gms | German Medical Science

48th Meeting of the Particle Therapy Co-Operative Group

Particle Therapy Co-Operative Group (PTCOG)

28.09. - 03.10.2009, Heidelberg

Generalization of the Local Effect Model – Biological Treatment Planning from Protons to Carbon Ions

Meeting Abstract

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  • T. Elsässer - GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt
  • M. Durante - GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt
  • M. Scholz - GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt

PTCOG 48. Meeting of the Particle Therapy Co-Operative Group. Heidelberg, 28.09.-03.10.2009. Düsseldorf: German Medical Science GMS Publishing House; 2009. Doc09ptcog054

DOI: 10.3205/09ptcog054, URN: urn:nbn:de:0183-09ptcog0544

Published: September 24, 2009

© 2009 Elsässer et al.
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Outline

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Background: Charged particle cancer therapy exploits the beneficial physical and biological properties of ions for a highly tumour conform irradiation. A major hindrance for a widespread use of this radiotherapy modality is the uncertainty in the biological effectiveness of the ions. The Local Effect Model (LEM) developed at GSI is currently the only model in therapeutic use that takes into account the dependences of the RBE on energy and LET of the primary beam and all its fragments, the tissue under consideration and the dose level. It calculates the RBE of different ions for cell lines or tissues starting from the corresponding experimental/clinical photon data and an amorphous track structure model. In recent years, we focused on the application in carbon ion therapy, where we found a good accuracy. Here, we present a substantial generalization of the LEM while keeping its main ideas.

Materials and methods: According to LEM, the biological response of ions can be determined by a transfer of the photon dose response. Instead of transferring the biological effect based on the local dose, in the novel approach the distribution of double strand breaks (DSBs) is considered, thus additionally taking into account interactions on a few hundred nanometer scale. Specifically, we distribute the DSBs around ion tracks according to the local dose and the photon response curve for the induction of DSBs. Then, we score the number of DSB pairs within a certain distance. We found that the number of DSB pairs relative to the deposited dose is a good quantity to relate different radiation qualities. By application of the (clinical) photon dose response curve, we can exploit the well established linear-quadratic parameters a and b to determine the dose response after particle irradiation. We compare this novel approach to n-vitro and in-vivo data. Finally, we apply the generalized LEM to calculate the biologically effective dose along a spread-out Bragg peak.

Results: By comparison with a large number of in-vitro experiments, we find good agreement between measurements and predictions of the generalized LEM for the entire range of ion species and ion energies relevant for radiotherapy. Also for extended volumes typically irradiated in particle therapy, the model calculations show the same accuracy for protons, helium ions and carbon ions. Especially the therapeutic ratio, namely the ratio of the high RBE in the target volume and the low RBE in the entrance channel, is nicely reproduced for all these particles. It was also applied for an analysis of the RBE in the distal part of the SOBP of proton and helium irradiations.

Conclusions: The generalized LEM facilitates accurate RBE predictions from protons up to oxygen ions with the same approach. Therefore, its range of applicability is largely extended. Importantly, it is suitable to directly compare the RBE-weighted dose for different ion species and will thus help to optimize ion tumor therapy.