Systemically delivered targeted radioisotopes are being used ever-increasingly for cancer therapy. Compared with conventional external beam radiation therapy, the radiation dose-rate associated with these novel systemic therapies is approximately two orders of magnitude lower in intensity (dose-rate). Hence it is referred to as low dose-rate (LDR) radiotherapy. An awareness of the unique radiobiology associated with LDR-based therapy provides a basis for cautious enthusiasm as these new therapies are explored.