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Simplified material assignment for cone beam computed tomography-based dose calculations of prostate radiotherapy with hip prostheses

Published online by Cambridge University Press:  21 April 2016

Turki Almatani ,
Richard P. Hugtenburg ,
Ryan Lewis ,
Susan Barley  and
Mark Edwards
Turki Almatani*
Affiliation:
College of Medicine, Swansea University, Swansea, Wales, UK
Richard P. Hugtenburg
Affiliation:
College of Medicine, Swansea University, Swansea, Wales, UK Department of Medical Physics and Clinical Engineering, Singleton Hospital, ABM University Health Board, Swansea, Wales, UK
Ryan Lewis
Affiliation:
Department of Medical Physics and Clinical Engineering, Singleton Hospital, ABM University Health Board, Swansea, Wales, UK
Susan Barley
Affiliation:
Oncology Systems Limited, Shrewsbury, UK
Mark Edwards
Affiliation:
Department of Medical Physics and Clinical Engineering, Singleton Hospital, ABM University Health Board, Swansea, Wales, UK
*
Correspondence to: Turki Almatani, College of Medicine, Swansea University, Singleton Park, Swansea SA2 8PP, UK. Tel: 44 7423 414495. E-mail: turkialmatani@gmail.com
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Abstract

Objective

Cone beam computed tomography (CBCT) images contain more scatter than a conventional computed tomography (CT) image and therefore provide inaccurate Hounsfield units (HUs). Consequently, CBCT images cannot be used directly for dose calculation. The aim of this study is to enable dose calculations to be performed with the use of CBCT images taken during radiotherapy and potentially avoid the necessity of re-planning.

Methodology

A phantom and prostate cancer patient with a metallic prosthetic hip replacement were imaged using both CT and CBCT. The multilevel threshold algorithm was used to categorise pixel values in the CBCT images into segments of homogeneous HU. The variation in HU with position in the CBCT images was taken into consideration and the benefit of using a larger number of materials than typically used in previous work has been explored. This segmentation method relies upon the operator dividing the CBCT data into a set of volumes where the variation in the relationship between pixel values and HUs is small. A field-in-field treatment plan was generated from the CT of the phantom. An intensity-modulated radiation therapy plan was generated from CT images of the patient. These plans were then copied to the segmented CBCT datasets with identical settings and the doses were recalculated and compared.

Results

In the phantom study, γ evaluation showed that the percentage of points falling in planning target volume, rectum and bladder with γ<1 (3%/3 mm) was 100%. In the patient study, increasing the number of bins to define the material type from seven materials to eight materials required 50% more operator time to improve the accuracy by 0·01% using pencil beam and collapsed cone and 0·05% when using Monte Carlo algorithms.

Conclusion

The segmentation of CBCT images using the method in this study can be used for dose calculation. For a simple phantom, 2 values of HU were needed to improve dose calculation accuracy. In challenging circumstances such as that of a prostate patient with hip prosthesis, 5 values of HU were found to be needed, giving a reasonable balance between dose accuracy and operator time.

Keywords

ARTCBCT-based dose calculationMC dose calculationmultilevel threshold algorithm
Type
Original Articles
Information
Journal of Radiotherapy in Practice , Volume 15 , Issue 2 , June 2016 , pp. 170 - 180
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Copyright
© Cambridge University Press 2016 

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