rational to believe that the detection of small mammographically occult cancers should reduce overall morbidity and mortality. In addition, many insurance companies will not reimburse for this exam. Moreover, to date, there are no uniform standards for the performance of screening breast ultrasound.
Although screening breast ultrasound has not been widely accepted in the United States, more patients and physicians are requesting this exam. In 2009, Connecticut enacted unprecedented legislation mandating radiologists to inform women with dense breasts on mammography that they may benefit from supplemental screening breast ultrasound. Texas, Virginia and New York have since followed with similar laws and in 2012, nearly a dozen addition states have also introduced similar legislation. As a result, the use of screening breast ultrasound will likely rise exponentially. Future research will be needed determine the optimal screening applications and screening interval. The performance of automated whole breast screening ultrasound will also need to be determined and uniform standards, similar to the Mammography Quality Assurance Act, will need to be established.