Synchrotron microbeam radiotherapy in a commercially available treatment planning system

Abstract

In this paper we use a research licensed version of the EclipseTM treatment planning system (TPS) from Varian Medical Systems, Inc. to develop a novel treatment planning environment for synchrotron microbeam radiotherapy (MRT). This research license allows for customised dose calculation algorithms to be integrated with the clinical work-flows in Eclipse that are typical to modern radiotherapy. In this way, all the functionality of the Eclipse TPS familiar to many radiation oncology professionals is retained for application to MRT. Our TPS is designed for the dynamic MRT modality that has been developed for the imaging and medical beamline (IMBL) at the Australian synchrotron. The TPS uses a very simple algorithm for primary dose calculation, and we estimate the peak to valley dose ratio (PVDR) and hence the valley dose from data derived from a separate Monte Carlo simulation. At this stage the algorithm is a fast approximation with only simple radiological path length corrections to account for tissue inhomogeneities. However, the algorithm can highlight to oncology clinicians the strengths and limitations of synchrotron MRT, and furthermore, serves as a proof of concept and a start-point for implementing more advanced dose calculation algorithms. For the treatment itself, the incident synchrotron broad beam is collimated to a 30 mm wide by 1 mm high field which illuminates the MRT collimator, which in turn produces 50 μm wide vertical microbeams separated at 400 μm centre-to-centre. The sample and a mask is then dynamically swept through this array of microbeams, producing a dose of radiation in the sample that is conformal to a patient specific mask aperture. All properties of this irradiation methodology can be configured within the TPS and used for the design of MRT treatments. We present implementation details of our TPS for MRT and describe the hyperbolic relationship between PVDR and depth, and use it for calculating the water equivalent PVDR at different radiological depths.