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Image comparison techniques for use with megavoltage imaging systems

P. M. Evans, DPhil ^* J. Q. Gildersleve, FRCR E. J. Morton, PhD ^*, W. Swindell, DSc ^* R. Coles, MSc ^*, M. Ferraro, BSc ^*,¶ C. Rawlings, DCR Z. R. Xiao, MPhil ^* and J. Dyer, DCR

^* Joint Department of Physics, Royal Marsden Hospital and Institute of Cancer Research, Downs Road, Sutton, Surrey SM2 5PT, UK Department of Radiotherapy, Royal Marsden Hospital and Institute of Cancer Research, Downs Road, Sutton, Surrey SM2 5PT, UK

In this paper we describe software facilities for enabling patient positioning studies using the megavoltage imaging system developed at the Royal Marsden Hospital and Institute of Cancer Research. The study focuses on the use of the system for three purposes: patient position verification (by comparing images taken at treatment simulation with megavoltage images taken at treatment time); reproducibility studies (by analysing a set of megavoltage images); and set-up correction (by adjusting the setup until the megavoltage image obtained at treatment registers with the simulation image). The need is discussed for suitably presented simulator images, a method of determining field boundaries and the possibility of delineating soft-tissue interfaces. Several algorithms of different types, developed specifically for the purpose of intercomparison of planar projection images, are presented. The techniques employed and their usefulness, in both the qualitative and the quantitative sense, are discussed. The results are presented of a phantom and clinical study, to evaluate the rigour and reproducibility of the algorithms. These results indicate that measurements can be made to an accuracy of about 1–2 mm, with a similar value for interobserver reproducibility for the best image comparison techniques available.

Key Words: Radiotherapy imaging • Patient position verification • Image comparison

Present address: Department of Radiation Oncology, University of Michigan, Ann Arbor, MI, USA.

Present address: Department of Medical Physics and Bioengineering, University Hospital of Wales, Cardiff CF4 4XW, UK.

^¶ Present address: Physics Department, Brunei University, Uxbridge, Middlesex UB8 3PH, UK.

Received for publication October 9, 1991. Revision received January 23, 1991. Accepted for publication February 12, 1992.