Linear Energy Transfer (LET)

• Energy deposited per unit length of track (keV/µm).

• Low LET: photons, electrons (sparse ionizations).

• High LET: neutrons, α-particles, carbon ions (dense ionizations).

Oxygen Effect and LET

• At high LET, the oxygen effect is reduced or eliminated (less dependence on O₂).

• Makes heavy ions more effective against hypoxic tumors.

Relative Biological Effectiveness (RBE) Variability

• RBE increases with LET up to ~100 keV/µm, then decreases (“overkill” effect).

• RBE depends on endpoint (cell survival vs. late toxicity).

• Clinical challenge: RBE not constant → uncertainty in treatment planning.