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Fluence correction factors and stopping power ratios for clinical ion beams.

Lühr, A*; Hansen, D C*; Sobolevsky, N*; Palmans, H; Rossomme, S*; Bassler, N* (2011) Fluence correction factors and stopping power ratios for clinical ion beams. Acta Oncol., 50 (6). pp. 797-805.

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Background: In radiotherapy, the quantity in terms of which treatments are prescribed is absorbed dose to water. However, if positioning accuracy is very critical, e.g. for low particle energies, it could be more convenient to perform the dose measurement in plastic phantoms as recommended in dosimetry protocols. Moreover, graphite calorimeter is developed as a (partially) independent primary standard for dose to water dosimetry. Dose conversion is also required in the comparison of dose distributions from Monte Carlo calculation and pencil beam algorithms.
Material and Methods: In the conversion of absorbed dose to phantom material to absorbed dose to water the water-to-material stopping power ratios (STPR) and the fluence correction factors (FCF) for the full charged particle spectra are needed. We determined STPR as well as FCF for water to graphite, bone, and PMMA as a function of water equivalent depth, zw, with the Monte Carlo code SHIELD-HIT10A. Simulations considering all secondary ions were performed for primary protons, carbon and oxygen ions with a total range of 3, 14.5, and 27 cm as well as for two spread-out Bragg-peaks (SOBP). STPR results are also compared to a recently proposed analytical formula.
Results: The STPR are of the order of 1.022, 1.070, and 1.112 for PMMA, bone, and graphite, respectively. STPR vary only little with depth except close to the total range of the ion and they can be accurately approximated with an analytical formula. The amplitude of the FCF depends on the non-elastic nuclear interactions and it is unity if these interactions are turned off in the simulation. Fluence corrections are of the order of a percent becoming more pronounced for larger depths resulting in dose difference of the order of 5% around 25 cm. The same order of magnitude is observed for SOBP.
Conclusions: We conclude that for ions with small total range (zw-eq < 3cm) dosimetry without applying FCF could in principle be performed in phantoms of materials other than water without a dramatic loss of accuracy. However, in clinical high-energy ion beams with penetration depths zw-eq > 15 cm, where accurate positioning in water is not an issue, absorbed dose measurements should be directly performed in water or accurate values of FCF need to be established.

Item Type: Article
Keywords: Proton, ion, radiotherapy, fluence correction, stopping power, muclear interaction, Monte Carlo, SHIELD-HIT
Subjects: Ionising Radiation
Ionising Radiation > Dosimetry
Identification number/DOI: 10.3109/0284186X.2011.581691
Last Modified: 02 Feb 2018 13:14
URI: http://eprintspublications.npl.co.uk/id/eprint/5187

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