Purpose Radiotherapy with antiprotons is still being investigated as a possible new beam modality. Antiprotons behave much like protons until they come to rest, where they will annihilate with a target nucleus, thereby releasing additional energy. This can potentially lead to a favourable depth-dose distributions and an increased biological effect in the target region from the production of secondary nuclear fragments with increased LET. Earlier it has been speculated how the target material will affect the depth-dose curve of antiprotons and secondary particle production. Intuitively, the presence of elements with higher Z, may lead to heavier fragments, which in turn may increase the LET and be beneficial in radiotherapy context. Also, it was speculated whether the addition of elements with high thermal neutron cross section to the target material may or may not boost the locally deposited energy from the annihilation process. Materials We have investigated the impact of substituting the target material on the depth-dose distribution of pristine and spread out antiproton beams using the FLUKA Monte Carlo transport program. Classical ICRP targets are compared to water phantoms. In addition, track average unrestricted LET is calculated for all configurations. Finally, we investigate which concentrations of gadolinium and boron are needed in a water target in order to observe a significant change in the antiproton depth-dose distribution. Results Results indicate, that there is no significant change in the depth-dose distribution and average LET when substituting the materials. Adding boron and gadolinium up to concentrations of 1 per 1000 atoms to a water phantom, did not change the depth-dose profile nor the average LET. Conclusions According to our FLUKA calculations, antiproton neutron capture therapy is unlikely to yield any clinical enhancement since unrealistic high concentrations are required in order to observe a beneficial effect. Substituting a water target with the aforementioned ICRP tissues has only minor effect on the depth-dose distribution and the LET-distribution. However, benchmarking experiments are still necessary in order to validate these results.
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10th BIENNIAL ESTRO CONFERENCE ON PHYSICS AND RADIATION TECHNOLOGY FOR CLINICAL RADIOTHERAPY, 2009