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British Journal of Radiology (2006) 79, S87-S98
© 2006 British Institute of Radiology
doi: 10.1259/bjr/60612178
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Dose-guided radiation therapy with megavoltage cone-beam CT

J Chen, PhD O Morin, BSc M Aubin, Eng-MSc M K Bucci, MD C F Chuang, PhD and J Pouliot, PhD

UCSF Comprehensive Cancer Center, Department of Radiation Oncology, University of California San Francisco, 1600 Divisadero Street, Suite H1031, San Francisco, CA 94143, USA


Figure 1
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  		Figure 1. A general view of adaptive radiation therapy. The large grey arrow represents the conventional flow of treatment, and the small arrows indicate the possible points of feedback into the process.

 

Figure 2
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  		Figure 2. Overview of proposed dose reconstruction procedure using MV CBCT imaging and treatment-time portal imaging.

 

Figure 3
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  		Figure 3. Depiction of the geometric calibration of the linear accelerator/electronic portal imaging device (EPID) system for cone beam CT (CBCT) imaging and for dose reconstruction. The result of the calibration is a set of projection matrices (P) that map a point in space (RXYZ) to the projected point on the detector plane (Ruv).

 

Figure 4
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  		Figure 4. Examples of megavoltage cone beam CT(MV CBCT) images at different exposure levels, from 2.9 cGy to 10 cGy.

 

Figure 5
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  		Figure 5. Registration of a patient megavoltage cone beam CT(MV CBCT) (grey) with the kV CT (colour) used for treatment planning. A large difference in the arching of the neck causes a considerable deviation in the spinal cord position.

 

Figure 6
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  		Figure 6. Megavoltage cone beam CT(MV CBCT) intensity as a function of electron density for tissue-equivalent inserts in a CT calibration phantom (pictured in above left).

 

Figure 7
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  		Figure 7. Radial(top row) and axial (bottom row) profiles through the megavoltage cone beam CT (MV CBCT) images of a large cylinder filled with water. The unmodified CBCT (left) exhibits a large cupping artefact as a result of scattered radiation reaching the electronic portal imaging device (EPID). Using a simple 3D cupping model effectively reduces the artefact (right). The radial and axial slices of the MV CBCT images (insets) are displayed using the same windowing level.

 

Figure 8
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  		Figure 8. Comparisons of measured dose profiles(line) in water and calibrated electronic portal imaging device (EPID) profiles (circle with dot) for a 10 cm square field through a Rando head phantom.

 

Figure 9
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  		Figure 9. Comparisons between planned isodose contours calculated using the patient kV CT image and a commercial treatment planning system(left) and reconstructed isodose contours calculated using the megavoltage cone beam CT (MV CBCT), the treatment-time portal images, and an in-house dose reconstruction program (right).

 




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