Welcome back to the RadOnc Smart Review Physics Series! In P2, we explored radioactive decay, where unstable nuclei emit particles like alphas and betas. Now, in Episode P3: Particulate Interactions, we'll follow those particles – and others like electrons and protons accelerated in our machines, plus neutrons – as they travel through matter. How do they lose energy? What factors determine how far they go? And what makes protons deposit their dose so differently?