This is Episode 18, Part A: Fractionation - The Four Rs of Radiobiology.In our last episode, we discussed the therapeutic ratio and the ongoing quest to predict how tumors and normal tissues will respond to radiation. One of the oldest and still most powerful strategies we use to widen this therapeutic window – to maximize tumor kill while minimizing normal tissue damage – is fractionation. That means instead of giving one massive dose of radiation, we divide it into multiple smaller doses delivered over days or weeks. The success of fractionated radiotherapy isn't just a happy accident; it's deeply rooted in fundamental radiobiological principles. Today, we're diving into the cornerstone of this understanding: the "Four Rs" of radiobiology. These are Repair, Reassortment, Repopulation, and Reoxygenation. Let’s unpack each one.