What are the differential diagnoses for a lump in an arm or leg?

Differential Diagnosis of Extremity Lumps

The most common extremity lump is a benign lipoma, but the differential diagnosis must systematically distinguish between benign and malignant soft tissue masses based on clinical features, location, and imaging characteristics. 1

Clinical Categorization Framework

Benign Lesions (Most Common - 96% of superficial masses)

Lipomatous lesions:

• Lipoma - the most common soft tissue mass of limbs and torso, typically superficial, mobile, and soft 1
• Atypical lipomatous tumors (well-differentiated liposarcoma) - larger, deep-seated, typically in lower limb; locally recurrent but rarely metastasize 1

Cystic lesions:

• Ganglion cyst - fluid-filled, commonly around joints and tendons 1
• Epidermoid cyst - superficial, encapsulated 1

Vascular lesions:

• Hemangioma - may contain phleboliths on radiograph 1
• Vascular malformations - can show pathognomonic features on ultrasound 1

Other benign masses:

• Nerve sheath tumors - well-defined, associated with nerve structures 1
• Myositis ossificans - shows characteristic peripheral mature ossification pattern 1
• Synovial chondromatosis - osteocartilaginous masses visible on radiograph 1

Malignant Lesions (Soft Tissue Sarcomas)

Key clinical features suggesting malignancy: 1

• Size >5 cm
• Deep location (subfascial)
• Increasing in size
• Painful mass

Common sarcoma subtypes in extremities:

• Liposarcoma (including atypical lipomatous tumors/well-differentiated) 1
• Synovial sarcoma - higher risk of lymph node involvement 1
• Clear cell sarcoma - higher risk of lymph node involvement 1
• Dermatofibrosarcoma protuberans (DFSP) - skin-based 1
• Atypical fibroxanthoma/pleomorphic dermal sarcoma - sun-exposed areas 1
• Angiosarcoma - can occur in sun-exposed areas or post-radiation 1

Borderline/Locally Aggressive Lesions

• Desmoid fibromatosis - locally aggressive but non-metastasizing, may be associated with FAP/Gardner syndrome 1

Inflammatory/Infectious Mimics

• Abscess/infection - can mimic malignancy 2, 3
• Tenosynovitis - fluid surrounding tendons 1
• Inflammatory lesions - various presentations 4, 3

Traumatic Lesions

• Hematoma - history of trauma 2, 3
• Myositis ossificans - post-traumatic heterotopic ossification 1

Critical Distinguishing Features

Location-based considerations: 4

• Cutaneous/dermal origin - skin appendage lesions, DFSP, AFX
• Subcutaneous - lipomas, epidermoid cysts (most common)
• Deep/subfascial - higher concern for sarcoma, atypical lipomatous tumors

Imaging characteristics on ultrasound (94.1% sensitivity, 99.7% specificity for superficial masses): 1

• Lipoma features: minimal acoustic shadowing, minimal vascularity, curved echogenic lines, encapsulated 1
• Solid vs cystic - ultrasound reliably differentiates 1
• Ganglion cyst - fluid content confirmed 1

Radiographic findings (positive in 62% of soft tissue masses): 1

• Calcification (27% of cases) - phleboliths suggest hemangioma 1
• Bone involvement (22% of cases) 1
• Intrinsic fat (11% of cases) 1

Common Pitfalls

Critical warning: Any mass that is increasing in size, >5 cm, deep-seated, or painful requires urgent evaluation to exclude sarcoma, even though benign lesions are far more common. 1

Lipoma vs atypical lipomatous tumor: MRI cannot reliably differentiate (only 69% accuracy); MDM-2 amplification testing via core needle biopsy is definitive. 1

Ultrasound limitations: Accuracy drops precipitously for deep masses and those outside subcutaneous tissue; atypical features mandate further imaging with MRI. 1