Breast cancer is the second most common cancer in women, after skin cancer, and represents 14% of all new cancers in the United States.[1] Breast cancer is most commonly diagnosed in women aged 55 to 64 years, and the risk increases with age.[2] Early diagnosis increases the chances that a patient may achieve a cure and also reduces the morbidity of the treatments. Breast cancer therapies continue to improve and have contributed to mortality reduction, but early diagnosis through mammographic screening has had a greater overall impact on mortality reductions.[3] The American College of Radiology recommends breast cancer screening for all average-risk women starting at the age of 40.[4] The diagnosis of breast cancer can be suggested by many different modalities, most commonly mammography, breast ultrasound, and magnetic resonance imaging (MRI).

However, tissue sampling is required for a definitive diagnosis. Breast cancer screening is performed with mammography, and patients with equivocal or suggestive mammographic screening results require further imaging with a diagnostic mammogram, ultrasound, breast MRI, biopsy, or combination thereof. This article will discuss the techniques and considerations when performing screening breast mammography.

The breast is composed of fat and fibroglandular tissue, with the breast parenchyma being composed of a vibrant network of exocrine glands, ducts, stromal connective tissues, vasculature, and nerves. This tissue extends from the second through sixth ribs and is bordered by the sternum and the midaxillary line. The breast tissue is commonly divided into quadrants for ease of localization. This is performed by segmenting the breast into medial/lateral and upper/lower components using the nipple as a reference point. It should be noted that the upper outer quadrants of the breasts often contain breast tissue that extends into the axilla. This is relevant for mammographic technique, as this region of breast tissue may be difficult to image fully without adequate patient positioning. Furthermore, one of the primary lymphatic drainage networks of the breast passes through the axilla, and abnormal lymph nodes may be incidentally discovered by imaging this region.

Mammography can be used to evaluate the breast based on the differential attenuation characteristics of the tissues. Specifically, fat attenuates fewer x-rays than fibroglandular tissue and stromal elements and appears gray on mammography. Dense mineral deposits can be easily identified and appear bright white, such as skin calcifications or calcifications within a neoplastic lesion. Superimposition poses a challenge in mammography, as tissue structures that overlie or overlap one another may obscure a small mass. To try and combat this, screening mammograms are performed in at least two standard views, which include the craniocaudal view (CC) and the mediolateral oblique (MLO). By comparing these two different viewpoints, structures that are superimposed upon one another in one view may be separated in the other. In cases of uncertainty, additional diagnostic views with spot compression paddles or variant angles, such as a mediolateral (ML) view, may be obtained. Additionally, newer technologies such as tomosynthesis imaging allow the radiologist to scroll through the breast in slices, decreasing the superimposition effect of tissues. A typical mammogram can depict the skin surface, the nipple, the main breast ducts, fibroglandular tissue, veins, adipose tissue, muscle, and occasionally lymph nodes.

The relative abundance of fibroglandular tissue versus fat that a patient has in their breast can vary considerably from person to person, and even within the same patient depending on hormonal factors and weight changes. For example, the hormonal effects of pregnancy and lactation can increase the abundance of glandular tissue, and similarly, cessation of lactation causes fibroglandular tissue to regress. Dense breast tissue attenuates a higher proportion of x-rays and appears whiter on a mammogram and may pose a diagnostic challenge when trying to detect breast cancer due to the masking effect. This is further complicated by the fact that patients with dense breast tissue are at increased risk of developing breast cancer.[5] As such, great care must be taken when imaging patients with dense breast tissue, and if extremely dense breast tissue limits screening mammography, supplemental evaluation with breast ultrasound or MRI may be considered.

Every screening mammogram is evaluated to ensure the technical quality, including adequate positioning as breast cancers may be missed if not enough tissue is imaged. The breast tissue overlies the pectoralis major muscles, and these muscles can be partially seen on mammographic images. The pectoralis demonstrates a striated appearance on the MLO view that is denser than the overlying fat and fibroglandular tissue. As a rule of thumb, the MLO view is not adequate if the pectoralis is not intersected by a line drawn orthogonally from the nipple posteriorly, also known as the posterior-nipple line (PNL). The CC view is not adequate if the PNL is not within 1 cm of the MLO posterior-nipple line.

Several professional societies offer recommendations on breast cancer screening. They include the American College of Obstetrics and Gynecology, the American College of Radiology, the Society for Breast Imaging, the American Cancer Society, the American Medical Association, the National Comprehensive Cancer Network, and the United States Preventative Service Task Force (USPSTF).[6][7][8][4][9] An in-depth discussion of the differences in the recommendations between these professional societies is beyond the scope of this article. According to the American College of Radiology, annual mammographic screening is recommended beginning at age 40 for all women with an average risk of developing breast cancer and should continue for women with a life expectancy of at least 5 to 7 years as long as the patient is willing to undergo further testing/biopsy and treatment. Specifically, there is no age cutoff for when screening mammography should be discontinued. Instead, a patient's health status should dictate whether screening measures continue to be warranted. Although breast cancer is most commonly diagnosed in the 50 to 60 age range, risk increases with age and screening mammography offer an opportunity to detect early malignant lesions or even precancerous lesions before they become otherwise clinically evident. Average risk patients are those with less than a 15% lifetime risk of developing breast cancer.[10]

