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What makes width matter is unclear.","posed_since":"1959","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.","problem_ref":null,"references":"","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.","status_note":"Microbeam sparing has been reproduced repeatedly since the 1990s, with no consensus mechanism as of 2025.","title":"Why narrow beams spare tissue that wide beams 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