RAPT Implementation on the GEMSS Grid

Composition of a RAPT Job

The Grid-enabled radiosurgery application uses RAPT as a front end to the EGS Monte Carlo engine to model ionising radiation transport through the head of the patient. The simulation requires:

Definition of patient geometry
Specification and distribution of material types contained within the geometry
Position of beam isocentre (i.e. beam focus)
Beam properties - intensity profile, spectrum
Beam distribution - number of beams and their arrangement
Quality of simulation parameters - total number of photons, interaction types

This data is specified by the contents of numerous text files that are loaded into the EGS solver at startup. The files are not native to EGS but are created for EGS through use of the RAPT input GUI. All input files are stored in a default Input working directory, zipped, encrypted and dispatched to the Grid. The simulation process simulates the dose given to the region of interest by modelling millions of photons, and following their paths, employing information from photon scattering data, to correctly give the photons their positions, angles of deflection, and energies or to absorb them in the tissue as they interact with the atoms of the target. The energy distribution within the geometry equates to the dose distribution of the model. Thus the output from the modelling process is:

3D patient mesh geometry
Accumulated dose at each element of the solution mesh
Flux variance at each element of the solution mesh

The results archive is pulled from the Grid in encrypted form and unzipped on the client within the working Output directory. This data is accessible to the user as a set of text files, but a visualization application has been provided to enable the user to explore the distribution of radiation dose within the treatment volume. The dose data can be represented as contours on specified planes through the volume mesh or visualised as isosurfaces in 3D. The job submission process is outlined in this figure.

RAPT interface

Treatment set-up

RAPT interface

Bubble head set-up

RAPT interface

Computed dose contours

Grid Resources

The primary role of the Grid in this application is to provide sufficient accessible computing power for the radiosurgery simulation to be completed in a fraction of an hour. However, in addition to compute issues, the GEMSS Grid has also implemented a means of costing and billing the user in accordance with the degree of Grid utilisation. Radiosurgery planning is a time critical application and therefore the process of job submission includes specification of the deadline by which the solution is to be completed, accompanied by suitable guarantees from the Grid provider (accommodated by the Quality of Service (QoS) component of the GEMSS middleware). Further requirements are associated with integrity and security of data transfer. These are themes that are common to other Grid applications and are key goals that have been addressed by the GEMSS consortium (e.g. adoption of X.509). In this application, the anticipated end-user is the clinical radiosurgery department, with the RAPT software operated by a technical expert (physicist). The Monte Carlo simulation is used to verify the treatment doses calculated by the more traditional (and more approximate) methods of GammaPlan. In order that the application be accessible, the Grid enabled application has been designed to address user-interface, accessibility, bandwidth, processor scalability and storage issues in such a way that it provides an effective means of solving real world problems in the desired timescale.

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