Which nanoscale observable predicts biological effect
In plain words
Dose alone does not predict biological damage for different particle types. The question is which microscopic measure of the energy pattern does.
Precise statement
Identify a nanodosimetric quantity, such as the ionization cluster size distribution in a volume of 2 to 10 nm, lineal energy on a 1 micron site, or a DNA-weighted energy-deposition metric, that collapses measured cell-survival and chromosome-aberration data for photons, protons and ions of LET from 0.2 to 200 keV/micron onto a single curve. An answer names the quantity and shows the collapse with residuals.
What would settle it
Joint nanodosimeter and radiobiology measurements on identical beams showing one metric that orders all endpoints consistently.
Status in the literature
Cluster-size based metrics outperform LET in several comparisons, but no single observable has been shown to be sufficient.