Lymph nodes are small, bean-shaped components of the immune system that filter lymphatic fluid, trapping foreign particles, infections, and abnormal cells. Imaging reports often note the presence of a “fatty hilum,” which can cause confusion regarding its significance. This article clarifies the meaning of this finding and contrasts it with features that might suggest malignancy.
Understanding the Lymph Node Hilum
A lymph node is a small, encapsulated, typically oval or kidney-shaped organ. Its primary function is to monitor and purify lymph fluid, essential for immune surveillance against threats like bacteria, viruses, and cancer cells.
The hilum is an anatomical indentation on the concave side of the lymph node. This central region serves as the entry and exit point for the node’s infrastructure, allowing blood vessels and efferent lymphatic vessels to pass through. On imaging studies like ultrasound or CT scans, this area appears as a distinct internal feature.
The Meaning of a Fatty Hilum
The term “fatty hilum” refers to the presence of adipose tissue (fat) within the lymph node’s central hub. On imaging, this fat appears bright on ultrasound or registers as fat density on a CT scan. Radiologists consider a preserved, fatty hilum a reassuring sign, demonstrating the lymph node has maintained its normal, functional structure.
This fatty tissue is not a sign of disease. It is often the result of fatty replacement or lipomatosis, a natural biological process. As a lymph node ages or returns to a quiescent state after an immune response, lymphoid tissue may involute and be replaced by fat. This phenomenon increases with age and is a normal part of the node’s involution.
The presence of this central fatty area confirms the preservation of the node’s architecture. Reactive lymph nodes, responding to infection or inflammation, typically maintain this structure. Since metastatic cancer or lymphoma tends to destroy the node’s normal organization, an intact fatty hilum strongly indicates the node is benign.
Concerning Features That Suggest Malignancy
While a fatty hilum is a positive finding, other radiological features raise suspicion of malignancy. The most significant concerning sign is the loss or complete effacement of the fatty hilum. This absence suggests that normal lymph tissue has been replaced or disorganized by an infiltrating process, such as a metastatic tumor.
A change in the node’s shape to a more rounded configuration is also a red flag. Radiologists calculate the short-axis to long-axis ratio; a value greater than 0.5 (meaning the node is more round than flat) is associated with a higher likelihood of malignancy. Malignant nodes may also show irregular cortical thickening, where the outer layer becomes unevenly enlarged, often exceeding 3 millimeters.
Abnormal blood flow patterns identified on Doppler ultrasound are another feature. Benign nodes usually exhibit centralized blood flow at the hilum, but malignant nodes often display peripheral or mixed vascularity. Other suspicious findings include:
- Necrosis (dark, non-vascularized areas within the node).
- Microcalcifications (tiny specks of calcium).
Next Steps for Further Evaluation
When a lymph node exhibits concerning features, a physician determines the next course of action based on the patient’s clinical picture and risk factors. If features are mildly suspicious, the first step is often observation with follow-up imaging. This involves repeating the ultrasound or CT scan in a few weeks or months to see if the node has resolved or grown.
If the node is highly suspicious, persists, or enlarges over time, a definitive diagnosis requires tissue sampling. This is typically done through Fine Needle Aspiration (FNA), which uses a thin needle to withdraw a small sample of cells for analysis. Alternatively, a core needle biopsy or an excisional biopsy, which removes the entire node, may be performed for a more complete tissue sample.
A definitive diagnosis of malignancy cannot be made without a biopsy, even with highly suspicious imaging features. Imaging findings, especially when a preserved fatty hilum is present, are used only to guide the need for further, more invasive testing. Consulting a healthcare provider who can evaluate the full medical history and imaging report is necessary for personalized guidance.