Patients with an increased risk of developing breast cancer require special consideration since they may develop breast cancer at an earlier age or require supplemental screening modalities beyond routine mammograms. Patients are considered to have an intermediate risk or 15% to 20% lifetime risk of developing breast cancer if they have any of the following:

1. Personal history of breast cancer
2. Atypical ductal hyperplasia on a prior breast biopsy
3. Lobular neoplasia on a prior breast biopsy

Patients with intermediate-risk should receive a screening mammogram annually and may benefit from supplemental screening with breast ultrasound and possibly breast MRI. Ultrasound is particularly useful in the setting of dense breast fibroglandular tissue, as a high-quality ultrasound examination is not impeded by increased breast density.[11]

Finally, patients with a 20% or higher lifetime risk of developing breast cancer are considered high-risk. In addition to supplemental screening modalities, these patients also benefit from initiating breast cancer screening at a younger age. High-risk patients are those with the following:

1. Women with specific gene mutations, including BRCA 1 and 2.
2. Those with a strong family history of breast cancer, even in the absence of a known gene mutation.
3. Patients that have received radiation therapy of the chest between 10 to 30 years of age.

Patients with a family history of breast cancer should begin screening mammography ten years before the age at which the youngest first-degree relative developed breast cancer; however, with the caveat that these patients should not start screening before the age of 30, as there are concerns that these patients may have an increased sensitivity to ionizing radiation, which mammography relies upon.[4] In addition to mammography, screening breast MRI is recommended for these patients, given its increased sensitivity.[12]

It is recommended that women undergo a risk assessment by age 30 to determine the appropriate screening timeline. Screening mammography is not recommended for male patients with breast tissue, known as gynecomastia.

There are no absolute contraindications to screening mammography, but relative contraindications do exist. First, any woman with signs or symptoms that are concerning for breast cancer, such as the presence of a palpable or enlarging breast mass, should undergo diagnostic mammographic and sonographic imaging rather than screening imaging alone. The distinction between screening and diagnostic mammography is made by the ability to utilize additional imaging techniques, which may include spot compression, supplementary angles, or magnification views. Diagnostic breast ultrasound adds complementary information, particularly when concerning lesions are identified through palpation.

As described above, for asymptomatic patients with an average risk of developing breast cancer, screening mammography is not recommended before the age of 40. Patients of this age group generally have increased breast density and mammography has decreased sensitivity for detecting breast cancer in this setting. There is also a risk of false-positive mammographic findings, potentially necessitating follow-up imaging, and possibly a tissue biopsy.[13] However, it is recommended that all women undergo risk assessment at age 30 to see if earlier screening is appropriate.

Mammography utilizes X-ray radiation to generate diagnostic images, a form of ionizing radiation. The X-rays used in mammography are lower energy than those used in CT or plain radiography, but there are at least theoretical risks with any degree of ionizing radiation exposure.[14] Thus, patients who are pregnant or who are lactating deserve special consideration. The radiation dose to the fetus during pregnancy is negligible and is well below the known thresholds for adverse pregnancy outcomes;[15] however, mammography should be tailored to the individual patient needs. When possible, dose-reduction techniques should be employed. Pregnancy should not preclude the evaluation of suspected pregnancy-associated breast cancer, but screening mammography may be deferred until the end of pregnancy for asymptomatic patients at average risk of developing breast cancer.[16]

Patients that are lactating deserve additional considerations, as the milk-producing lobules attenuate more X-rays, increasing the breast tissue density and potentially obscuring premalignant or malignant lesions. However, breastfeeding or pumping before the study may decrease breast parenchymal density and increase the sensitivity of mammography.[17] Thus, lactation does not contraindicate screening mammography, but patients should be given advance guidance before any imaging exams.[16]

As briefly described previously, mammography utilizes x-rays to evaluate the different anatomic structures within the breast. Initially, mammograms were performed using traditional film-based radiographic techniques. However, film mammography has been replaced by systems that use phosphor storage technology or direct digital capture. The phosphor storage systems use a crystal that transforms x-rays into visible light, which is subsequently digitized. The direct digital capture systems utilize a CCD array that can directly transfer the x-ray into a digital image. There are many advantages to these newer technologies over film mammography, most notably, the dynamic range and contrast resolution can be adjusted after image capture; this allows radiologists to assess subtle differences within tissues better and optimize image rendering at the workstation.[18]

Recently, digital breast tomosynthesis (DBT, also known as "3D mammography") has gained popularity. Briefly, DBT is performed by obtaining multiple mammographic projections per view, and these projections are acquired serially along an arc. The images are then reconstructed via post-processing techniques into a series of stacked images of 1mm slice thickness. The image stacks combat the challenges of superimposed tissues seen in single-projection mammography by allowing the breast to be assessed in the X, Y, and Z dimensions.[19] DBT has been shown to increase cancer detection and decrease false-positive rates. Furthermore, this increased sensitivity offers the chance to identify lower grade cancers, thereby improving treatment options and prognosis. Given these advantages, the use of DBT in both screening and diagnostic mammography is rapidly becoming the standard of care.[20]

The essential mammography components include an x-ray generator, an image detector, and a breast compression paddle. The x-ray generator and image detector are set at a fixed distance and are oriented orthogonally to one another, but the entire unit can be rotated to obtain the required mammographic views. Additional protective equipment is necessary for the technical personnel who take the mammograms; this includes a protective shield with a remote control station to operate the equipment.