Mammography captures images of breast tissue to detect early signs of cancer. Interpretation sometimes reveals suspicious findings that do not represent true disease, known as radiographic artifacts. The summation artifact is one of the most common and benign occurrences that can lead to patient concern and the need for further testing.
The Mechanism of Summation Artifacts
The formation of a summation artifact is a direct consequence of projecting a three-dimensional organ onto a two-dimensional image plate. Breast tissue is a complex mix of varying densities, including glandular, connective tissue, and fat, which are distributed throughout the breast volume.
When the breast is compressed for a standard mammogram, normal structures are stacked along the path of the X-ray beam. If multiple dense structures, such as fibroglandular tissue, align perfectly, their combined density creates a shadow that looks like a single, dense mass. This apparent density is called a “pseudomass” or summation shadow.
This superimposition of normal breast structures is why the finding is termed a summation artifact. It is an optical illusion of density, not an actual volumetric lump. It is estimated that more than 75% of findings categorized as asymmetries or pseudomasses are eventually confirmed to be summation artifacts.
Distinguishing Artifacts from True Lesions
Radiologists use distinct visual and behavioral characteristics to differentiate a summation artifact from a genuine lesion. A true mass is a three-dimensional volumetric structure that maintains its shape and appearance across different mammographic views. In contrast, a summation artifact is merely a shadow created by overlapping structures, meaning its appearance depends highly on the angle from which the image is taken.
A suspicious, true mass often exhibits features suggestive of malignancy, such as an irregular shape or spiculated margins. These masses also tend to be denser at the center and resist the compression applied during the mammogram, making them appear whiter on the image.
Conversely, the pseudomass created by a summation artifact typically presents with ill-defined or fuzzy margins. The most defining feature is its behavior across different projections; a true mass will be visible on both the craniocaudal (top-to-bottom) and mediolateral oblique (side-angle) views. A summation artifact, being a chance alignment, will often be visible on only one of the two standard views, indicating it is benign.
Resolving Ambiguity: Next Steps in Diagnosis
Once a radiologist suspects a summation artifact, supplemental imaging is used to definitively confirm the finding is benign. This often involves performing additional mammographic views, specifically spot compression views. These focused images use a smaller paddle to apply localized pressure and image the area from a slightly different angle. If the apparent mass disperses, changes shape dramatically, or completely disappears, it confirms the finding was merely a superimposition of normal tissue.
The primary tool for resolving this ambiguity is Digital Breast Tomosynthesis (DBT), or 3D mammography. Unlike standard 2D mammography, DBT acquires a series of low-dose X-ray images as the tube moves in an arc around the breast. A computer then reconstructs these images into thin slices, effectively separating the overlapping tissue planes. This slicing technique allows the radiologist to look through the breast layer by layer, confirming the apparent mass was just a projection and not an actual lump.
If the finding persists or remains equivocal after these specialized mammographic views, an ultrasound is often performed as a secondary measure. Because a summation artifact is a two-dimensional imaging illusion and not a physical lump, it will not be visible on the ultrasound. The absence of a corresponding lesion confirms the benign nature of the initial mammographic finding, allowing the patient to return to routine screening.