Revision 1

Accepted · first revision · record

Title: Why narrow beams spare tissue that wide beams destroy
Plain: Tissue survives a huge dose when it is delivered in stripes tens of micrometres wide, but not when the same dose covers a broad area. What makes width matter is unclear.
Precise: 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.
Settled by: 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.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: open
Status note: Microbeam sparing has been reproduced repeatedly since the 1990s, with no consensus mechanism as of 2025.
Posed since: 1959