How should I evaluate a patient with an infiltrative lesion of the right upper‑arm musculature, considering soft‑tissue sarcoma, lymphoma, or aggressive inflammatory myopathy as differentials?

Evaluation of Infiltrative Right Upper Arm Musculature Lesion

Obtain MRI of the right upper arm without and with IV contrast as the primary imaging modality, followed by image-guided core needle biopsy of the most metabolically active or enhancing region to establish definitive histologic diagnosis. 1

Initial Imaging Approach

MRI is the gold standard for evaluating deep soft tissue masses of the extremity. 1 For this infiltrative upper arm process, the imaging protocol should include:

• MRI without and with IV contrast to characterize tissue planes, assess compartmental involvement, and identify areas of viable tumor versus necrosis 1
• Baseline chest imaging (chest radiograph or CT) to evaluate for pulmonary metastases, as soft tissue sarcomas most commonly metastasize to the lungs 1

The ACR Appropriateness Criteria specifically recommend MRI with and without contrast for deep masses or when initial evaluation is nondiagnostic. 1

Key MRI Features to Assess

Evaluate the MRI for distinguishing features among your differentials:

Soft Tissue Sarcoma Features:

• Well-defined margins with variable signal characteristics depending on histologic subtype 1
• Compartmental localization (extremity sarcomas often respect fascial boundaries initially) 1

Lymphoma Features:

• Intermediate signal intensity on both T1 and T2-weighted images (85% show intermediate T2 signal, which is atypical for most soft tissue masses) 2
• Poorly defined margins with peritumoral edema in approximately 40% of cases 2
• Involvement of multiple muscle compartments (50% of cases) and subcutaneous extension when located just deep to fascia 2
• Extension along neurovascular bundles (29% of cases) 2
• Look for associated lymphadenopathy, particularly confluent nodes, which strongly favors lymphoma over sarcoma 3

Inflammatory Myopathy Features:

• Typically bilateral and symmetric (though aggressive variants can be focal)
• Associated muscle edema and enhancement

Clinical Red Flags for Lymphoma

Assess for clinical features that favor lymphoma over sarcoma:

• Pain and tenderness at presentation (more common in lymphoma) 3
• Ipsilateral extremity swelling 3
• Elevated lactate dehydrogenase (LDH) - check this laboratory value 3
• Palpable lymphadenopathy 3
• Older age (mean 72.5 years in one series of soft tissue lymphomas) 3

Tissue Diagnosis Strategy

Core needle biopsy is essential and should be performed by or in consultation with the treating surgical oncologist. 1 Critical principles include:

• Biopsy tract placement must be planned along the line of definitive surgical resection to avoid contaminating additional compartments 1
• Target the most enhancing or metabolically active areas on MRI to maximize diagnostic yield and avoid necrotic regions 1
• Obtain adequate tissue for histology, immunohistochemistry, and molecular studies 1

For lymphoma diagnosis, immunohistochemistry should include CD20, CD3, CD68, and potentially CD30 and ALK, as most soft tissue lymphomas are B-cell non-Hodgkin lymphomas. 2, 3

For sarcoma, molecular testing may identify targetable alterations (ALK, ROS1, NTRK, RET fusions in inflammatory myofibroblastic tumors; various tyrosine kinase fusions including PDGFRB and PDGFRA). 4, 5

Multidisciplinary Evaluation

Before initiating treatment, refer to a multidisciplinary sarcoma team with demonstrated expertise, as mandated by NCCN guidelines. 1 This is critical because:

• Adherence to evidence-based protocols significantly impacts outcomes in these rare tumors 1
• Treatment planning requires coordination between surgical oncology, medical oncology, radiation oncology, and specialized pathology 1

Staging Considerations

Once histologic diagnosis is established:

• For sarcoma: Chest CT for metastatic evaluation (lungs are primary metastatic site) 1
• For lymphoma: Full staging workup including CT chest/abdomen/pelvis, bone marrow biopsy, and consideration of PET/CT for systemic disease assessment 3

Common Pitfall

Do not assume this is a benign inflammatory process without tissue diagnosis. Even with steroid-responsive symptoms, aggressive lymphomas and inflammatory myofibroblastic sarcomas can show temporary response to corticosteroids, delaying definitive diagnosis and worsening prognosis. 6, 4