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

Background and purposeRelative to X-ray beams, proton [1H] and carbon ion [12C] beams provide superior distributions due primarily to their finite range. The principal differences are LET, low for 1H and high for 12C, and a narrower penumbra of 12C beams. Were 12C to yield a higher TCP for a defined NTCP than 1H therapy, would LET, fractionation or penumbra width be the basis?MethodsCritical factors of physics, radiation biology of 1H and 12C ion beams, neutron therapy and selected reports of TCP and NTCP from 1H and 12C irradiation of nine tumor categories are reviewed.ResultsOutcome results are based on low dose per fraction 1H and high dose per fraction 12C 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 1H and 12C TCPs overlaps. Frequencies of GIII NTCPs were higher after 1H than 12C treatment.ConclusionsAssessment of the efficacy of 1H vs12C 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 1H and 12C ion TCPs at defined BED ranges indicate that TCPs are determined in large measure by dose, BED. Late GIII NTCP was higher in 1H than 12C patients, indicating LET as a significant factor. We recommend trials of 1H vs12C 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.