Camptothecin-based compounds are useful for treating a neoplasm in mammalian subjects by administering such compound to the subjects in combination with radiotherapy, i.e., the treatment of tumors with radioactive substances or radiation from a source external to the subject. Camptothecin-based compounds are modified by positioning at least one electron-affinic group around the camptothecin structure to enhance their value in combination with radiotherapy. New Camptothecin-based compounds are disclosed that are useful for treating cancer by administering the novel compounds alone or in combination with radiotherapy.