General
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• Rare neoplastic-like villonodular hyperplasia of synovium and tendon sheaths in young adults composed of mononuclear cells and multinuclear giant cells with hemosiderin deposition (bonetumor.org, Am Fam Physician 1999;60:1404)
• May be similar to diffuse form of tenosynovial giant cell tumor

Clinical features
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• Develops in synovial lining of joints, tendon sheaths and bursae, usually of knee (80%), ankle, hip, shoulder, elbow joint; nodular variant occurs in hands and wrists
• Almost always monoarticular
• Occasionally invades underlying bone
• May cause bone cyst formation, loss of bone and cartilage
• Consists of CD68+ histiocytes with osteoclastic giant cell differentiation, may be hyperplastic, not neoplastic (Hum Pathol 2003;34:65)
• Locally aggressive; may recur, but only rarely has malignant behavior (Am J Surg Pathol 1997;21:153)

Case reports
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• 15 year old with new onset of fatigue, diffuse arthralgia, butterfly rash, tenosynovitis of wrist, lymphopenia and thrombocytopenia (J Med Case Rep 2011;5:443)
• 28 year old woman with painful swelling in ankle (Intern Med 2013;52:1149)
• 36 year old man with tumor at temporomandibular joint (Arch Pathol Lab Med 2002;126:195)
• 38 year old man with 3 year history of nontraumatic pain in knee joint (J Ultrasound 2011;14:167)
• 56 year old woman with malignant tumor (Open Orthop J 2011;5:13)
• 62 year old man with gradually worsening pain and stiffness in knee (J Med Case Rep 2009;3:121)
• 66 year old woman with long history of back pain (Turk Neurosurg 2010;20:251)
• 2 cases involving vertebral column (Arch Pathol Lab Med 1984;108:228)
• 2 rare cases of malignant behavior (Arch Pathol Lab Med 2000;124:1636)

Treatment
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• Excision, but may recur locally if not completely excised; postoperative external beam radiotherapy may be useful for recurrences (Acta Orthop 2012;83:256)
• Also Imatinib mesylate (Cancer 2012;118:1649)

Clinical images
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Knee arthroscopy

Stained synovium

Gross description
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• Focal or diffuse; brown-yellow spongy tissue with variable color and firm nodular consistency

Gross images
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Brownish synovium

Micro description
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• Hyperplastic synovium with papillary projections composed of foamy cells and hemosiderin containing macrophages
• Also large clefts, pseudoglandular or alveolar spaces lined by synovial cells, multinucleated (10-70 nuclei) giant cells, epithelioid cells
• Abundant collagen may be present, but lymphocytes and plasma cells are sparse
• No/rare mitotic figures
• Malignant if nodular and solid invasive growth pattern are coupled with large round or oval cells with large nuclei, prominent nucleoli, necrotic areas, atypical mitotic figures

Micro images
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Pseudoglandular or cleft-like spaces

Proliferation of polygonal cells

Multinucleated giant cells

Hemosiderin-laden macrophages

Hemosiderin-laden and lipid-laden macrophages

Lipid-laden macrophages

Low power

Medium power

Malignant cases

Hemosiderin deposits, foamy histiocytes and giant cells

Extensive histiocytic proliferation

Virtual slides
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48 year old woman with synovial polyp

54 year old man with tissue from knee joint

Cytology description
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• Round, spindled and multinucleated cells without atypia
• Round cells have minimal cytoplasm and eccentric nuclei
• Occasional cytoplasmic hemosiderin
• No mitotic figures

Cytology images
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H&E

Positive stains
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• CD68 (stromal and giant cells), HAM56, and vimentin

Negative stains
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• S100, CD45/LCA, EMA, keratin, HMB45, CD34, desmin, smooth muscle actin

Molecular / cytogenetics description
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• Trisomy 5 and 7
• Translocation and expression of CSF1 (Stanford University)

Molecular / cytogenetics images
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FISH: trisomy 5 and 7

Differential diagnosis
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• Fibrosarcoma
• Hemosiderotic synovitis: associated with hemophilia and intraarticular bleeding, no mononuclear or giant cell nodular cellular proliferation, hemosiderin primarily in synovial lining cells
• Synovial sarcoma

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