SMPD1 and neoplasm: Radiosensitizing Ad5H2E-PPE1(3x)-ASMase or Ad5H2E-mVEGFR2-ASMase gene therapy, which does not affect the radiosensitivity of normal tissues, might hypothetically enable SDRT dose de-escalation to levels below the inherent threshold for tumor-adjacent normal tissue toxicity, while still affording curative radiation dosing.