What is the differential diagnosis for an ovoid iso/hypoechoic (having the same or lower echogenicity than surrounding tissues) solid striated mass in the right lower back with difficult to define margins, that reduces with ultrasound (US) probe pressure and has grown approximately 2 centimeters since the previous US, with the radiologist suggesting a possible lumbar hernia?

Differential Diagnosis of Growing Solid Striated Mass in Right Lower Back

The most likely diagnosis is a lumbar hernia (specifically Grynfeltt-Lesshaft hernia), but soft tissue sarcoma must be urgently excluded given the documented 2 cm growth and difficult-to-define margins.

Primary Diagnostic Considerations

Lumbar Hernia (Most Likely)

• Lumbar hernias present as ovoid, soft, reducible masses in the lumbar region that are impulsive with coughing or increased abdominal pressure 1, 2
• The reduction with probe pressure strongly supports this diagnosis, as hernias characteristically reduce with external pressure 1
• Grynfeltt-Lesshaft hernias occur in the superior lumbar triangle and can appear as hypoechoic masses on ultrasound with identifiable defects in the abdominal wall 1, 2
• The striated appearance likely represents herniated fat or peritoneal contents rather than solid tissue 2

Soft Tissue Sarcoma (Must Exclude)

• Growth of 2 cm from previous ultrasound is the single most concerning feature requiring immediate action, and difficult-to-define (indistinct) margins independently predict malignancy 3
• Well-differentiated liposarcoma, undifferentiated pleomorphic sarcoma, and leiomyosarcoma can all present as iso/hypoechoic masses with irregular margins 3
• All soft tissue masses that are increasing in size require urgent evaluation 3

Other Differential Diagnoses

• Lipoma: However, lipomas typically appear hyperechoic on ultrasound, not iso/hypoechoic, and the documented growth makes this less likely 3, 4
• Hematoma or abscess: These would typically have more acute presentation and different clinical context 5

Critical Diagnostic Algorithm

Immediate Next Steps

1. Obtain MRI of the lumbar region to better characterize the lesion, assess depth, evaluate for fascial defects (confirming hernia), and assess for features of malignancy 3

   • MRI is superior to ultrasound for evaluating deep soft tissue masses and defining margins 3

2. Perform dynamic ultrasound evaluation with Valsalva maneuver or coughing to assess for herniation through a fascial defect 2

   • If the mass increases with Valsalva and reduces completely, this strongly supports lumbar hernia 1, 2

3. If imaging is equivocal or suggests solid tissue rather than hernia, proceed to core needle biopsy 3

   • Core needle biopsy is preferred over fine needle aspiration for soft tissue masses 3
   • All discrete masses on ultrasound that are growing require histologic diagnosis without delay 3

Key Distinguishing Features

Features Favoring Lumbar Hernia

• Reducibility with probe pressure (present in this case) 1, 2
• Impulsivity with coughing or Valsalva (needs clinical assessment) 1
• Identification of fascial defect on imaging 1, 2
• Location corresponding to superior or inferior lumbar triangle 1, 2

Features Concerning for Malignancy

• Documented growth of 2 cm (present in this case) 3
• Difficult to define margins (present in this case) 3
• Iso/hypoechoic echogenicity represents a wide diagnostic spectrum including malignant mesenchymal tumors 3
• Solid appearance rather than cystic 6

Critical Pitfalls to Avoid

• Do not assume benign lipoma based on clinical impression alone, as lumbar hernias are frequently misdiagnosed as lipomas, leading to delayed treatment and increased morbidity 4
• Do not perform simple excision without proper imaging and tissue diagnosis if there is any concern for malignancy 3
• All palpable soft-tissue masses that appear discrete on sonogram should be diagnosed without delay by biopsy if hernia is excluded 6
• Refer to a specialist sarcoma multidisciplinary team before any surgical intervention if imaging suggests sarcoma 3

Management Based on Findings

If Lumbar Hernia Confirmed

• Surgical repair (herniorrhaphy) is indicated to prevent complications and progression 1, 2
• Both open and laparoscopic approaches are described 2

If Sarcoma Cannot Be Excluded

• Refer to specialized sarcoma center before any intervention 3
• Obtain tissue diagnosis via core needle biopsy 3
• Stage appropriately with cross-sectional imaging 3