First published online August 17, 2006
British Journal of Radiology (2006) 79, 918-921
© 2006 British Institute of Radiology
doi: 10.1259/bjr/19559792
The use of megavoltage cone-beam CT to complement CT for target definition in pelvic radiotherapy in the presence of hip replacement
M Aubin, MSc,
O Morin, BSc,
J Chen, PhD,
A Gillis, MD,
B Pickett, MSc,
J F Aubry, MSc,
C Akazawa, CMD,
J Speight, MD,
M Roach, III, MD and
J Pouliot, PhD
Department of Radiation Oncology, University of California San Francisco, Comprehensive Cancer Center, San Francisco, CA 94143, USA
Correspondence: Dr Jean Pouliot, Department of Radiation Oncology, University of California San Francisco, 1600 Divisadero Street, Suite H1031, San Francisco, CA 94941-1708, USA. E-mail: pouliot{at}radonc17.ucsf.edu.
 |
Abstract
|
|---|
In Europe and the USA combined, over half a million people had a hip joint replaced in 2005, contributing to the increasing number of radiotherapy patients with metallic hip prostheses. The treatment plan for external beam radiation therapy is based on the delineation of the anatomy in the planning CT scan. When implanted objects of high atomic number (Z) material are present, however, severe image artefacts are generated in conventional CT, strongly hindering the ability to delineate some organs. This is particularly the case for the planning of prostate patients with hip prostheses. This short communication presents the use of a new imaging modality, megavoltage cone-beam CT, to complement the regular CT for target definition of prostate cancer treatment of patients with hip replacements.
|
Introduction
|
|---|
Treatment planning for external beam radiation therapy is based on the delineation of the anatomy visualized in the planning CT scan. However, image artefacts caused by the presence of a hip replacement often render CT images useless for prostate delineation (Figure 1
, left) and preclude precise dose calculation.
View larger version (117K):
[in this window]
[in a new window]
|
Figure 1. Comparison of a conventional CT(left) and megavoltage cone beam CT (MV CBCT) (right). (A) Axial and (B) coronal views are shown for a unilateral hip replacement. (C) Axial view for a bilateral hip replacement.
|
|
It was recently suggested that CT-MR image registration could facilitate target definition for a prostate patient with hip replacements [1]. Effectively, MR images provide an accurate definition of the clinical target volume (CTV) and better visualization of normal structures. A number of factors, however, were found to affect image quality and/or accuracy of target definition. The standard MR couch, different from a CT or linac treatment couch, might result in different patient position, and the presence of the metallic implants may create significant distortion. Finally, in the presence of a hip replacement, neither the regular CT nor the MR can provide accurate electron density information for heterogeneous dose calculation. Promising artefact reduction techniques on regular CT are being developed to minimize the impact of streaking artefacts [2]. At that time, they may require manual image post-processing and most CT scanners available in radiation oncology departments are not equipped with these features.
We report on the use of megavoltage cone-beam CT (MV CBCT) to complement the conventional CT for target organ definition in seven patients treated for prostate cancer with external beam radiation therapy. In this study, we exploited the predominantly Compton scattering of high-energy photons delivered in MV CBCT systems to obtain 3D images of the anatomy in the presence of unilateral or bilateral hip replacements and complement the planning CT during the target delineation process.
|
Method
|
|---|
A cone-beam CT image is reconstructed from a large set of projection images acquired at various angles around a patient in a process similar to that of conventional CT. In cone-beam CT, a two-dimensional (2D) array of detectors, in our case a portal imager attached to the linear accelerator (linac), is used to reconstruct a three-dimensional (3D) image. For MV CBCT, the 6 MV treatment beam of the accelerator, containing photons primarily in the MeV range, is used for the imaging.
A MV CBCT system [3, 4] integrated onto an ONCOR clinical Linac (Siemens Oncology Care Systems, Concord, CA) was used to acquire 3D images in treatment position for seven prostate patients with unilateral or bilateral hip prostheses. MV CBCT acquisitions were performed by rotating the linac in a continuous 200° arc (270° to 110°, clockwise) acquiring one portal image for each degree. Because MV CBCT uses the treatment beam, the treatment planning system (Pinnacle, Phillips, Best, The Netherlands) was used to evaluate the dose delivered during an MV CBCT acquisition. For a typical acquisition procedure, the dose at the isocentre was 0.05 Gy and the acquisition lasted 45 s. The reconstructed image, a typical 256x256x274 volume (27.4 cmx27.4 cmx27.4 cm), was available less than 2 min after the acquisition of the first portal image. A first order scatter correction was applied to facilitate the adjustment of window/levels. For each patient, the MV CBCT images were imported into the treatment planning system and registered with the original planning CT using bony anatomy contoured on each image set. The target volumes and organs at risk for prostate treatment were contoured using both the CT and the MV CBCT for single hip replacement, and using only the MV CBCT for bi-lateral hip prostheses.
