PVNS means pigmented villonodular synovitis, an older name for what is now generally classified within tenosynovial giant cell tumor (TGCT). It arises from synovial tissue -- the lining of joints, bursae, and tendon sheaths.

It is usually benign in the metastatic-cancer sense, but "benign" undersells the local problem. Diffuse TGCT can infiltrate a joint, recur after surgery, restrict motion, erode cartilage/bone, and eventually produce secondary arthritis.

Big idea: the memorable mechanism is not just "a tumor in the joint." A relatively small population of neoplastic cells can send a signal that recruits a much larger inflammatory-looking cell population.

The strange tumor biology

A small minority of neoplastic cells can overproduce CSF1 (colony-stimulating factor 1) because of genetic rearrangements involving the CSF1 locus. CSF1 then recruits large numbers of macrophages and related cells.

So much of the visible tumor mass is not the original neoplastic clone at all -- it is a cellular crowd recruited by the tumor's signal. This has been called a landscape effect.

That mechanism explains why the CSF1/CSF1R pathway became a drug target in difficult unresectable/recurrent disease.

Why "pigmented"?

Repeated bleeding and iron-containing hemosiderin deposition give affected synovium its characteristic brownish pigmentation and distinctive MRI behavior.

The knee is one of the most common large joints affected.