Structure Creation Using Limited Point Topology-Evidence in Radiotherapy

Shape Variability Analysis and Image Segmentation in Radiotherapy

Introduction to SCULPTER

The SCULPTER project addresses an urgent problem: to cut unacceptably high intra and inter-observer variability of tumour and OAR delineation in radiotherapy, of the kind recently reported for the breast, prostate, lung, rectum and cervix.

These sites encompass high incidence cancers, which could otherwise benefit from improved effectiveness, comparison and adoption of new cancer treatment techniques. SCULPTER will create and use a radically new deformable clinical target volume paradigm supported by real-time 3D computation and visualisation. The proposal focuses on a delineation approach that combines a range of expert sources of information and presents these to the radiotherapy planning process in a dynamic and interactive way. Results will emerge sequentially to build a unique body of tumour and organ knowledge and an evolving delineation tool for parallel clinical investigations and potentially wide ranging exploitation.

With the above points in mind the SCULPTER proposal has the following specific aims.

  • To develop novel 3Dt structure creation consistent with the demands of international protocols.
  • To provide evidence based point definition with local and global control in 3D.
  • To develop self-mapping, expert-topology primitives for diverse anatomical sites using RBFs.
  • To capture and exploit expert range and confidence indices for evidence based points.
  • To develop topology transfer between volumes visible in dynamic data sets.
  • To analyse the evidence basis and suggest objective criteria for standardised clinical practice.
  • Reduce variability of delineation when using large scale static and dynamic imaging studies.

How does SCULPTER work

Sculpter works by interacting with 2D slices of the image dataset (CT/MR). The first and last slice on which the bio-structure (organ to delineate) is visible. The operator then defines a number of initial (evidence based) points that define the surface of the bio-structure. Normally three or four slices are picked and often no more than 8 points are needed on each slice. A mapping process is then used to "shrink wrap" a primitive object over the defined points.

Results are extremely good for the simple GTV/CTV/PTV structures in the pelvis e.g. prostate, because the organ's principal axes are appropriately aligned. Investigation of bladder and rectum suggests itself. SCULPTER will be extended to complex structures with very angular topology, such as the pelvis bones. The femoral heads are often irradiated in prostate treatments and so form potentially mobile but non-deformable OAR. The pelvis also forms the cage-like constraints essential for modelling the motion and deformation patterns of the prostate-bladder-rectum.

In figure 2, (below) the left hand side shows two views of a GTV defined by an expert observer (data kindly provided by William Beaumont Hospital, Detroit). The right hand side shows the same two views of a surface defined using SCULPTER with < 40 evidence based points. The intra observer variability shown by the crinkle cut nature of the surface defined by the expert user is not present in the surface defined using the SCULPTER tool.

In figures 3a (below) and 3b (below) the white boxes indicate the position of the evidence based points.

These have been defined on only five slices. The green contour shows the results of SCULPTER's mapping process. The blue contour shows the expert observers definition of the bio-structure. In figure 3a the fifth slice shows the expert user has decided the posterior border is concave. This is inconsistent with the adjacent slices as suggested by the contour calculated by the SCULPTER tool.

This is just one example of how definition of a minimal set of points for which the observer has a high degree of confidence can produce a more consistent bio-structure definition than if the observer is forced to make a decision for hundreds of points or describe the structure using a free-hand graphics tool.

Research Systems International (Kodak)

The SCULPTER tool is being written in IDL (Interactive Data Language, RSI). RSI have taken great interest in this work and are supporting the research of the developing technologies section for its development.

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