Intramammary lymph nodes
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The intramammary lymph nodes (IMLN) (a.k.a. intramammary nodes) 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.
Intramammary lymph nodes are seen in ~5% of mammographic studies 2.
They can occur in any portion of the breast, although there may be a predilection for the upper outer quadrant (towards the axillary tail) 2,3.
The lymph nodes are usually less than 1 cm in greatest diameter. They are seen as a circumscribed oval or reniform non-calcified structure 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 in size on follow up mammograms but generally they can be tracked on follow up mammograms and stay constant in size and position. They have been know to disappear but in practice you will see them every time the patient is imaged.
There are two critical issues in evaluating intramammary lymph nodes on mammograms:
- the nodes are - without exception - 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 nodes are low density lesions. If they are projected over the profile of the muscle, you will see the muscle through the node. If this feature disappears or the "node" increases in density on follow up you are not dealing with a simple lymph node. To judge the density of a node, look for one 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.
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 color Doppler interrogation.
Some authors suggest that intramammary nodes with classical features are pathognomonic. Other authors however suggest that a lesion with radiographic features of an intramammary lymph node may rarely hide an early breast cancer 6.
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 1 cm in diameter may be seen. See main article: metastatic intramammary lymph node.
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