Biophysical Reality: Why Whole-Organ Averaging Fails for Internal Emitters
The International Commission on Radiological Protection (ICRP) absorbed dose formula $D = E / m$ calculates energy deposited ($E$) divided by total tissue mass ($m$). When a tiny plutonium or radon alpha particle is inhaled into the lung, its energy is not evenly distributed across 1 kilogram of tissue. Instead, high Linear Energy Transfer (LET ~100–300 keV/µm) helium nuclei deposit their entire kinetic energy in a cylinder of tissue only 40 micrometers long.
Cells situated directly along this track suffer massive, clustered double-strand breaks (DSB) and dense hydroxyl free-radical ($OH\cdot$) plumes from water radiolysis. Surrounding cells receive zero radiation. Averaging this concentrated destruction across the whole organ artificially lowers the computed Gray value, disguising severe local genomic toxicity and carcinogenesis risk.