In this review, we highlight some of recent studies underscoring the importance of the tumor microenvironment, especially the role of bone marrow-derived myeloid cells, in restoring tumor growth after irradiation. Myeloid cells are hematopoietic cells that give rise to monocytes and macrophages in the peripheral blood and tissues. These cells have been shown to be proangiogenic in tumors promoting tumor growth. We also discuss our previously unpublished results on the effect of irradiation on the tumor vasculature including pericyte and basement membrane coverage to the endothelium of tumor blood vessels. We summarize the clinical significance of these studies including the use of MMP-9 inhibitors, administering white blood cell boosters, or planning safety margin of tumor volumes, in order to improve overall clinical benefits in cancer patients treated with radiotherapy.