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Track structure test on therapeutic carbon ion radiation using the PTB nanodosimeter

23.12.2021

As part of a joint project with the Heidelberg Ion‑Beam Therapy Center (HIT), the track structure of carbon ions of therapeutic energy after penetration of a layer of simulated tissue was investigated in a first beam time using the PTB nanodosimeter. For this purpose, a PMMA absorber of 124 mm thickness, which simulated the layer of tissue, was irradiated with carbon ions of an energy of 3.5 GeV (292 MeV/u). After entering the vacuum chamber of the nanodosimeter, the energy of the carbon ions was reduced to approximately 1 GeV (83 MeV/u). The generated mixed radiation field contained not only the primary carbon ions but also all of the secondaries (ions and electrons) and fragments generated during the penetration of the simulated tissue. The figure shows the mean number of ionizations M1(d) generated in the target volume as a function of the distance d of the particle track from the target volume for the generated mixed radiation field and, for comparison, for a monoenergetic carbon ion beam of 150 MeV (12.5 MeV/u). The target gas in both cases is 1.2 mbar C3H8. The ratio of the mean number of ionizations at d = 0 mm (that is to say in the center of the target volume) is an initial indicator for additional ionizations due to secondaries. In a first approximation, the ratio of M1(0) was expected to approximately match the ratio of the stopping powers of both ion energies S(1 GeV) / S(150 MeV) ≅ 0.21; the ratio actually measured was M1(0 mm, 1 GeV) / M1(0 mm, 150 MeV) ≅ 0.28. The mean number of ionizations in the target volume is larger than expected for the mixed radiation field with carbon ions of 1 GeV , which indicates additional ionizations due to secondaries. The difference in the decrease of M1(d) with increasing distance d for both ion energies is another indicator. While M1(d) constantly decreases for the monoenergetic ion beam with 150 MeV with increasing distance d, M1(d) becomes almost constant with increasing distance d for the mixed radiation field with carbon ions of 1 GeV, indicating a background of additional ionizations due to secondaries.

After completion of the work in progress, the “Nanodosimetry” field will be discontinued at PTB.

Figure: Mean number M1(d) of ionizations generated in the target volume for the mixed radiation field with carbon ions of 1 GeV and for a monoenergetic carbon ion beam of 150 MeV in 1.2 mbar C3H8.

Contact

Opens local program for sending emailG. Hilgers, Department 6.3, Working Group 6.36