What is the recommended workup for a 33-year-old patient with an irregular mass in their posterior thigh?

Workup for Irregular Posterior Thigh Mass in a 33-Year-Old

Obtain MRI of the thigh with IV contrast as the primary imaging modality, followed by core needle biopsy if the mass appears suspicious for malignancy. 1, 2

Initial Imaging Strategy

MRI with contrast is the definitive imaging study for characterizing soft tissue masses in the thigh. 2 This modality provides superior soft tissue contrast resolution compared to other imaging options and achieves 91% accuracy in distinguishing benign from malignant masses. 2 MRI can accurately determine the mass's origin, assess its relationship to neurovascular structures and deep fascial planes, and identify features suggestive of malignancy such as solid components with enhancement, irregular borders, and invasion of adjacent structures. 2

Why MRI Over Other Modalities

• Ultrasound has limited utility for deep soft tissue masses in the thigh, though it may help determine if a superficial mass is solid or cystic. 3
• CT is not appropriate for initial evaluation of soft tissue masses due to inferior soft tissue discrimination compared to MRI. 2
• PET-CT is not indicated for initial workup of soft tissue masses. 2

Biopsy Approach

If MRI demonstrates features concerning for sarcoma, proceed with image-guided core needle biopsy (multiple cores to maximize diagnostic yield). 1 The biopsy tract must be planned so it can be excised during definitive surgery if malignancy is confirmed. 1

Key Features Suggesting Malignancy on MRI

• Size >5 cm 1
• Deep location (subfascial) 1
• Heterogeneous signal intensity 2
• Irregular margins 2
• Significant contrast enhancement in solid components 2

Staging if Sarcoma is Confirmed

For confirmed or highly suspected soft tissue sarcoma, obtain CT chest to exclude pulmonary metastases. 1 Most patients with confirmed intermediate or high-grade soft tissue sarcomas require chest CT as the lungs are the most common site of metastatic disease. 1

Consider CT abdomen and pelvis for high-grade sarcomas of the lower extremities, as visceral metastases can occur. 1

Critical Pitfalls to Avoid

• Never perform excisional biopsy as the initial diagnostic approach for masses suspicious for sarcoma, as this compromises subsequent definitive surgical margins. 1
• Do not delay biopsy if imaging is highly suspicious for malignancy—histologic diagnosis should be established before definitive treatment. 1
• Avoid fine needle aspiration as the primary diagnostic modality, as it provides insufficient tissue for accurate sarcoma diagnosis and grading. 1
• Plan the biopsy tract carefully—it must be placed so the entire tract can be excised during definitive resection if sarcoma is confirmed. 1

Alternative Diagnoses to Consider

While sarcoma is the primary concern for an irregular posterior thigh mass in this age group, the differential includes:

• Lipoma or atypical lipomatous tumor (though these are typically well-circumscribed, not irregular) 4, 5
• Popliteal artery aneurysm if the mass is pulsatile or located in the popliteal fossa—ultrasound with Doppler can rapidly confirm this diagnosis 1
• Hematoma or abscess if there is recent trauma or infection 1

The irregular nature of the mass raises significant concern for malignancy and mandates prompt imaging with MRI and likely biopsy. 1, 2