The Influence of Stopping Powers upon Dosimetry for Radiation Therapy with Energetic Ions

Abstract

Abstract Following a recent recommendation from the International Atomic Energy Agency (IAEA), air filled ionization chambers (calibrated in terms of absorbed dose to water) should be used for the dosimetry in radiation therapy with fast ions. According to IAEA, the main source of uncertainty in the dose determination is resulting from the stopping power ratio water to air, which is introduced in order to convert the dose measured in the air cavity to the dose to water, which is used as the standard reference medium. We show that our knowledge of suitable stopping power data is very limited, but that the dependence of the stopping power ratio on the mean ionization energies I water and I air is dominating this quantity over a large energy range. We discuss the I-values used in ICRU Reports 37, 49, and 73, and we show how the various choices affect the ratio of stopping powers and the stopping power ratio. In doing so, we also investigate a choice of I-values differing from the ICRU recommendations. The stopping power ratio is calculated as the fluence-weighted average ratio of stopping powers using the Monte Carlo program SHIELD-HIT v2, for primary carbon ions at 50 and 400 MeV/nucleon, including the effect of secondary fragments produced by nuclear reactions. Using a single set of I-values for all primary and secondary particles, we find that the stopping power ratio hardly differs from the simple ratio of stopping powers for C ions over a large energy range. Compared to an earlier result [O. Geithner et al., Phys. Med. Biol.51 (2006) 2279] there are some minor differences, arising from a combination of different I-values from different stopping power tables (ICRU 49 for protons and alphas, ICRU 73 for the heavier ions). For the very low energy region, which is important for dosimetrical measurements close to the Bragg peak, the simple ratio of stopping powers is no longer valid. When using a consistent set of I-values it is shown that the deviation of the stopping power ratio (including nuclear fragmentation) from the recommendation of IAEA is very small at high energies, but increases up to 3% in the stopping region. Concerning future investigations, we think it is worthwhile to reanalyze the various sources of I-values taking into account not only stopping power data but also precision range measurements, since the calculated ranges critically depend on the selected I-value.