https://www.sciencedirect.com/topics/immunology-and-microbio...
Lymph Node Metastasis∗
S.D. Nathanson, ... M. Burke, in Introduction to Cancer Metastasis, 2017

4.2.1 Mammography
Level I axillary LNs are seen in about half of all mammograms. Normal axillary LNs are round or oval shaped and are variable in size. A radiolucent fatty hilum is frequently visible. A combination of features that suggest LNMets include round shape, irregular margins, increased node density, and loss of the fatty hilum.

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Note added at 53 mins (2020-10-24 12:24:00 GMT)
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http://www.themammopress.com/lymph-node-non-cancerous-breast...
The fatty hilum is the prominent component of the normal lymph node. This fatty hilum can easily blend into the normal fatty tissue of the breast. If the cortex is thin enough, it is nearly impossible to distinguish the normal lymph node from the surrounding fatty breast tissue.

https://ruj.uj.edu.pl/xmlui/bitstream/handle/item/148475/and...
Loss of fatty hilum is also a morphological
feature suggestive of malignancy.