The relative biological effectiveness of antiprotons.

BACKGROUND AND PURPOSE: Aside from the enhancement of physical dose deposited by antiprotons annihilating in tissue-like material compared to protons of the same range a further increase of biological effective dose has been demonstrated. This enhancement can be expressed in an increase of the relative biological effectiveness (RBE) of antiprotons near the end of range. We have performed the first-ever direct measurement of the RBE of antiprotons both at rest and in flight.

MATERIALS AND METHODS: Experimental data were generated on the RBE of an antiproton beam entering a tissue-like target consisting of V79 cells embedded in gelatin with an energy providing a range of approximately 10cm.

RESULTS: The RBE in the entrance channel (the "plateau") is only slightly above the value for a comparable proton beam, and remains low until the proximal edge of the spread-out Bragg peak (SOBP). A steep increase of RBE is seen starting from the onset of the SOBP.

CONCLUSIONS: This paper reports the final results of the experiment AD-4/ACE at CERN on the first-ever direct measurement of RBE of antiprotons and constitutes the first step toward developing treatment plans.