Necrotic granulomatous pseudotumor following metal-on-metal hip arthroplasty: a potential mimic of sarcoma on fine needle aspiration cytology.

A 69-year-old female presented with left leg pain 4 months after total hip replacement for left hip joint protrusio acetabuli. A "cystic" mass lesion was identified radiologically in relation to the arthroplasty and an initial CT-guided core needle biopsy showed a spindle cell proliferation with associated necrosis, interpreted as suspicious for malignancy. A repeat CT-guided fine needle aspiration showed necrosis, "ghost" spindle cells, aggregates of histiocytes, giant cells, and inflammatory cells as well as scattered large atypical spindle cells. The simultaneously obtained core biopsy showed extensive necrosis with ghost spindle cells that was surrounded by CD68+ histiocytes, which in turn were surrounded by a predominantly CD3+, CD4+ lymphocytic infiltrate. A diagnosis of necrotic granulomatous pseudotumor was made after a diagnosis of sarcoma was initially entertained. This unusual tissue response is rarely seen after metal-on-metal arthroplasty, occurs more often in females and may represent a type-IV hypersensitivity reaction incited by very small-sized particulate metallic debris that forms haptens in association with serum proteins.