Oligometastases: Defined by prognosis and evaluated by cure

Recently, the state of oligometastases has been spotlighted in the treatment strategy for metastases. Aggressive local treatment for oligometastases, including pulmonary resection, stereotactic body radiotherapy (SBRT), radiofrequency ablation, and cryoablation has been the subject of research. Among studies on the local treatment, those on SBRT more often evaluated local control as the primary outcome, and those on pulmonary metastasectomy more often evaluated overall survival as the primary outcome. Oligometastases is a disease concept that is defined by a state of limited systemic metastatic tumors for which local ablative therapy could be curative. By definition, the purpose of local treatment for oligometastases is cure, and the primary outcome to be analyzed should be disease-free survival. As systemic adjuvant therapy in addition to local treatment with complete ablation has some effect on micrometastases, in clinical research on oligometastases, the only treatment modality under evaluation should be local ablation. There are multiple discrete indications for the local treatment of metastatic lesions. The purposes of these indications are (a) the intent to cure oligometastases, (b) the intent to prolong survival as a part of multidisciplinary therapy, and (c) local control for palliative care. In order to appropriately evaluate the significance of local treatment, the outcomes should depend on the indication for treatment. The corresponding outcomes to consider are (a) disease-free survival, (b) overall survival, and (c) local control. Factorial analysis of each outcome corresponding to each indication for local therapy would yield information on each clinical presentation to help decide treatment.