Skin nontumor
Panniculitis
Encapsulated fat necrosis
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PubMed Search: Encapsulated fat necrosis
Table of Contents
Definition / general | Essential features | Terminology | ICD coding | Epidemiology | Sites | Pathophysiology | Etiology | Clinical features | Diagnosis | Laboratory | Radiology description | Radiology images | Prognostic factors | Case reports | Treatment | Clinical images | Gross description | Gross images | Microscopic (histologic) description | Microscopic (histologic) images | Virtual slides | Positive stains | Negative stains | Videos | Sample pathology report | Differential diagnosis | Board review style question #1 | Board review style answer #1 | Board review style question #2 | Board review style answer #2Cite this page: Griffith M, Schukow C, Ahmed A. Encapsulated fat necrosis. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/skinnontumorencapsulatedfatnecrosis.html. Accessed December 4th, 2024.
Definition / general
- Encapsulated fat necrosis is a degenerative process characterized by nodules of necrotic adipose tissue in a fibrous or connective tissue capsule
Essential features
- Nonneoplastic inflammatory reaction that is commonly secondary to adipose tissue injury
- Trauma interrupts local blood supply to affected adipose tissue, causing necrosis and local inflammation and resulting in fibrosis and encapsulation
- Primarily affects lower extremities or other body regions subject to repeated trauma
- Late lesions may become calcified
Terminology
- Well circumscribed fat necrosis
- Nodular cystic fat necrosis
- Mobile encapsulated lipoma
- Nodular fat necrosis
- Encapsulated necrosis
- Posttraumatic fat degeneration
- Reference: Br J Plast Surg 2001;54:643
ICD coding
- ICD-10: L98.9 - disorder of the skin and subcutaneous tissue, unspecified
Epidemiology
- Can affect patients of any age (J Am Acad Dermatol 1989;21:493)
- Average reported age: 40.9 years (J Cutan Pathol 2000;27:19)
- Women reportedly affected more than men (F:M = 31:18) (J Cutan Pathol 2000;27:19)
Sites
- Encapsulated fat necrosis can occur on any part of the body but most commonly occurs on lower extremities or fat rich areas exposed to repetitive trauma (Br J Plast Surg 2001;54:643, Med Ultrason 2020;22:397)
Pathophysiology
- Trauma or interrupted blood flow leads to (Med Ultrason 2020;22:397)
- Infarction of adipose tissue
- Subsequent inflammation involving local proinflammatory cytokine release
- Increased macrophage activity after initial insult that creates fibrotic strands separating injured necrotic tissue from surrounding healthy tissue
- The overall fibrocellular response leads to the formation of a fibrous capsule and a cleavage plane between the lesion and surrounding adipose (Br J Plast Surg 2001;54:643)
- Late lesions may develop dystrophic calcification, which can be seen on imaging (Br J Plast Surg 2001;54:643)
Etiology
- Lesions may arise secondary to trauma or interrupted blood supply
Clinical features
- Single or multiple subcutaneous nodules that may be painful (Med Ultrason 2020;22:397, Skeletal Radiol 2013;42:1465)
- Nodules are usually mobile in areas with more adipose tissue (Br J Plast Surg 2001;54:643)
- Often range in size from 1 to 35 mm
Diagnosis
- Diagnosis made using suggestive clinical (i.e., recent history of local trauma) and microscopic findings (Skeletal Radiol 2013;42:1465)
- Radiographic imaging may show calcifications in chronic lesions but does not rule out malignancy
- Microscopy (i.e., biopsy) required to confirm diagnosis (Skeletal Radiol 2013;42:1465)
Laboratory
- Complete blood count, urinalysis, liver function tests, electrocardiogram and stool examinations are likely to be normal (J Dermatol 1998;25:616)
Radiology description
- MRI (Skeletal Radiol 2013;42:1465)
- Lesions often demonstrate heterogeneous, high signal intensity compared to muscle tissue but may appear isointense compared to healthy adipose tissue
- Fat suppression visible on T2 weighted images
- Fibrous capsule may demonstrate contrast enhancement
- PET: heterogenous mass with focal fat encapsulation (Skeletal Radiol 2013;42:1465)
Radiology images
Images hosted on other servers:
Encapsulated abdominal mass
MRI
CT
Prognostic factors
- Generally low recurrence rates (Skeletal Radiol 2013;42:1465)
- Morbid obesity or previous surgery may lead to increased risk of recurrence (J Cutan Pathol 2004;31:565)
Case reports
- 47 year old woman with multiple nodular lesions on back (Med Ultrason 2020;22:397)
- 59 year old woman with left arm mass increasing in size, mimicking subcutaneous liposarcoma (Skeletal Radiol 2013;42:1465)
- 67 year old woman with recurrence of previously excised fat necrosis that originally developed in appendectomy scar (J Cutan Pathol 2004;31:565)
Treatment
- Surgical excision is curative (Med Ultrason 2020;22:397, J Dermatol 1998;25:616)
Clinical images
Images hosted on other servers:
Subcutaneous nodule of right thigh
Mobile nodule with unusual mobility
Gross description
- Well demarcated, white-yellow smooth nodule lacking vascular connections (Br J Plast Surg 2001;54:643, J Cutan Pathol 2000;27:19)
- Lesion can contain single or multiple nodules (J Cutan Pathol 1995;22:551)
- Cut surface may demonstrate fluid fatty tissue (J Cutan Pathol 2000;27:19)
Gross images
Images hosted on other servers:
Subcutaneous encapsulated fat necrosis
Cut section of the gross lesion
Microscopic (histologic) description
- Well circumscribed, necrotic adipose tissue encapsulated by
- Fibrous septa (Med Ultrason 2020;22:397)
- Thin connective tissue capsule (J Cutan Pathol 2000;27:19)
- Types of necrosis include (J Cutan Pathol 2000;27:19)
- Macro or microcystic
- Coagulation
- Membranous
- Thickness of the fibrous cap is variable (Br J Plast Surg 2001;54:643)
- Late stage lesions may demonstrate lymphocytic, histocytic and lipophagic inflammation and possible dystrophic calcification of fibrous cap (Br J Plast Surg 2001;54:643, J Cutan Pathol 2000;27:19)
Microscopic (histologic) images
Contributed by Aadil Ahmed, M.D.
