Disclosed herein are methods and compositions for treating cancer by increasing radiation-induced damage to cancer without increasing radiation-induced side effects by increasing secretory ASMase levels specifically in tumor endothelium, and inducing apoptosis of tumor endothelial cells by treating the tumor with radiation. ASMase levels are increased in tumor endothelium by administration of a recombinant DNA construct comprising a region coding for a functional ASMase linked to particular transcriptional regulatory sequences that confer tissue-specific expression of ASMase.