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By how much is not agreed.","posed_since":"2000","precise":"Quantify the shift in the biologically effective range at the distal edge of a spread-out Bragg peak, where $\\mathrm{LET}_d$ exceeds about $5\\,\\mathrm{keV}/\\mathrm{micron}$, for normal tissues of $\\alpha/\\beta$ near $200\\,\\mathrm{rad}$. Express it as a range extension in mm of water-equivalent thickness as a function of beam energy and modulation width. An answer is a number with uncertainty, per endpoint.","problem_ref":null,"references":"","settled_by":"Serial imaging or functional measurement of tissue change just beyond the planned range in patients, correlated with computed $\\mathrm{LET}_d$ and dose.","status_note":"Estimates of roughly 1 to 3 mm come from imaging studies, but the value remains endpoint dependent and unvalidated for most sites.","title":"Effective range extension at the distal 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