Overlapping breast tissue is a common mammogram finding. It refers to the visual effect created when normal tissues are compressed and stacked during the imaging process. This is not an abnormality or disease, but an artifact that occurs because a mammogram captures a three-dimensional organ in a two-dimensional image. The combined densities of glandular and fibrous tissue create a shadow that mimics a solid mass or suspicious area. While receiving this report can be alarming, it usually means the radiologist needs to confirm the dense spot is simply overlapping tissue and not a true lesion.
Why Breast Tissue Appears Overlapping
The appearance of overlapping tissue is a direct consequence of how mammography images the breast structure. The breast is a three-dimensional volume composed of fat, ducts, and fibroglandular tissue. Standard mammography flattens this volume to create a single, flat X-ray image, projecting structures lying at various depths onto the same plane.
Denser glandular and fibrous tissues appear white on a mammogram. When these tissues line up, their shadows combine, or summate, to look like a single, solid lump. This is often called a “summation shadow” or a pseudo-mass, and it is a known limitation of two-dimensional mammography. Compression aims to spread the tissue and reduce this effect, but it cannot eliminate it entirely, especially in dense breasts.
How Radiologists Differentiate Overlap from a True Lesion
Radiologists determine if a dense area is a benign overlap artifact or a concerning mass using specific diagnostic methods. The initial step involves analyzing standard screening mammogram images, which are taken from two different angles: the Cranio-Caudal (CC) view and the Medio-Lateral Oblique (MLO) view.
The primary method relies on observing how the perceived abnormality behaves across these projections. A true lesion is a fixed three-dimensional structure that maintains a consistent shape and density regardless of the viewing angle. Conversely, overlapping tissue will typically shift, change shape significantly, or even disappear entirely when the breast is positioned for the second view.
Radiologists also examine the edges of the dense area. Overlap artifacts, or summation shadows, tend to have indistinct, hazy, or ill-defined margins because they are combined shadows of multiple normal structures. True masses, especially suspicious ones, often possess defined, sometimes irregular or spiculated, margins. If an apparent mass is seen on only one of the two standard views, it strongly suggests a benign summation shadow.
Standard Follow-Up Tests
When initial mammogram views cannot definitively resolve whether a density is a true mass or an overlap, additional imaging is required. These follow-up tests are often referred to as a diagnostic mammogram, performed to obtain a clearer view of the area in question.
Spot Compression and Magnification
One common procedure is a spot compression or magnification view. This uses a smaller paddle to apply more focused pressure directly over the area of concern. This targeted compression attempts to spread the tissue further apart; if the density resolves or dissipates on this new image, it confirms the finding was merely overlapping tissue.
Breast Ultrasound
Another frequent next step is a breast ultrasound, an imaging modality that uses high-frequency sound waves rather than X-rays. Ultrasound is effective at determining the composition of a density, specifically whether it is solid tissue or a simple fluid-filled sac, known as a cyst. If the area is found to be a cyst or appears to be normal tissue, no further investigation is required.