What is the differential diagnosis for an adult patient with an ovoid, iso/hypoechoic (isoechogenic/hypoechogenic), solid, and striated mass in the right lower back, with difficult-to-define margins, that reduces in size with probe pressure and has grown approximately 2 cm since the previous ultrasound (US) examination, initially thought to be a lipoma?

Differential Diagnosis for Ovoid Iso/Hypoechoic Striated Mass in Right Lower Back

This mass requires urgent tissue diagnosis via core needle biopsy or surgical excision because the combination of growth (2 cm increase), difficult-to-define margins, and solid striated appearance raises significant concern for soft tissue sarcoma, particularly well-differentiated liposarcoma (atypical lipomatous tumor), despite initial suspicion for lipoma. 1, 2

Primary Differential Considerations

Most Concerning: Soft Tissue Sarcoma

• Well-differentiated liposarcoma (atypical lipomatous tumor) is the most critical diagnosis to exclude given the growth pattern and imaging characteristics 3, 4
• These lesions are often initially mistaken for lipomas but demonstrate growth, deep location, and can exceed 5 cm 1, 5
• The iso/hypoechoic appearance with striated pattern and indistinct margins are concerning features that deviate from typical benign lipoma characteristics 2, 6
• All soft tissue masses that are increasing in size require urgent evaluation per UK sarcoma guidelines 1

Other Malignant Considerations

• Undifferentiated pleomorphic sarcoma (UPS) presents as hypoechoic masses with irregular margins 1
• Leiomyosarcoma can appear as solid hypoechoic masses in soft tissues 1
• Malignant lesions consistently show discrete echo patterns on ultrasound with clearly defined margins, though margins may appear indistinct due to infiltration 6

Benign Differential (Less Likely Given Growth)

• Intramuscular lipoma can appear atypical with striated appearance due to muscle fiber involvement 5, 3
• Fibrolipoma contains fibrous tissue that creates internal striations and may appear less hyperechoic than typical lipomas 7, 5
• However, the 2 cm growth significantly argues against benign lipoma, which are characteristically slow-growing 7, 5

Critical Imaging Features Analysis

Features Suggesting Malignancy

• Growth of 2 cm from previous ultrasound is the single most concerning feature requiring immediate action 1, 6
• Difficult-to-define (indistinct) margins independently predict malignancy and suggest infiltrative behavior 2
• Iso/hypoechoic echogenicity represents a wide diagnostic spectrum including malignant mesenchymal tumors, contrasting with the hyperechoic appearance expected in benign lipomas 2, 8
• Solid appearance with striated pattern suggests heterogeneous tissue composition, which strongly suggests malignancy rather than homogeneous fat 2

Atypical Features for Lipoma

• True lipomas appear intensely hyperechoic on ultrasound, not iso/hypoechoic 2, 8
• Lipomas have well-circumscribed margins with minimal internal vascularity, not difficult-to-define margins 8
• The reduction with probe pressure is non-specific and can occur with various soft tissue masses 1

Recommended Diagnostic Algorithm

Immediate Next Steps

1. Obtain MRI of the affected region to better characterize the lesion, assess depth, and evaluate for features of malignancy 1

   • MRI is superior to ultrasound for evaluating deep soft tissue masses and defining margins 1
   • Look for heterogeneous signal intensity, infiltrative margins, and relationship to fascial planes 1

2. Perform core needle biopsy (preferred) or surgical excision for tissue diagnosis 1, 6

   • All discrete masses on ultrasound that are growing require histologic diagnosis without delay 6
   • Core needle biopsy is preferred over fine needle aspiration for soft tissue masses 1

3. Refer to specialist sarcoma multidisciplinary team (MDT) before any surgical intervention if imaging suggests sarcoma 1

   • Any deep-seated mass with concerning features should be managed at a specialized sarcoma center 1

Critical Pitfalls to Avoid

• Do not assume this is a benign lipoma based on initial clinical impression - the imaging characteristics and documented growth contradict this diagnosis 1, 8
• Do not perform simple excision without proper imaging and tissue diagnosis - inadequate initial surgery compromises outcomes for sarcomas 1
• Do not rely on ultrasound alone - while useful for initial triage, ultrasound has only 64% sensitivity for predicting malignant potential and is operator-dependent 2
• Physical examination correctly identifies only 85% of lipomas, highlighting the critical importance of imaging and tissue diagnosis 8
• Atypical lipomatous tumors (well-differentiated liposarcomas) can have similar pathological features to lipomas but demonstrate high local recurrence rates and require specialized management 3, 4

Size and Location Considerations

• Lesions in the lower back/trunk region that are deep-seated and >5 cm have higher probability of being atypical lipomatous tumors rather than benign lipomas 1, 5
• The term "giant" applies to lipomas ≥10 cm, but any growing mass warrants concern regardless of absolute size 7
• Retroperitoneal and deep truncal locations have worse prognosis if malignant and higher recurrence rates 3