Whilst primary giant cell tumors (GCT) of bone are often considered benign in nature (albeit locally aggressive), distant metastases occur in a small minority (between 1-9%, depending on the study), with pulmonary metastases being the most common site. 

Unlike most other distant metastatic diseases, metastatic GCT is not necessarily associated with a poor prognostic outcome, with metastasectomy (where possible) being curative in many patients. 

In the setting of known primary GCT this diagnosis is relatively straightforward, however, in the absence of such a history, a wider differential diagnosis must be considered.