Fibrous capsule surrounding
adipose tissue
Histiocyte within adipose tissue
Focal lipomembranous change
Virtual slides
Images hosted on other servers:
Subcutaneous fat necrosis of the skin
Positive stains
- Special or immunostains are not needed
- Von Kossa can highlight calcium in calcified lesions (J Dermatol 1998;25:616)
- PAS is positive in cases with lipomembranous fat necrosis (J Cutan Pathol 2004;31:565)
Negative stains
Videos
Posttraumatic fat necrosis (encapsulated subcutaneous fat necrosis) by Dr. Jerad Gardner
Sample pathology report
- Left shin nodule, wide local excision:
- Encapsulated fat necrosis (see comment)
- Comment: Microscopic examination reveals necrosis of mature adipose tissue encapsulated by fibrous strands of dense collagen. Affected adipocytes display eosinophilic cytoplasm. A mixed, scattered inflammatory infiltrate is present within the necrosis, including macrophages, lymphocytes and plasma cells. No evidence of nuclear atypia, malignancy or invasion is identified.
Differential diagnosis
- Lipoma:
- Proliferation of mature adipose tissue without any significant necrosis
- Paucicellular fibrous septa may be present
- Liposarcoma:
- Multilobulated with infiltrative lipoblasts on histology
- Angiolipoma:
- Composed of mature adipocytes and branching capillaries / vessels, which often contain fibrin thrombi
- Membranous fat necrosis:
- Nonencapsulated variant of fat necrosis with cystic foci that are lined by eosinophilic membrane that projects within the cyst, creating a pseudopapilla or arabesque appearance
- Positive periodic acid-Schiff stain
- Pancreatic panniculitis:
- Necrotic adipocytes with amorphous or granular blue-gray substance, consistent with the presence of ghost cells
- Infiltrate is predominantly neutrophilic
- Amyloidoma:
- Homogeneous, pale pink deposition that will be positive for Congo red stain
Board review style question #1
A 36 year old woman presents with a gradually enlarging, painless mobile nodule on the right anterior shin. She sustained an injury to the same area 10 years ago. The team decides to proceed with surgical excision of the lesion. Histology is shown in the image. What is the most likely diagnosis?
- Encapsulated fat necrosis
- Lipoma
- Liposarcoma
- Pancreatic panniculitis
Board review style answer #1
A. Encapsulated fat necrosis demonstrates mature adipose tissue with central necrosis within a fibrous capsule. The capsule may become calcified over time. This lesion can be preceded by trauma to the area. Answer B is incorrect because lipoma would demonstrate mature adipocytes without necrosis due to intact blood supply. Larger lipomas can exhibit focal necrosis but without any capsule formation. Answer C is incorrect because liposarcoma would demonstrate multilobulated adipose tissue with infiltrative lipoblasts. Answer D is incorrect because pancreatic panniculitis would exhibit neutrophilic infiltrate with characteristic ghost cells.
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Reference: Encapsulated fat necrosis
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Board review style question #2
A 50 year old man presented for excision of a 1 cm, painless subcutaneous nodule of the left lower extremity. Upon microscopic review of the specimen, the lesion was diagnosed as encapsuled fat necrosis. Which stain would confirm the presence of a lipomembranous necrosis process?
- Giemsa
- Oil red O
- Periodic acid-Schiff
- Von Kossa
Board review style answer #2
C. Periodic acid-Schiff. PAS is specific to lipomembranous necrosis. Answer A is incorrect because Giemsa is commonly used to stain peripheral blood and bone marrow aspirate smears. Answer B is incorrect because oil red O is a hydrophobic, fat soluble dye that highlights fat droplets in lipid storage disease, steatosis or for detection of fat embolism. Answer D is incorrect because this stain is used to detect mineral deposits in tissues, such as calcium.
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Reference: Encapsulated fat necrosis
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