Calculated LET spectrum from antiproton beams stopping in water.

INTRODUCTION Antiprotons have been proposed as a potential modality for radiotherapy because the annihilation at the end of range leads to roughly a doubling of physical dose in the Bragg peak region. So far it has been anticipated that the radiobiology of antiproton beams is similar to that of protons in the entry region of the beam, but very different in the annihilation region, due to the expected high-LET components resulting from the annihilation. On closer inspection we find that calculations of dose averaged LET in the entry region may suggest that the RBE of antiprotons in the plateau region could significantly differ from unity, which seems to warrant closer inspection of the radiobiology in this region. MATERIALS AND METHODS Monte Carlo simulations using FLUKA were performed for calculating the entire particle spectrum of a beam of 126 MeV antiprotons hitting a water phantom. RESULTS AND DISCUSSION In the plateau region of the simulated antiproton beam we observe a dose-averaged unrestricted LET of about 4 keV/microm, which is very different from the expected 0.6 keV/microm of an equivalent primary proton beam. Even though the fluence of secondaries is a magnitude less than the fluence of primary particles, the increased stopping power of the secondary particles causes an increase in the dose averaged LET which is expected to result in a RBE different from unity.