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Proceedings Paper

X-ray scatter in quantitative megavoltage computed tomography: implications for adaptive radiation therapy
Author(s): George Hajdok; Jerry J. Battista; Ian A. Cunningham; Tomas Kron
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Paper Abstract

The emergence of helical tomotherapy has provided a unique opportunity to combine aspects of diagnostic computed tomography and radiation treatment. Daily megavoltage computed tomography (MVCT) scans of a patient in the treatment position provide an ideal input for adaptive radiation therapy, whereby the quantitative CT knowledge of a patient from a treatment fraction combined with the knowledge of the therapy dose distribution can be used to alter and correct for the dose delivery in subsequent fractions. In order for adaptive radiotherapy to be successful, the quantitative information from the CT scan must be as accurate as possible in geometric and dosimetric information. One potential impediment to the accuracy of the CT data values is x-ray scatter. In our study, we quantify the magnitude of x-ray scatter in the tomotherapy (fan-beam) MVCT system, based on Monte Carlo simulations of the scatter-to-primary ratio (SPR) as a function of incident x-ray energy, fan-beam slice thickness, patient size, and air gap distance. Furthermore, based on these SPR values, the impact on CT number accuracy is shown, and the implications for adaptive radiotherapy (i.e. dose reconstruction) are discussed. Under conditions common to tomotherapy MVCT scanning, SPR values range from 0.02 to 0.16 (depending on the size of the phantom), and are generally lower than those encountered in diagnostic cone-beam CT and megavoltage portal imaging. These SPR values are sufficient enough to introduce CT number errors as high as 5 HU in soft-tissue and 100 HU in bone. The implication of this inaccuracy for adaptive radiotherapy would be to cause potential dose calculation errors during dose reconstruction and treatment re-planning.

Paper Details

Date Published: 6 May 2004
PDF: 10 pages
Proc. SPIE 5368, Medical Imaging 2004: Physics of Medical Imaging, (6 May 2004); doi: 10.1117/12.535988
Show Author Affiliations
George Hajdok, Robarts Research Institute (Canada)
London Regional Cancer Ctr. (Canada)
Univ. of Western Ontario (Canada)
Jerry J. Battista, London Regional Cancer Ctr. (Canada)
Univ. of Western Ontario (Canada)
Ian A. Cunningham, Robarts Research Institute (Canada)
Univ. of Western Ontario (Canada)
Tomas Kron, London Regional Cancer Ctr. (Canada)
Univ. of Western Ontario (Canada)

Published in SPIE Proceedings Vol. 5368:
Medical Imaging 2004: Physics of Medical Imaging
Martin J. Yaffe; Michael J. Flynn, Editor(s)

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