Roy and Niels

Roy and Niels

Sunday, November 7, 2010

One Stopping Power Library to Rule Them All: libdEdx

Stopping powers describe the energy loss of charged particles traversing matter. In particle therapy, stopping powers are an essential ingredient for calculating the dose distribution of ion beams.

A direct way of calculating the stopping power is using the Bethe equation. However, this equation requires a good knowledge of the ionization potential (in particle therapy jargon: "the I-value") and for compounds this value is pretty ill researched. The I-value can be found experimentally, but there are only few experimental data available for compounds relevant for particle therapy, such as various tissue types and even for water. Our postdoc. Armin L├╝hr has recently submitted a paper where he takes a closer look on these issues which is available on the archive: http://arxiv.org/abs/1010.5356

Several programs provide stopping powers, either as analytic calculations or inter/extrapolated experimental data. The list below may be incomplete, but these are the codes I have been in touch with so far:
  • ESTAR: for electrons. (Fortran77)
  • PSTAR: for protons. (Fortran77)
  • ASTAR: for alpha particles (Fortran77)
  • MSTAR: for alpha particles and heavier ions up to Z=18. Basically MSTAR is scaling ASTAR data, where the scaling factors are fitted to experimental data. (Fortran77)
  • ATIMA: code developed by GSI. (Java wrapping a Fortran core)
  • TRIM/SRIM: Application which can simulate any ion on any compound.
And various tables provided by the ICRU:
  • ICRU 49: proton data and alphas, equivalent with PSTAR/ASTAR.
  • ICRU 73: ions heavier than helium, old version
  • ICRU 73: new version after errata, quite similar to MSTAR, even if calculated differently.
and in summary they don't agree too well at lower energies (say below 10 MeV/u).

Nonetheless: from an application developer point of view this leaves you in a dilemma of choosing the proper code for your application, and most likely we will see more updates, since the issues are not solved yet with the erratum of the ICRU73.

In addition, from a technical point of view most stopping power codes are not written in convenient ways which allows a clean integration into your application. This is a common thing which happens when physicists develop code: the code will always be optimized to work nicely on the developers computer and fulfill their specific needs. Platform portability, an API, and install scripts following de-facto standards are mostly absent.

This is why we started the development of libdEdx.

libdEdx is meant to be a platform independent stopping power library which contains multiple tables to choose from, and can be extended with additional algorithms/tables. Currently libdEdx includes ESTAR,PSTAR,ASTAR,MSTAR and ICRU73 data as well as an implementation of the Bethe-equation akin to that found in SHIELD-HIT.
The material database is based on the extensive ICRU set found in the ESTAR table, and using Braggs additivity rule libdEdx can extend the MSTAR/ICRU73 data set to cover the ESTAR material list.

libdEdx comes along with an installer based on cmake, and the first official release should be able to run on Windoze just as well as Unix/Posix/Linux compliant platforms.

Surely, Geant4 also offers a range of stopping power tables to choose from, but you do not want to install entire Geant4 just to access the stopping power values.

Our bachelor student Jakob Toftegaard (who very conveniently has a background in physics and computer science) did most of the coding. A very early experimental pre-release is available on sourceforge, but if you want to experiment with it and even contribute you can also grab the most recent version from the SVN repository.

P.S.: Armin and David will present libdEdx on the MC2010 conference in Stockholm on the 9th to the 12th November. Posters can be found here.

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