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Intramammary lymph nodes

Intramammary lymph nodes (IMLN) are lymph nodes within the breast tissue. In breast imaging, they generally fall into BIRADS II lesions 7. They can be solitary or multiple. This article discusses normal (physiological) intramammary lymph nodes.

Epidemiology

Intramammary lymph nodes are seen in ~5% of mammographic studies 2.

Gross anatomy

They can occur in any portion of the breast. There may be predilection for the upper outer quadrant (towards the axillary tail2-3

Radiographic features

Mammography

Nodes are usually less than 1cm in greatest diameter. They are seen  seen as a circumscribed oval or reniform non-calcified mass with a central or peripheral lucency that represents fat within the hilum. The IMLN is often projected over the pectoral muscle profile as a well defined, smoothly circumscribed low density lesion with no spiculation. The majority (~78%) of lesions have a lower density at the center than at the periphery on mammography 3. Sometimes a "hilar notch" may be seen. The node may change slightly is size on follow up mammograms but generally they can be tracked on follow up mammograms and stay constant in size an position. They have been know to disappear but in practcie you will see them every time the patient is imaged.

There are two critical issues in evaluating an IMLN on mammogram: the first is that without exception the nodes are smoothly circumscribed and well defined. If the lesion is not very neatly and clearly outlined or loses this feature on follow up studies, it is not a simple node. The second is these are low density lesions. If it is projected over the profile of the muscle, you will see the muscle through the node. If this feature disappears or the "node" increases with density on follow up you are not dealing with a simple lymphnode. To judge the density of a node. look for one up in the tail of the breast on the MLO view - they should have similar densities.

If these two criteria are kept in mind, these lesions will rarely cause confusion or misdiagnosis.

Ultrasound

Usually detected as a solid reniform lesion which is hypoechoic 5 to the remainder of the breast tissue and with a similar shape to that described on mammography. The outer margin is typically well defined. A hyperechoic central area resulting from the hilar fat may be seen. Sometimes a nearby blood vessel may be seen with some flow entering the hilum on colour Doppler interrogation.

Significance

Some authors suggest that intra mammary nodes with classical features as pathognomonic. Other authors however suggest that a lesion with radiographic features of an intramammary lymph node may rarely hide an early breast cancer 6.

Related pathology

Normal intramammary lymph nodes may appear or enlarge in the breast as a reaction to inflammatory conditions of the skin but also as a reaction to metastatic involvement. When they do so, a round, well defined, homogeneous, high-density mass, larger than 1cm in diameter may be seen. See main article: metastatic intramammary lymph node.


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