Protons

• Deposit energy at a sharp peak (the Bragg peak), sparing distal normal tissue.

• LET is generally low–intermediate → biological effects similar to photons.

Carbon ions (and other heavy ions)

• Higher LET → dense ionization tracks, complex DNA damage.

• Cause more clustered, irreparable double-strand breaks.

• More effective against hypoxic or radioresistant tumors.

Relative Biological Effectiveness (RBE)

• RBE = Dose of reference radiation (X-rays) / Dose of test radiation (e.g., protons) to achieve same biological effect.

• For protons: average RBE ≈ 1.1.

• For carbon ions: RBE can range 2–5, depending on tissue, endpoint, and LET.