Modern Evidence and Future Prospects of External Body Radiation Therapy for Lung Oligometastases of Breast Cancer

After Hellman and Weichselbaum defined "Oligometastasis" in 1995, several local therapies for lung oligometastases including surgical resection and external body radiation therapy were reported that improved local control (LC) and progression-free survival, overall survival, and quality of life. This suggests that oligometastases is a potentially curable state. Modern advances in radiation therapy such as stereotactic body radiation therapy (SBRT) in which high dose coverage of target lesion without exposure of normal organ is possible, and are widely used to treat solitary or a limited number of primary lung cancer and metastases. Several reports showed that SBRT was a useful treatment method for lung oligometastases, and the LC rate of SBRT was 80-90% in 2 years and less invasive than surgical resection. SBRT is a safe and effective especially for small and peripheral lung metastases. However, if the metastatic lesion is big or centrally located, careful treatment is necessary to prevent radiation pneumonitis. After SBRT, it is sometimes difficult to differentiate local recurrence and pulmonary injury, especially in the early phase. However, it is important to detect local recurrence especially in patients who require further local therapy such as surgical resection and re-irradiation or systemic therapy. The diagnosis can be improved by determining the natural course after SBRT and local recurrence with computed tomography imaging and F-fluorodeoxyglucose positron emission tomography, respectively. Moreover, radiation therapy may have both local and systemic effects that are related to the enhancement of immune-response after radiation. Currently, several trials evaluating the benefits of SBRT for oligometastatic breast cancer are underway. However, the adaption of SBRT for lung metastases including other treatment strategies should be carefully discussed by the radiation oncologist and a multi-disciplinary team comprising a breast surgeon, medical oncologist, diagnostic radiologist, and radiation oncologist, among others.