What is the recommended diagnostic work‑up and initial management for a patient with an abdominal mass who is experiencing thigh and back pain?

Recommended Diagnostic Work-Up and Initial Management

For a patient with an abdominal mass and thigh/back pain, immediately obtain CT abdomen and pelvis with IV contrast as the initial imaging study to evaluate for retroperitoneal pathology, potential malignancy, or complications causing referred pain. 1

Clinical Reasoning

The combination of an abdominal mass with thigh and back pain is highly concerning for retroperitoneal pathology. The back pain suggests posterior extension or retroperitoneal involvement, while thigh pain may indicate:

• Psoas muscle involvement or irritation
• Nerve compression (lumbar plexus, femoral nerve)
• Referred pain from retroperitoneal structures
• Potential hemorrhage or mass effect

This presentation differs fundamentally from simple adnexal or intra-abdominal masses without these symptoms, requiring a broader diagnostic approach.

Initial Imaging Strategy

CT Abdomen and Pelvis with IV Contrast (First-Line)

This is the single most appropriate initial study because it:

• Provides comprehensive evaluation of retroperitoneal structures, including psoas muscles
• Identifies mass characteristics (solid vs. cystic, enhancement patterns)
• Detects complications: hemorrhage, abscess, or active bleeding
• Evaluates for malignancy with staging information
• Assesses vascular involvement and organ displacement
• Can identify nerve compression or infiltration

1 The ACR Appropriateness Criteria for acute nonlocalized abdominal pain emphasizes that CT with IV contrast is the preferred imaging option when the differential is broad and clinical acuity demands fast, definitive diagnosis. CT changed the leading diagnosis in 49% of patients and altered surgical plans in 25% in emergency settings.

Alternative if CT Contraindicated

If iodinated contrast is contraindicated:

• MRI abdomen without and with IV contrast provides excellent soft tissue characterization
• Particularly useful for distinguishing tumor from hemorrhage
• Can characterize bleeding acuity and identify underlying neoplasm 2

Critical Diagnostic Considerations

Retroperitoneal Pathology (Most Likely Given Symptoms)

The thigh and back pain pattern strongly suggests:

• Retroperitoneal sarcoma (most common in adults 40-60 years)
• Psoas abscess (if febrile)
• Retroperitoneal hemorrhage (if anticoagulated or trauma history)
• Lymphoma with retroperitoneal involvement
• Metastatic disease to retroperitoneal lymph nodes

3, 4 For retroperitoneal/intra-abdominal soft tissue masses, CT is the preferred imaging modality. The differential includes sarcomas, GISTs, lymphomas, germ cell tumors, and desmoid tumors.

When to Obtain Biopsy

Do NOT biopsy before imaging is complete. After CT characterization:

• Image-guided core needle biopsy is required if:

  • Neoadjuvant therapy is being considered
  • Suspicion for lymphoma or germ cell tumor (to avoid inappropriate major resection)
  • Diagnosis unclear and would change management

• Avoid open surgical biopsy - use CT or ultrasound-guided core needle biopsy 3

Immediate Management Priorities

Pain Control

• Administer opioid analgesics immediately - they do NOT affect diagnostic accuracy of physical examination 5
• Pain intensity reduction improves patient cooperation with imaging

Fluid Resuscitation

• Aggressive IV fluid substitution regardless of diagnostic workflow 6
• Critical if retroperitoneal hemorrhage suspected

Antibiotics (If Indicated)

• Start broad-spectrum antibiotics within 1 hour if sepsis recognized 5
• Consider early initiation if abscess suspected with fever

Physical Examination Specifics to Document

Look for and document:

• Psoas sign (pain with hip extension) - suggests psoas involvement
• Femoral nerve stretch test - thigh pain with hip extension
• Flank fullness or mass - retroperitoneal location
• Loss of psoas shadow on any available radiographs
• Fever - suggests infection/abscess
• Hemodynamic stability - hemorrhage consideration
• Neurologic deficits in lower extremity - nerve compression

Common Pitfalls to Avoid

1. Do not start with ultrasound - it has limited ability to evaluate retroperitoneal structures and the entire abdomen in acute settings 2

2. Do not obtain plain radiographs - they have low sensitivity (radiographs show psoas obscuration only with large hematomas) and delay definitive imaging 2, 1

3. Do not assume gynecologic origin in females - the back/thigh pain pattern makes retroperitoneal pathology more likely than simple adnexal mass 7, 8

4. Do not delay imaging for extensive laboratory workup - CRP and WBC alone cannot discriminate urgent from non-urgent causes 5

Subsequent Steps Based on CT Findings

If Malignancy Suspected:

• Chest CT for staging (lung metastases)
• Consider brain MRI if alveolar soft part sarcoma, clear cell sarcoma, or angiosarcoma 4
• Multidisciplinary sarcoma team consultation before biopsy 3

If Retroperitoneal Sarcoma Confirmed:

• Refer to specialized sarcoma center
• Core needle biopsy for histologic subtype and grade
• Consider neoadjuvant radiation therapy discussion
• Surgical planning for en bloc resection with involved organs 3

If Abscess Identified:

• Percutaneous drainage if feasible
• Blood cultures and targeted antibiotic therapy
• Source control within 12 hours

If Hemorrhage Detected:

• Assess for active extravasation on CT
• Consider CT angiography if active bleeding suspected
• Interventional radiology consultation for possible embolization
• Correct coagulopathy

The key principle: thigh and back pain with abdominal mass demands evaluation of the retroperitoneum first, making CT with IV contrast the essential initial diagnostic study. 1