What imaging modalities (e.g. ultrasound, Magnetic Resonance Imaging (MRI), Computed Tomography (CT) scan) should be ordered for a painful nodule in the soft tissue of the thigh?

Imaging for Painful Nodule in Thigh Soft Tissue

Start with plain radiographs followed immediately by ultrasound, and proceed to MRI if ultrasound is uncertain or suggests concerning features. 1

Initial Imaging Algorithm

Step 1: Plain Radiographs

• Obtain radiographs first to identify bone involvement, risk of fracture, or calcification patterns that may guide further workup 1
• Radiographs detect mineralization in approximately 11% of soft-tissue masses and can reveal pathognomonic calcification patterns 1, 2
• However, radiographs have limited sensitivity (54%) and specificity (68%) for soft tissue masses and may be unrewarding for small, deep-seated, or non-mineralized lesions 3

Step 2: Ultrasound Evaluation

• Ultrasound should be performed as the primary triage tool after radiographs for any soft tissue nodule in the thigh 1, 3
• Ultrasound demonstrates high diagnostic accuracy with sensitivity of 94.1% and specificity of 99.7% for superficial soft tissue masses 1, 3, 2
• The examination can differentiate solid from cystic lesions, identify benign lipomas, and detect concerning features requiring further investigation 1

Critical caveat: Ultrasound accuracy drops precipitously for deep-seated masses, and the examination is highly user-dependent 1

Step 3: MRI When Indicated

Proceed directly to MRI if any of the following apply:

• Ultrasound findings are uncertain or atypical 1, 2
• The nodule is deep to the fascia 1, 4
• Size greater than 5 cm 1, 4
• Rapid growth or increasing pain 2, 4
• Ultrasound suggests features concerning for soft tissue sarcoma 1

MRI provides the most accurate information for diagnosis and surgical/radiotherapy planning for soft-tissue tumors affecting the extremity 1

Red Flags Requiring Urgent Specialist Referral

Any painful thigh nodule with the following features warrants suspected cancer pathway referral (within 2 weeks) 1, 4:

• Deep location (below the fascia) 1, 4
• Size >5 cm 1, 4
• Increasing in size 1
• Ultrasound or MRI findings suggestive of soft tissue sarcoma 1

Tests NOT Recommended for Initial Evaluation

• CT is not appropriate as initial imaging for a thigh soft tissue nodule unless there is suspected complex osseous involvement 1
• PET-CT has no role in initial evaluation of a painful soft tissue nodule 1
• MRI should not be the first test due to its inability to adequately identify mineralization and cost considerations 1

Common Pitfalls to Avoid

• Do not rely solely on clinical examination to exclude malignancy; painful nodules can represent sarcoma 1
• Do not assume deep location automatically means benign (such as Baker's cyst); deep masses in the thigh require MRI evaluation 1
• Do not perform excisional biopsy outside a sarcoma center if imaging raises any concern for malignancy, as this increases local recurrence risk 4
• Do not dismiss ultrasound findings of a "lipoma" if the mass is deep-seated or in the lower limb, as atypical lipomatous tumors are more common in these locations 1, 2