This invention provides a combination of a DNA strand/fragment and isotope therapy that is applied to a cancerous tissue to selectively kill cancer cells with minimal negative effects on surrounding non-cancerous cells. The mechanism of cancer cells uptake of large DNA fragment (larger than 30 bp) is endocytosis which does not occur in normal organ cells. We are the first to identify this difference and use the selectivity to deliver isotope P-32 to treat cancer. The DNA fragments with labeled isotope are able to be absorbed by the tumor cells and bind the tumor cell's DNA through recombination, and then the isotope kills the tumor cells. Illustratively, a gene or a DNA fragment is employed as a carrier to deliver the P-32 which can kill cancer cells through radioactive emission. Appropriate doses are provided to patients as part of a medical treatment method.