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ESSENTIAL INFORMATION

Key Differential Diagnosis Issues
- Given limitations of clinical examination, imaging plays important role in differentiating true soft tissue masses from hernias, vascular abnormalities, and normal variants
- Most soft tissue masses have nonspecific appearance and may require biopsy or excision for diagnosis
Helpful Clues for Common Diagnoses
- Abdominal Wall Hernias - Inguinal hernia - Most common external hernia, which extends into groin anterior to horizontal plane of pubic tubercle - Divided into direct (arises anteromedial to inferior epigastric vessels) and indirect (arises superolateral to inferior epigastric vessels) subtypes - Ventral hernia - Broad term describing acquired or congenital hernias through anterior and lateral abdominal wall - Midline hernias include epigastric (above umbilicus) and hypogastric (below umbilicus) hernias - Incisional hernias occur at prior surgical incision sites - Umbilical hernia - Hernias arising at midline in upper 1/2 of umbilical ring, which can be congenital or acquired - Very common incidental finding on imaging, although usually small and asymptomatic - Spigelian hernia - Hernia through defect lateral to rectus sheath (inferior and lateral to umbilicus) often covered by external oblique muscle and aponeurosis - Femoral hernia - Groin hernia extending medial to femoral vessels with frequent compression of femoral vein - Most common in older female patients with very high risk of strangulation and incarceration - Lumbar hernia - Hernia through defect in lumbar muscle or thoracolumbar fascia - Can be congenital or acquired with many acquired due to incisions in flank region for renal surgery
- Abdominal Wall Abscess - Loculated fluid collection (± internal gas) with peripheral enhancement and surrounding edema/fat stranding - Differentiate drainable abscess from diffuse, nondrainable edema/fluid (cellulitis/phlegmon) - Presence of gas-containing abdominal wall abscess in close contiguity with bowel tethered to abdominal wall raises possibility of enterocutaneous fistula
- Sebaceous Cyst - Common incidental finding, appearing as small, round/oval, well-encapsulated cyst near skin surface - Should be low density and nonenhancing without surrounding subcutaneous edema/fat stranding
- Lipoma - Common incidental mass in subcutaneous tissues and between muscle planes, demonstrating uniform fat density with no internal soft tissue component - Differentiate from liposarcoma, which demonstrates internal complexity and soft tissue component - Confident diagnosis may be difficult on US, but mass should have similar echogenicity to subcutaneous fat
- Keloid - Benign fibrotic scar tissue or tissue overgrowth at site of soft tissue injury (i.e., surgical incision or trauma) - Usually asymptomatic but can be painful or pruritic - No clear imaging features to allow differentiation of large keloid from other soft tissue masses
- Hematoma - Heterogeneous, high-density blood products, which gradually evolve and become lower in density over time - More diffuse subcutaneous blood products may reflect subcutaneous ecchymosis
- Paraumbilical Varices - Common portosystemic collaterals in patients with severe cirrhosis and portal hypertension - Serpiginous enhancing structures that connect to recanalized paraumbilical vein near falciform ligament - May be visible/palpable at skin (i.e., caput medusae)
- Injection Site - Common incidental finding usually secondary to injection of heparin, insulin, or other medications - Small nodular foci with ectopic gas, blood, or fluid - May chronically evolve into injection granulomas, appearing as rounded or linear foci of soft tissue or calcification (most common in buttocks)
- Calcified Scar - Heterotopic ossification (myositis ossificans traumatica) can occur at abdominal incision sites and is most common in linear alba after midline abdominal incision - Ossified scar in incision can resemble rib (with both cortex and medulla)
- Muscle Asymmetry (Mimic) - May be mistaken for mass and are common secondary to prior surgery, paralysis, myopathy, etc.
- Melanoma - 5th most common new cancer in US, but imaging typically not utilized for diagnosis of primary tumor - Most commonly multiple small subcutaneous nodules, although rarely presents as solitary abdominal wall mass - Homogeneous enhancement ± hyperintense on T1 MR
Helpful Clues for Less Common Diagnoses
- Endometriosis - Endometriosis implants may be seen within incision sites after prior C-section or hysterectomy - Typically appears as solid, spiculated subcutaneous mass with variable enhancement (usually hypointense on T1 and hyperintense on T2 MR) - May be associated with clinical history of cyclical pain (corresponding with menstruation) at incision site
- Calcinosis Syndromes - Dystrophic: Calcifications may be due to tissue injury response, such as implanted medical device, connective tissue diseases (scleroderma, dermatomyositis, CREST), severe pancreatitis, or fat necrosis - Metastatic: Most often in patients with calcium-phosphate imbalance (renal failure, milk-alkali syndrome) - Tumoral calcification: Large globular deposits of calcification near joints
- Soft Tissue Metastases - Most common malignancies to metastasize to soft tissues are melanoma and renal cell carcinoma - Soft tissue nodule or mass(es) in subcutaneous fat or muscle with enhancement similar to primary tumor - Easily overlooked on CT if careful survey of soft tissues not undertaken, but often more apparent on PET - Tumor may also be implanted at site of surgery (probably more common with laparoscopic surgery) or biopsy - Surgical seeding can also occur with benign lesions, including uterine fibroids and ectopic splenic tissue
- Lymphoma and Leukemia - Cutaneous T-cell lymphoma (a.k.a. mycosis fungoides or Sézary syndrome) - Skin 2nd most common site of extranodal lymphoma (after GI tract) - Skin involvement may be difficult to appreciate on imaging unless unusually nodular or mass-like - Subcutaneous panniculitis-like T-cell lymphoma - Manifests as site of soft tissue induration/infiltration or as discrete nodules - Leukemia cutis (i.e., chloroma or granulocytic sarcoma) - Primary B-cell cutaneous lymphomas more likely to present as solitary isolated skin lesion - Posttransplant lymphoproliferative disorders (PTLD) can rarely manifest in subcutaneous soft tissues
- Desmoid - Benign locally aggressive neoplasm, which can be intraabdominal or extraabdominal (e.g., abdominal wall) - Abdominal wall lesions most frequently arise from rectus or oblique muscles, especially at incision sites - Major risk factors include prior surgery, trauma, Gardner syndrome, and familial adenomatous polyposis - Variable appearance but typically solid, well-defined, hypoenhancing, heterogeneously high signal on T2 and low signal on T1 MR
- Sarcoma - Malignant mesenchymal soft tissue tumors, which encompass wide range of different histologic subtypes - May be difficult to differentiate from other soft tissue masses based on imaging alone, although most sarcomas tend to be larger and more heterogeneous with frequent necrosis (± distant metastatic disease) - Different subtypes of sarcomas cannot be differentiated on imaging with any accuracy
- Rhabdomyolysis - Muscle necrosis in response to wide variety of causes, including crush injury, seizures, statin medications, etc. - Involved muscles on CT generally appear either normal or abnormally hypodense (due to edema) - MR more sensitive, with muscles demonstrating T2 hyperintensity and enlargement, as well as hyperenhancement (can appear ring-like or mass-like) - Commonly leads to severe renal damage due to release of myoglobin into bloodstream
- Pancreatic Panniculitis - Subcutaneous fat necrosis seen with pancreatitis and pancreatic adenocarcinoma (due to ↑ serum lipase) - Manifest as small nodular foci of predominantly fat density on CT and hyperechoic on US
- Kaposi Sarcoma - Most common AID-related vascular neoplasm in Western world, presenting as either diffuse infiltration of skin or discrete subcutaneous nodules

