Welcome back to the RadOnc Smart Review Physics Series! In E12a, we mastered the formulas for calculating Monitor Units to deliver a specific dose to a point. But radiation therapy treats volumes. How do we define these volumes precisely, accounting for not just the visible tumor but also microscopic spread and uncertainties in setup and motion? In Episode 12b: Treatment Planning Volumes & Concepts, we introduce the standardized language defined by the ICRU (International Commission on Radiation Units and Measurements) for describing target volumes and organs at risk, laying the foundation for evaluating treatment plans.