Why narrow beams spare tissue that wide beams destroy

Explain the dose-volume effect in which normal tissue tolerates peak doses of several times $10^{4}\,\mathrm{rad}$ in beams of $25\ \text{to}\ 100\,\mathrm{micron}$ width with spacing of $200\ \text{to}\ 400\,\mathrm{micron}$, while tolerating only a few thousand rad in broad fields. Candidate mechanisms: preferential survival and migration of cells from unirradiated valleys, differential microvascular response, bystander signalling range, and immune recruitment. An answer is a mechanism predicting tolerance as a function of beam width, spacing and valley dose.

Experiments varying beam width and spacing independently at fixed peak and valley dose, with direct observation of cell and vessel survival in valleys and peaks.