References

Selected References

Images

Selected Images

Abdominal Wall Hernias Axial CECT shows a right inguinal hernia containing loops of nonobstructed small bowel.

Sebaceous Cyst Axial CECT shows an encapsulated, near water density mass in the left buttock. Sebaceous cysts are a common incidental finding and, when demonstrating a classic appearance, do not require further follow-up or evaluation.

Lipoma Coronal CECT shows a large, fat-containing mass within the right lateral abdominal wall, compatible with a simple lipoma. Note the absence of any complexity or soft tissue component within the mass.

Hematoma Axial CECT shows an acute, high-density subcutaneous hematoma in a patient with recent trauma.

Hematoma Axial CECT in a patient with cirrhosis and portal hypertension shows subcutaneous varices overlying the anterior abdominal wall, representing a caput medusae.

Paraumbilical Varices Axial T1 C+ MR shows an enhancing mass in the left anterior pelvic wall, found to represent a scar endometrioma in this patient status post prior laparoscopic pelvic surgery.

Endometriosis Sagittal CECT shows a soft tissue mass intimately associated with the umbilicus, ultimately found at biopsy to represent endometriosis.

Soft Tissue Metastases Axial CECT shows a hypodense mass in the midline anterior abdominal wall, proven to represent a metastasis from the patient's known primary colon cancer.

Soft Tissue Metastases Axial CECT shows a hypodense mass in the abdominal wall musculature, representing a metastasis from the patient's known colon cancer.

Lymphoma and Leukemia Axial CECT shows a biopsy-proven chloroma in the right anterior abdominal wall in a patient with known leukemia.

Desmoid Axial CECT shows multiple large, hypodense masses in the pelvic subcutaneous soft tissues in a patient with known familial polyposis, representing desmoid tumors.

Desmoid Axial CECT shows a hypodense mass in the right anterior abdominal wall, ultimately found to represent a desmoid tumor.

Sarcoma Axial CECT shows a large, rapidly growing mass in the left anterior abdominal wall, representing a primary soft tissue sarcoma (malignant fibrous histiocytoma).

Sarcoma Axial T1 C+ FS MR shows a highly invasive, large tumor in the buttock , which enhances significantly. This lesion proved on biopsy to be a high-grade epithelioid sarcoma.

Additional Images

Paraumbilical Varices Axial CECT shows a colostomy with extensive varices in the parastomal region. These develop in patients with portal hypertension (e.g., following colectomy for primary sclerosing cholangitis with cirrhosis).

Paraumbilical Varices Axial CECT shows prominent parastomal varices .

Paraumbilical Varices Axial CECT shows a cirrhotic liver and a large parumbilical varix .

Paraumbilical Varices Axial CECT shows continuation of the parumbilical varix with collaterals in the rectus muscles and subcutaneous fat (caput medusae).

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