What is a fatty hilum in a lymph node or other structures?

What is a Fatty Hilum?

A fatty hilum is the normal central echogenic (bright) or low-density fat-containing region within a lymph node that represents the entry and exit point for blood vessels and lymphatic channels—its presence is a key indicator of benign lymph node architecture. 1

Anatomic and Imaging Characteristics

The fatty hilum appears differently depending on the imaging modality used:

On Ultrasound

• Appears as an echogenic (bright/hyperechoic) central region within the lymph node that represents normal fatty tissue, blood vessels, and lymphatic sinuses 2
• The presence of this echogenic fatty hilum is one of the most reliable signs of a benign lymph node 1
• Absence of the fatty hilum has a positive predictive value of 90-93% for malignancy in the context of breast cancer staging 1

On CT Imaging

• Appears as a central low-density area (measuring fat density, typically -50 to -150 Hounsfield units) within the lymph node 2
• Pathologic examination confirms this low-density hilum contains arteries, veins, lymphatic sinuses, and fatty tissue 2
• The peripheral high-density cortex surrounding the hilum contains lymphatic tissue 2

On MRI

• Described as loss of the normal fatty hilum when abnormal, appearing as rounded shape, heterogeneous signal, or loss of the characteristic fat signal 1
• Normal nodes display characteristic fat signal in the hilar region on T1-weighted images 1

Clinical Significance in Malignancy Assessment

The fatty hilum serves as a critical discriminator between benign and malignant lymph nodes across multiple cancer types:

Breast Cancer

• Absence of fatty hilum is the single most predictive ultrasound feature for axillary metastases with PPV of 90-93% 1
• Other suspicious features include cortical thickness >0.3 cm and short-axis diameter >1 cm, but loss of fatty hilum is most specific 1
• Even hypermetabolic lymph nodes on PET/CT that retain a fatty hilum may be reactive (such as post-vaccination) rather than metastatic 3

Vulvar Cancer

• Absence of fatty hilum combined with irregular shape, cortical thickness ≥4 mm, and peripheral vascularization indicates lymph node metastases with sensitivity 76.3% and specificity 91.3% 1
• Absent echogenic fatty hilum is a key criterion for selecting lymph nodes for fine-needle aspiration biopsy 1

Cervical Cancer

• Loss of normal fatty hilum on MRI, along with rounded shape and heterogeneous signal, indicates abnormal pelvic or para-aortic lymph nodes 1
• These morphologic criteria perform comparably to PET imaging for nodal staging 1

Oral Cavity Squamous Cell Carcinoma

• Disrupted fatty hilum is independently predictive of occult nodal metastases (OR = 3.31, p = .04) even in sub-centimeter lymph nodes 4
• When combined with round/oval shape and asymmetric number, disrupted fatty hilum achieves 93.8% positive predictive value for metastatic disease 4

Important Clinical Pitfalls

When Fatty Hilum is Present

• Lymph nodes with preserved fatty hilum and benign morphology require no imaging follow-up or biopsy, as malignancy risk is extremely low 5
• Nodes <10 mm in short axis with fatty hilum are consistently benign 5
• Even if nodes are hypermetabolic on PET/CT, preserved fatty hilum suggests reactive rather than metastatic etiology—always obtain vaccination history 3

When Fatty Hilum is Absent

• Absence of fatty hilum warrants percutaneous biopsy in the appropriate clinical context (known primary malignancy) 1
• This finding alone has the highest positive predictive value among ultrasound features for malignancy 1
• Must be interpreted with other features: cortical thickness, shape (round vs. oval), size, and vascular pattern 1

Rare Exceptions

• Abundant macroscopic fat replacement of lymph nodes can occur in chronic lymphocytic leukemia, but this represents diseased rather than normal nodes 6
• Fatty infiltration of lymph nodes can occur with renal angiomyolipoma, representing local aggressive behavior rather than malignancy 7
• These scenarios are distinctly different from the normal fatty hilum and represent pathologic fat deposition 6, 7

Practical Assessment Algorithm

For any lymph node evaluation:

1. First assess for presence of fatty hilum on ultrasound (echogenic center) or CT (low-density center) 1, 2
2. If fatty hilum is present AND node is <10-15 mm AND oval-shaped: benign, no further workup needed 5
3. If fatty hilum is absent OR disrupted: assess cortical thickness, shape, size, and vascularity 1, 4
4. If multiple suspicious features present: proceed to biopsy in patients with known malignancy 1, 4

1, 5, 3, 4, 2