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Radiobiological studies of a high‐energy modulated proton beam utilizing cultured mammalian cells

The modulated, 160‐MeV proton beam produced by the Harvard Cyclotron has been examined in detail for its ability to kill mammalian cells as assayed by colony forming ability. Using two different cell exposure techniques, the characteristics of position and total dose in producing cell death in two aneuploid cell lines selected for their radiobiological relevance have been determined. The parameters which describe the survival curves after proton or 60Co irradiation show no statistical differences, except on the distal portion of the final Bragg peak of the proton beam, where RBE increases to approximately 1.4. This increase in RBE results in extending the cell killing effect of the beam by approximately 2 mm. This effect may be of practical significance in the irradiation of tissue such as the pituitary where the position of the Bragg peak is of great importance. The overall killing efficiency ratio between the modulated high‐energy proton beam and 60Co γ rays was 1.00 ± .01 (standard error).