Extracranial oligometastases: a subset of metastases curable with stereotactic radiotherapy.

The term oligometastases, introduced in 1995 and detailed more recently, describes an intermediate state of cancer spread between localized disease and widespread metastases. Metastases from solid tumors are regarded as representative of disseminated cancer and are not considered curable, with the rare exception, such as germ cell tumors. By contrast, evidence has emerged that patients with limited metastatic disease, such as liver metastasis from colon or rectal cancer, can be cured by removal of the metastasis, drawing increased focus on the potential for intermediate states of metastatic cancer involvement. The implication of the concept of an oligometastatic state is that metastatic disease may be cured with metastasis-directed therapy. As a further conceptual refinement, Niibe et al have suggested the concept of oligorecurrence to consider patients with a limited number of metastases and controlled primary tumors as a group with an improved prognosis as compared with patients with limited metastasis and uncontrolled primary tumors. The oligometastatic hypothesis is distinct from other potentially important uses of radiotherapy and surgery in metastatic disease, such as consolidation of chemotherapy responses or as an application of the Norton-Simon hypothesis, which predicts that effectiveness of chemotherapy is proportional to the growth rate of the tumor and that the fastest growth rates occur in nonbulky tumors. Aggressive local therapy to metastatic lesions can downsize tumors, and the remaining cells might therefore be more sensitive to chemotherapy. Our review will focus on extracranial oligometastases because intracranial oligometastasis is an established clinical entity where surgery, radiotherapy, and radiosurgery have defined roles supported by phase III randomized trials and detailed outcome analyses.