Which ion species is optimal for therapy
In plain words
Beyond protons, beams of helium, carbon, oxygen and neon are possible, each with a different balance of sharpness, biological effect and unwanted fragments. Which one is best for which tumour is unsettled.
Precise statement
Rank ion species by atomic number Z from 1 to 10 against the combined criteria of lateral and distal penumbra, dose-averaged LET in target versus normal tissue, fragmentation tail dose beyond the Bragg peak, and oxygen enhancement ratio in hypoxic targets. An answer is a classification of tumour and site classes to the preferred ion, with the physics criteria that decide each case.
What would settle it
Comparative planning plus measured fragmentation yields and in vivo effectiveness for each species on a common endpoint set.
Status in the literature
Unverified note
Helium returned to clinical use at HIT Heidelberg in 2021, after the Berkeley helium programme of 1957 to 1992; oxygen and multi-ion schemes were under study in 2025 without a settled ranking.