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

Abstract 

Background and purpose

Relative 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?

Methods

Critical 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.

Results

Outcome 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.

Conclusions

Assessment of the efficacy of ¹H vs ¹²C 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 vs ¹²C 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.

Keywords: Protons, Carbon ions, TCP, NTCP