Welcome back to Rad Onc Smart Review, Radiation Biology edition is Episode 20: Brachytherapy Radiobiology. In our previous discussions, we've explored the principles of fractionation, primarily in the context of external beam radiotherapy, where doses are delivered in discrete, short bursts. Today, we're shifting gears to a fascinating and very different world of radiation delivery: brachytherapy. The term "brachy" comes from the Greek word for "short," and that’s exactly what it is – short-range therapy, where radioactive sources are placed directly into or very close to the tumor. This proximity allows for a highly conformal, high-dose treatment to the tumor while rapidly sparing surrounding normal tissues. But it also introduces unique radiobiological considerations due to the continuous, and often low-dose-rate, exposure. We'll unravel how principles like the LQ model, repair, and dose rate apply in this setting, compare low dose rate (LDR) with high dose rate (HDR) brachytherapy, look at the isotopes commonly used, and discuss the clinical advantages of that characteristic steep dose gradient.