Solitary Metastases: Illusion Versus Reality

“Suddenly a solitary horseman appeared on the horizon, then another, then another…in a few moments a whole crowd of horsemen swooped down upon him.”—Leacock

The illusion of solitary metastases is counterintuitive but has generated a sizable literature on the subject. The reality is that there are more metastatic deaths each year than the total number of true long-term survivors of solitary metastases combining all organ sites in the literature of the past century up to the present time. The largest number of solitary metastases survivors had metastases primarily in the lung and/or liver. With innovations in molecular imaging and advances in molecular oncology, the stage is set to detect truly solitary metastases early. Then, aggressive treatment by surgical excision, stereotactic body radiosurgery, targeted chemotherapy, or immunotherapy could eradicate the lesion. A comprehensive review of solitary metastases in a large variety of anatomic sites is presented. A broader staging system is recommended to encompass a solitary metastasis (M1) and oligometastases (M2) as distinct from multiple metastases (M3).