Our laboratory [11]–[14] demonstrated that tumor stem cell clonogen (SCC) lethality after SDRT, within the clinically-relevant range of 8–25 Gy, is conditionally-linked to an early wave (0.5–6 h) of acid sphingomyelinase (ASMase)-induced apoptosis in the microvascular endothelium of exposed tissue, and that the coupling of these events mediates SDRT tumor cure. This evidence concerns the gene SMPD1 and neoplasm.