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Accepted · first revision · record

Title: Effective range extension at the distal falloff
Plain: The last few millimetres of a proton beam are the most damaging per unit dose, which effectively pushes the treated region deeper than planned. By how much is not agreed.
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.
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.
Kind: well-posed
Answer type: value
References: (empty)
Literature status: partially-resolved
Status note: Estimates of roughly 1 to 3 mm come from imaging studies, but the value remains endpoint dependent and unvalidated for most sites.
Posed since: 2000