Effective range extension at the distal falloff
In plain words
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 statement
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.
What would settle it
Serial imaging or functional measurement of tissue change just beyond the planned range in patients, correlated with computed $\mathrm{LET}_d$ and dose.
Status in the literature
Estimates of roughly 1 to 3 mm come from imaging studies, but the value remains endpoint dependent and unvalidated for most sites.
Related problems
- Special case of Variable biological effectiveness of proton beams