Most Common Etiology of Leg Mass
The most common cause of a leg mass is a benign lipoma. 1
Epidemiological Evidence
By far the most common soft tissue mass of the limbs and torso seen in primary care is benign lipoma. 1 This is explicitly stated in the 2025 UK guidelines for soft tissue sarcomas published in the British Journal of Cancer, which represents the highest quality and most recent evidence available. The guidelines emphasize that despite large increases in referrals for suspected sarcomas, the percentage of patients who actually have sarcoma after direct referral from primary care remains low, precisely because lipomas vastly outnumber malignant lesions. 1
Clinical Context and Differential Diagnosis
While lipomas dominate the differential diagnosis, the clinical presentation matters:
Superficial masses: Lipomas are overwhelmingly the most common etiology, with ultrasound demonstrating 94.1% sensitivity and 99.7% specificity for identifying them. 1
Deep-seated masses: Atypical lipomatous tumors (well-differentiated liposarcomas) are manyfold less common than benign lipomas but tend to be larger, deep-seated, and preferentially located in the lower limb. 1
Malignant masses: Soft tissue sarcomas present most commonly as a painless enlarging mass, but they remain rare compared to benign abnormalities like lipomas and cysts. 1
Critical Red Flags That Change the Diagnosis
The following features should raise suspicion for malignancy rather than simple lipoma:
- Size >5 cm 1, 2, 3
- Deep location (beneath the investing fascia) 1, 2, 3
- Rapidly increasing in size 1, 2
- Painful mass 1
- Firm consistency (firmer than surrounding muscle) 2
Important caveat: Up to 32% of soft tissue sarcomas can present as small, superficial masses that mimic benign lesions, so clinical vigilance remains essential even for seemingly benign presentations. 2
Diagnostic Approach
Ultrasound is the most effective initial triage tool for evaluating leg masses, particularly for superficial lesions. 1 The majority of soft-tissue lumps will be diagnosed as benign lipomas, and patients can be safely reassured when ultrasound features are typical (well-circumscribed, hyperechoic, minimal vascularity). 1, 4
When ultrasound is uncertain or clinical concern persists, MRI provides the most accurate information for extremity masses and helps distinguish benign from malignant lesions. 1
Common pitfall: Physical examination alone correctly identifies only about 85% of lipomas, highlighting why diagnostic imaging is essential rather than relying on clinical impression alone. 4