What is the next investigation for a well-demarcated, red, painful lesion on the thigh?

Plain Radiographs in Two Planes Should Be the First Investigation

For a well-demarcated, red, painful lesion on the thigh, conventional radiographs in two planes must be obtained as the initial investigation to exclude underlying bone pathology, assess for soft tissue calcification, and guide subsequent management. 1

Rationale for Radiographic Evaluation First

• The European Society for Medical Oncology (ESMO) explicitly states that conventional radiographs in two planes should always be the first investigation for any persistent painful extremity lesion 2, 1
• Plain radiographs serve multiple critical functions: excluding bone involvement (osteomyelitis, tumor, metastatic disease), detecting soft tissue calcification that narrows the differential diagnosis, and identifying fractures even without trauma history 1
• Radiographic findings directly guide whether subsequent advanced imaging (MRI, CT) or tissue sampling is needed 2, 1

Key Diagnostic Considerations on Plain Films

• Bone pathology assessment: Look for cortical erosion, periosteal reaction, mixed lucency and sclerosis, or aggressive bone destruction that would suggest osteomyelitis, primary bone tumor, or metastatic disease 2
• Soft tissue evaluation: Identify calcification patterns (which may indicate myositis ossificans, calcified hematomas, or certain soft tissue tumors), gas in tissues (suggesting infection), or a visible soft tissue mass 1, 3
• Growth characteristics: The pattern of bone destruction (geographic vs. moth-eaten vs. permeated) and margin characteristics (sclerotic rim vs. ill-defined) indicate the biologic activity and aggressiveness of any lesion 4

Critical Pitfalls to Avoid

• Never assume simple cellulitis or abscess without radiographic evaluation to exclude deeper pathology, as this can delay diagnosis of serious conditions including malignancy 1
• Do not delay imaging for empiric treatment: Persistent non-mechanical pain lasting more than a few weeks warrants immediate radiographic investigation 1
• Never perform biopsy before imaging: This can compromise definitive surgical management if malignancy is present, as the biopsy tract becomes contaminated and must be excised with the tumor 2, 1, 5

Next Steps Based on Radiographic Findings

If Radiographs Show Concerning Features

• Aggressive bone changes (cortical destruction, aggressive periosteal reaction, soft tissue mass): Refer immediately to a specialized bone and soft tissue tumor center before any biopsy 2, 1
• Classic osteomyelitis changes (cortical erosion, periosteal reaction, mixed lucency/sclerosis): Treat for presumptive osteomyelitis after obtaining appropriate culture specimens 2

If Radiographs Are Normal or Equivocal

• MRI without IV contrast becomes the next appropriate study to evaluate soft tissue extent, characterize the lesion, and assess for bone marrow involvement that may not be visible on plain films 2, 1
• MRI is the most accurate imaging modality for defining bone infection and provides the most reliable assessment of deep soft-tissue infections 2
• For suspected osteomyelitis with normal initial radiographs, treat the soft-tissue infection for 2 weeks, then repeat plain radiography 2-4 weeks later if suspicion persists 2

If Soft Tissue Mass Without Bone Involvement

• Proceed to MRI of the entire compartment with adjacent joints for local staging, as this is the best modality for evaluating extremity tumors and soft tissue infections 2, 6
• CT may be appropriate for better visualization of calcification, periosteal bone formation, or cortical destruction when diagnostic problems exist 2, 6

Special Considerations

• Radiographs provide higher specificity than any other imaging modality for detecting tumor morphologic hallmarks and remain the cornerstone for differential diagnosis of skeletal tumors 4
• In 62% of patients with proven soft tissue tumors who had radiographs, positive findings were present including visible soft tissue mass, calcification, fat, or bone involvement 3
• The combination of clinical information with radiographic patterns of destruction, margins, matrix characteristics, and anatomic location allows determination of whether a lesion is neoplastic vs. non-neoplastic, benign vs. malignant, and primary vs. metastatic 4