Proton vs carbon ion beams in the definitive radiation treatment of cancer patients.

BACKGROUND AND PURPOSE Relative to X-ray beams, proton [(1)H] and carbon ion [(12)C] beams provide superior distributions due primarily to their finite range. The principal differences are LET, low for (1)H and high for (12)C, and a narrower penumbra of (12)C beams. Were (12)C to yield a higher TCP for a defined NTCP than (1)H therapy, would LET, fractionation or penumbra width be the basis? METHODS Critical factors of physics, radiation biology of (1)H and (12)C ion beams, neutron therapy and selected reports of TCP and NTCP from (1)H and (12)C irradiation of nine tumor categories are reviewed. RESULTS Outcome results are based on low dose per fraction (1)H and high dose per fraction (12)C therapy. Assessment of the role of LET and dose distribution vs dose fractionation is not now feasible. Available data indicate that TCP increases with BED with (1)H and (12)C TCPs overlaps. Frequencies of GIII NTCPs were higher after (1)H than (12)C treatment. CONCLUSIONS Assessment of the efficacy of (1)H vs(12)C therapy is not feasible, principally due to the dose fractionation differences. Further, there is no accepted policy for defining the CTV-GTV margin nor definition of TCP. Overlaps of (1)H and (12)C ion TCPs at defined BED ranges indicate that TCPs are determined in large measure by dose, BED. Late GIII NTCP was higher in (1)H than (12)C patients, indicating LET as a significant factor. We recommend trials of (1)H vs(12)C with one variable, i.e. LET. The resultant TCP vs NTCP relationship will indicate which beam yields higher TCP for a specified NTCP at a defined dose fractionation.