C.J. Dykstra

A TREATMENT PLANNING INTER-COMPARISON OF PROTON AND INTENSITY MODULATED PHOTON RADIOTHERAPY

PURPOSE A comparative treatment planning study has been undertaken between standard photon delivery techniques,b intensity modulated photon methods and spot scanned protons in order to investigate the merits and limitations of each of these treatment approaches. METHODS Plans for each modality were performed using CT scans and planning information for nine patients with varying indications and lesion sites and the results have been analysed using a variety of dose and volume based parameters. RESULTS Over all cases, it is predicted that the use of protons could lead to a reduction of the total integral dose by a factor three compared to standard photon techniques and a factor two compared to IM photon plans. In addition, in all but one Organ at Risk (OAR) for one case, protons are predicted to reduce both mean OAR dose and the irradiated volume at the 50% mean target dose level compared to both photon methods. However, when considering the volume of an OAR irradiated to 70% or more of the target dose, little difference could be shown between proton and intensity modulated photon plans. On comparing the magnitude of dose hot spots in OARs resulting from the proton and IM photon plans, more variation was observed, and the ranking of the plans was then found to be case and OAR dependent. CONCLUSIONS The use of protons has been found to reduce the medium to low dose load (below about 70% of the target dose) to OARs and all non-target tissues compared to both standard and inversely planned photons, but that the use of intensity modulated photons can result in similar levels of high dose conformation to that afforded by protons. However, the introduction of inverse planning methods for protons is necessary before general conclusions on the relative efficacy of photons and protons can be drawn.

Source distribution dependent scatter correction for PVI

Source distribution dependent scatter correction methods which incorporate different amounts of information about the source position and material distribution are developed and tested. The techniques use image to projection integral transformation, incorporating varying degrees of information on the distribution of scattering material, or convolution subtraction methods, with some information about the scattering material included in one of the convolution methods. To test the techniques, they are applied to data generated by Monte Carlo simulations which use geometric shapes or a voxelized density map to model the scattering material. Source position and material distribution are found to have some effect on scatter correction. An image to projection method which incorporates a density map produces accurate scatter correction but is computationally expensive. Simpler methods, both image to projection and convolution, can also provide effective scatter correction. >

Design of a volume-imaging positron emission tomograph

Progress is reported in several areas of design of a positron volume imaging tomograph. As a means of increasing the volume imaged and the detector packing fraction, a lens system of detector light coupling is considered. A prototype layered scintillator detector demonstrates improved spatial resolution due to a unique Compton rejection capability. The conceptual design of a novel mechanism for measuring scattered radiation during emission scans has been tested by Monte Carlo simulation. The problem of how to use effectively the resulting sampled scattered radiation projections is discussed. >

Characterization of dose distribution in radiation therapy plans

As a method of considering only significant radiation doses to different tissues, the ICRU Report 50 recommends taking the dose given to a significant tissue volume (minimum diameter greater then 15 mm) instead of choosing a single, potentially insignificant, voxel value. In order to find this significant volume, we have adapted an emission imaging analysis method to radiation therapy planning. The resulting method finds and characterizes the dose distribution in the volumes of interest in a way that includes spatial arrangement. The data can be used to signal significant hot or cold volumes in the dose plan and to score the plans based on significant dose to the tissues.

Imaginer: graphical user interface for automated analysis of emission images

Imaginer is a graphical user interface currently being developed for automated analysis of emission images. It will be the first application to implement a new feature extraction method of contiguous volume analysis on an unlimited number of image formats. Its development was prompted by the desire to simplify the steps involved in that analysis and to improve visualization and interpretation of results through a graphical user interface. This paper discusses difficulties that have arisen in generalizing the method of contiguous volume analysis to work with an unlimited number of image formats, as well as in abstracting the visualization techniques to effectively represent all types of data used during analysis. Issues in creating a flexible, intuitive, and extensible user interface for scientific investigation and clinical use are discussed, along with several usability issues that have arisen during development. Prototypes of Imaginer and its software components are described. Designed for ease of use, flexibility, extensibility and portability, Imaginer will enable users to assess the appropriateness of this method of feature extraction for various clinical and research purposes, and to use it in contexts for which it is found to be appropriate.

The effect of material distribution of scatter correction for PVI

Scatter correction methods for positron volume imaging (PVI) are often developed and tested using cylindrical phantoms or Monte Carlo simulations of water cylinders. To determine the effect of more complex shapes and nonhomogeneity, the authors apply scatter correction techniques which incorporate different amounts of information about the material distribution in the Monte Carlo simulated data. The simulations use either geometric shapes or a voxelized density map to model the scattering material. Complex shape and material distribution have been found to have an effect on scatter correction. A method which incorporates a density map produces accurate scatter correction but is computationally expensive. Simpler methods can also provide effective scatter correction.<<ETX>>