Several observations can be made based on the results of the current study. The majority of metastases in patients with RAIR PET-positive metastases are of a histologically aggressive subtype. However, well-differentiated RAIR metastatic disease is observable. Poorly differentiated disease is underrecognized in many cases if defined by architectural and nuclear features alone. The presence of tumor necrosis was found to be a strong predictor of aggressive behavior, even within this group of clinically aggressive tumors. Finally, there is a significant amount of histologic plasticity between primary tumors and metastases that may reflect the genetic instability of these tumors.