What is the role of ultrasound (USG) in the diagnosis of soft tissue sarcoma in adults, particularly those over 40 years old with a history of increasing size of the mass, deep location, or pain?

Role of Ultrasound in Diagnosing Soft Tissue Sarcoma

Any patient with an unexplained lump that is increasing in size should receive a direct access ultrasound scan within 2 weeks, as this is the single most important first-line diagnostic tool for triaging soft tissue masses suspicious for sarcoma. 1, 2

Primary Diagnostic Role of Ultrasound

Ultrasound serves as the mandatory initial imaging modality for suspected soft tissue sarcomas in adults presenting with red flag features (increasing size, >5 cm, deep location, or pain). 1, 2

Diagnostic Performance

• Ultrasound demonstrates high diagnostic accuracy with sensitivity of 86.87-94.1% and specificity of 95.95-99.7% for identifying soft tissue masses requiring further evaluation. 3
• In a prospective study of 358 consecutive patients, ultrasound correctly identified 79% of lesions as benign (categories 1-5), with zero malignancies missed on 24-30 month follow-up among those triaged as benign. 4
• Ultrasound effectively triaged patients, with only 1.68% of total patients ultimately having sarcoma, demonstrating its efficiency in avoiding unnecessary advanced imaging. 4

Algorithmic Approach to Ultrasound Findings

When Ultrasound Suggests Benign Disease

• Well-circumscribed, hyperechoic or isoechoic masses with minimal internal vascularity and no acoustic shadowing can be managed with clinical observation if <5 cm and superficial. 3
• These patients should be referred back to primary care or non-sarcoma specialists without requiring MRI. 4

When Ultrasound Mandates Urgent Referral

Consider a suspected cancer pathway referral (within 2 weeks) if: 1, 2

• Ultrasound findings are suggestive of soft tissue sarcoma (irregular margins, heterogeneous echogenicity, disorganized vascularity on Doppler)
• Ultrasound findings are uncertain and clinical concern persists (particularly with red flag features)
• Any mass is deep to the fascia, regardless of ultrasound appearance 2, 5
• Mass is >5 cm in diameter, even if ultrasound appears benign 2, 5

Critical Limitations of Ultrasound

Where Ultrasound Fails

• Ultrasound is considerably less accurate for deep lipomas compared to superficial ones, and cannot reliably exclude atypical lipomatous tumors in deep-seated or lower limb locations. 3
• For retroperitoneal or intra-abdominal masses, ultrasound is inadequate—these require CT or MRI and immediate referral to a specialist sarcoma MDT before any biopsy or surgical treatment. 1, 3
• Ultrasound cannot differentiate benign lipomas from atypical lipomatous tumors with sufficient accuracy, particularly when masses are increasing in size. 3

Mandatory Progression to Advanced Imaging

After positive or indeterminate ultrasound, patients require: 1, 5

• MRI of the primary site for definitive characterization and surgical planning (can differentiate benign from atypical lipomatous tumors in up to 69% of cases)
• CT chest for staging if sarcoma is confirmed or highly suspected
• Percutaneous core needle biopsy (≥16G needles, multiple cores) performed by the sarcoma service, not in primary care

Common Pitfalls to Avoid

• Never assume a "normal" ultrasound excludes sarcoma in patients with persistent red flag features—if clinical concern persists despite benign ultrasound, proceed directly to MRI and specialist referral. 2
• Never perform excisional biopsy or surgical excision based on ultrasound alone outside a sarcoma center, as this increases local recurrence risk and contaminates tissue planes. 5
• Never rely on ultrasound alone for deep-seated masses—all deep masses require MRI regardless of ultrasound appearance. 2, 3
• Do not dismiss masses with recent trauma history—trauma does not exclude sarcoma and should not delay ultrasound evaluation. 2

Special Considerations for Adults Over 40

• The median size at diagnosis remains over 9 cm, reflecting delayed recognition particularly for deep-seated lesions in this population. 2
• Prior therapeutic irradiation is the most important environmental risk factor in this age group, with radiation-induced sarcomas appearing many years after radiotherapy. 2
• For any enlarging lipomatous mass in adults over 40, ultrasound alone is insufficient—proceed directly to MRI with MDM-2 amplification analysis to exclude atypical lipomatous tumor. 3