Pigmented Villonodular Synovitis (PVNS)

The disease shows no hereditary pattern and no clear environmental or genetic risk factors have been identified. The hallmark is chronic monoarticular synovitis with recurrent hemarthrosis.

The key pathological distinction is localized versus diffuse forms, which have different prognosis and recurrence risk.

Joint Distribution:

• Knee (80% of cases) - most common site
• Hip (10-15%) - more challenging diagnosis and treatment
• Ankle, shoulder, elbow (rare, each under 5%)
• Temporomandibular joint (case reports)

The key clinical clue is monoarticular chronic synovitis with recurrent hemarthrosis in a young adult.

The critical distinction is benign (PVNS, chondromatosis) versus malignant (synovial sarcoma) - biopsy if any doubt.

The differential table and the Murphey evidence card both allude to "malignant PVNS", and the SafetyAlert warns to "exclude malignancy", but the topic never develops this rare, important entity - a genuine exam curveball for an otherwise "benign" disease.

• It exists, and it is rare. Malignant TGCT (malignant PVNS) is a sarcomatous transformation of, or a malignant tumour arising alongside, a diffuse-type giant cell tumour. It is very rare (well under 5% of TGCT) but carries a metastatic potential and mortality quite unlike benign PVNS - so a "benign" label must never be applied without adequate sampling.
• When to suspect it. Red flags are older age at onset, a large or rapidly enlarging mass, recurrent disease that becomes more aggressive, and destructive/atypical imaging (aggressive bone destruction and soft-tissue extension beyond the usual PVNS pattern). Most malignant cases arise in recurrent, previously-benign diffuse disease rather than at first presentation.
• How it is diagnosed. By histology: alongside the typical mononuclear/hemosiderin/giant-cell background there are overtly malignant features - marked cytological atypia, high mitotic activity, atypical mitoses, spindling into a sarcomatous pattern, necrosis, and enlarged mononuclear cells. This is why an atypical, aggressive or unexpectedly recurrent lesion warrants generous biopsy and expert sarcoma pathology, not just a diagnosis of "recurrent PVNS".
• Management is that of a sarcoma. Wide surgical resection (rather than simple synovectomy), with adjuvant radiotherapy/chemotherapy and staging for metastases (lungs, nodes) via a sarcoma MDT. Prognosis is markedly worse than benign disease, with a real risk of metastasis and death.

The topic covers external-beam radiotherapy and the Verspoor series explicitly lists "radiosynovectomy" among treatments received, but the intra-articular radioisotope option is never explained - a distinct, exam-relevant adjunct different from external-beam radiation.

• What it is. Radiosynovectomy (radiosynoviorthesis) is the intra-articular injection of a beta-emitting radionuclide that is taken up by the synovium and ablates residual diseased synovial lining from within - a "medical synovectomy". It targets microscopic residual disease that surgery leaves behind, which is the very reason diffuse PVNS recurs.
• Which isotope for which joint. The agent is matched to joint size by the radionuclide's tissue penetration: Yttrium-90 (⁹⁰Y) for the knee (large joint, deeper penetration), Rhenium-186 (¹⁸⁶Re) for medium joints (hip, shoulder, ankle, wrist, elbow), and Erbium-169 (¹⁶⁹Er) for small (finger/toe) joints. Colloid/particulate carriers keep the isotope in the joint.
• Where it fits. It is used mainly as an adjunct after surgical synovectomy in diffuse disease (to treat residual synovium and reduce recurrence), or for inaccessible/multiply-recurrent disease where repeat open surgery is unattractive - conceptually similar to adjuvant external-beam radiotherapy but delivered from inside the joint with less exposure to surrounding tissues. It is also long-established for haemophilic and inflammatory synovitis.
• Cautions. The joint must be sealed (no leak/fistula) to avoid radionuclide escape and lymphatic/systemic spread; post-injection rest/immobilisation limits leakage; and it is generally avoided in the very young (theoretical radiation risk). Availability is limited to centres with nuclear-medicine support.

The major complication of PVNS is recurrence, which drives need for surveillance and adjuvant therapy strategies.

Long-term surveillance is essential given high recurrence risk in diffuse PVNS.

The key message: PVNS is a chronic disease requiring long-term surveillance, but outcomes are generally good if recurrence is detected and treated early.

Management of Recurrent PVNS:

• First recurrence: Re-synovectomy + adjuvant radiation (20 Gy)
• Multiple recurrences: Consider CSF1R inhibitor (pexidartinib, imatinib)
• Unresectable disease: CSF1R inhibitor or arthroplasty

The goal is to balance aggressive treatment with preservation of joint function.

PVNS is a rare locally aggressive disease best managed through specialist multidisciplinary pathways worldwide; the principal global variation is access to advanced imaging, radiotherapy and CSF1R-targeted agents rather than differences in underlying clinical principles.