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

Conserving absorbed energy during accumulation of dose from a 4D geometry

Meeting Abstract

  • D. Tilly - Oncology, Radiology and Clinical Immunology, Uppsala, Sweden
  • C. Sj÷berg - Oncology, Radiology and Clinical Immunology, Uppsala, Sweden
  • N. Tilly - Oncology, Radiology and Clinical Immunology, Uppsala, Sweden
  • E. Traneus - Research, Nucletron Scandinavia AB, Uppsala, Sweden
  • A. Ahnesj÷ - Oncology, Radiology and Clinical Immunology, Uppsala, Sweden

PTCOG 48. Meeting of the Particle Therapy Co-Operative Group. Heidelberg, 28.09.-03.10.2009. DŘsseldorf: German Medical Science GMS Publishing House; 2009. Doc09ptcog203

DOI: 10.3205/09ptcog203, URN: urn:nbn:de:0183-09ptcog2036

Veröffentlicht: 24. September 2009

© 2009 Tilly et al.
Dieser Artikel ist ein Open Access-Artikel und steht unter den Creative Commons Lizenzbedingungen (http://creativecommons.org/licenses/by-nc-nd/3.0/deed.de). Er darf vervielfńltigt, verbreitet und ÷ffentlich zugńnglich gemacht werden, vorausgesetzt dass Autor und Quelle genannt werden.


Gliederung

Text

Purpose: The aim of this work is to account for changes in patient geometry by accumulating all dose to a single fixed geometry, I f , while preserving deposited energy in all moving geometries.

Materials: We consider a single instance I m, from the set of moving geometries where I m is related to I f by deformable registration. It is assumed that the mass is preserved between I m and I f . The energy absorbed by I m should then also be preserved which implies that the dose should not be the transferred quantity since a voxel in I f may consist of several I m voxels with different composition. Dose varies smoother than absorbed energy and may be a preferred subject for an interpolation scheme. The accumulated dose is by choice selected to be presented in I f , therefore the mass in I f should be preserved.

To fulfil these requirements for the dose in the fixed geometry, D f , an energy conservation interpolation method (ECI) is proposed according to
D f (r f ) = D m (r m ) ρ m (r m ) / ρ f (r f ), (1)
where D m (r m ) and ρ m (r m ) are interpolated quantities in I m and the position r f is related to r m by the registration. The energy in I m is approximately preserved and the interpolation is done in the smoother dose domain.

A post energy transfer method (PETM) is also proposed where each voxel in I m is divided into eight subvoxels and the energy absorbed by each subvoxel is transferred to I f using the registration. D f (r f ) is calculated as the sum of transferred energy divided by the mass in I f . The aim of PETM is to more accurately conserve the absorbed energy in I m into I f .

The ECI and PETM methods are applied to a lung case treated with a 6x6 cm2 proton field with a SOBP of 4 cm wed. I m has an artificial displacement of the target volume of ~9 mm compared to I f . The dose is calculated with Geant4 (Agostinelli, et al. Nucl Instrum Meth. 2003;A 506) in a 2x2x3 mm grid.

The transfer of dose from I m to I f is done using ECI and PETM and is compared to an energy transfer method (Siebers, et al. Med Phys. 2008;35) (ETM), and direct dose mapping (Rosu, et al. Med Phys. 2005;32) (DM) in Im.

The ETM can be considered to be accurate but is only applicable to Monte Carlo dose calculation and requires the less commonly existing deformation field from I m to I f . DM is a simple interpolation scheme but is sensitive to redistribution of mass between voxels.

Results: Figure 1 [Fig.á1] shows the CTV DVH in I f for the calculation of the same plan in I m where the dose is transferred to I f using ECI, PETM, DM and ETM. For the data set considered in this work, the ECI and PETM method shows a qualitatively better agreement with the ETM than the DM.

Conclusions: We present two new schemes for transferring a dose distribution from a moving to a fixed geometry. Both methods better preserve the energy absorbed by the moving geometry.