In vivo tumor endothelial cell (TEC) apoptosis was measured by double staining with anti-cleaved caspase 3, to detect apoptotic cells, and the endothelial cell surface marker MECA-32, to identify tumor endothelium. Briefly, tumor specimens were removed after SDRT and/or Pazopanib treatment at the indicated time points, fixed in 4% paraformaldehyde, embedded in paraffin, and 5-μm sections were stained. Red-Green–Blue–merged endothelial cell was counted as an apoptotic cell, and a minimum of 500 endothelial cells were evaluated per point.