What is the diagnostic approach for a suspected myofibroblastic mass around the knee?

Diagnostic Approach for Suspected Myofibroblastic Mass Around the Knee

For suspected myofibroblastic masses around the knee, the diagnostic approach should begin with radiographs followed by MRI, with ultrasound being appropriate for superficial lesions, and ultimately require tissue sampling for definitive diagnosis. 1

Initial Imaging Evaluation

• Plain radiographs should be the first imaging study for any suspected soft tissue mass around the knee to identify calcifications, bone involvement, or intrinsic fat 1
• Radiographs can demonstrate positive findings in 62% of soft tissue masses, including calcification (27% of cases), bone involvement (22%), and intrinsic fat (11%) 1
• For superficial or palpable masses, ultrasound is highly appropriate with reported sensitivity of 94.1% and specificity of 99.7%, particularly useful for distinguishing solid from cystic lesions 1
• Radiographs may have limitations when the mass is small, deep-seated, non-mineralized, or in an area with complex anatomy like the knee 1

Advanced Imaging

• MRI without and with IV contrast is the preferred next step if initial evaluation is nondiagnostic or for deep masses around the knee 1, 2
• On MRI, inflammatory myofibroblastic tumors (IMTs) typically show isointense to hypointense signal compared to skeletal muscle on T1-weighted images and heterogeneously high signal on T2-weighted images 3
• After contrast administration, IMTs demonstrate persistent heterogeneous enhancement, often with cystic regions 3
• CT may be useful to evaluate calcifications within the mass, which are present in some IMTs 4, 3

Tissue Sampling

• Core needle biopsy is the standard approach for establishing histopathological diagnosis of suspicious soft tissue masses 2, 5
• Multiple core samples should be taken under image guidance to maximize diagnostic yield 2, 5
• Excisional biopsy may be more practical for small subcutaneous lesions (<2 cm diameter) or superficial lesions <5 cm 2, 5
• Biopsy planning should ensure the biopsy tract can be safely removed during definitive surgery 5

Histopathological and Immunohistochemical Features of IMTs

• IMTs are characterized by spindle cell proliferation within a myxoid stroma with admixed plasma cells, lymphocytes, and eosinophils 6, 7
• Immunohistochemistry typically shows positivity for vimentin (87%), smooth muscle actin (SMA) (88.9%), and anaplastic lymphoma kinase (ALK) (44.4%) 3, 7
• Three basic histologic patterns may be recognized: myxoid/vascular areas resembling nodular fasciitis, compact spindle cells with inflammatory cells, and dense plate-like collagen 6

Management Considerations

• Complete surgical excision is the standard treatment for IMTs 8, 6
• IMTs are generally benign but can demonstrate local invasiveness in some cases (12 of 51 patients in one study) 3
• Recurrence can occur (reported in 13 of 53 patients in one series), typically within 1-24 months after initial treatment 6
• For masses with concerning features, referral to a specialized sarcoma center with a multidisciplinary team is recommended 1, 2

Clinical Pitfalls to Avoid

• Physical examination alone is insufficient for diagnosis, correctly identifying only about 85% of soft tissue tumors 1
• IMTs can mimic malignant lesions both clinically and radiologically, making histopathological confirmation essential 3, 7
• Laboratory abnormalities such as anemia, thrombocytosis, polyclonal hypergammaglobulinemia, and elevated erythrocyte sedimentation rate may be present in some patients with IMTs 6
• Follow-up imaging is necessary to detect potential recurrence 7