|
Results
|
|---|
The MV CBCT images could be used to visualize clearly the hip prosthesis and bony anatomy and provide sufficient soft-tissue contrast to help delineate the prostate, bladder and rectum. The artefacts on the regular CT obscure the border between the prostate and anterior wall of the rectum (Figure 1A
, left) and the interface between the prostate base and the bladder neck (Figure 1B, left). The MV CBCT images were particularly useful to help delineate these structures as well as the lateral extension of the prostate in the axial plane, the seminal vesicles and the lymph nodes. Also, normal anatomy such as pelvic bones, penile bulb, bladder, femoral heads, rectum and small bowel can be delineated with higher accuracy as well.
An example of organ segmentation is presented in Figure 2. The change of shape of the prostate (red), bladder (yellow) and rectum (blue) between the CT (left) and the MV CBCT (right) can be easily observed. In this study, the time intervals between the CT and the MV CBCT ranged from a few hours to 1 week. By itself, this can explain the change in rectum and bladder volumes due to different fillings. For the six patients with a single hip replacement, the posterior limit of the prostate was found to be more anterior, and therefore the prostate volumes contoured with the help of MV CBCT were generally smaller than the volumes that would have been estimated using only the regular CT containing severe artefacts. These smaller prostate volumes may prevent overdosage of the rectum. Target delineation for the patient with bilateral hip prostheses was entirely performed using the MV CBCT, since the relevant organs were totally obscured due to the severity of the artefacts on the conventional CT.
View larger version (147K):
[in this window]
[in a new window]
|
Figure 2. Segmentation of the bladder(yellow), prostate (red) and rectum (bule) using the conventional CT (left) and the megavoltage cone beam CT (MV CBCT) (right) shown on the (A) axial, (B) sagittal and (C) coronal views.
|
|
|
Discussion
|
|---|
Image artefacts (Figures 1 and 2, left) caused by the presence of hip replacements render regular CT images difficult to use for prostate delineation. In contrast, the presence of high-Z material has relatively little impact on the image quality of MV CBCT.
Tests performed on phantoms [5] showed that the presence of a metallic object strongly impacts on Hounsfield numbers (up to 70% error) of a conventional CT image and therefore disturbs the electron density even far away from the object, making CT inaccurate for dose calculation. For this reason, CT treatment plans of patients with hip replacements are generally produced without density correction. Similar tests performed with MV CBCT on a phantom with and without a metallic object demonstrated that Hounsfield numbers remain unchanged (within 3%) in the presence of metallic objects, allowing for significantly more accurate dose calculation. Thus, the next step after using MV CBCT for image segmentation will be to use MV CBCT for dose calculation. Research to develop calibration procedures to use MV CBCT for dose calculations is being performed. There is also ongoing technical development to increase the field of view of the current version of MV CBCT (27 cmx27 cm) to encompass the entire pelvic region.
The possibility of considering dose escalation protocols depends on the ability to identify the prostate volume and critical structures for treatment planning, and the accuracy of the dose calculation. Because of degradation of image quality in the presence of high-Z material, treatment planning and dose calculations are limited in these settings. Consequently, patients with hip prostheses may not be candidates for advanced treatment planning like intensity-modulated radiotherapy (IMRT). With the advance capability of using MV CBCT in the treatment planning software, it is now possible to deliver accurately higher doses of radiation to the prostate in patients with hip prostheses. While MV CBCT acquisition dose is two to three times higher than a conventional CT, it represents only a very small fraction (<0.1%) of the treatment dose. With dose escalation these patients may benefit from a decreased risk of treatment failure.
MV CBCT in the presence of high-Z material may improve treatment planning, allowing patients with pelvic malignancies and hip prostheses the most advanced form of radiation therapy.
|
Conclusion
|
|---|
MV CBCT provides 3D anatomical information of the patient in the treatment position, even in the presence of "CT non-compatible" objects. MV CBCT registered with the planning CT can complement missing information and facilitate segmentation for planning purposes when hip prostheses are present.
Received for publication January 20, 2006.
Revision received June 22, 2006.
Accepted for publication July 4, 2006.
|
References
|
|---|
- Charnley N, Morgan A, Thomas E, Wilson S, Bacon S, Wilson D, et al. The use of CT-MR image registration to define target volumes in pelvic radiotherapy in the presence of bilateral hip replacements. Br J Radiol 2005;78:634–6.[Abstract/Free Full Text]
- Yazdia M, Gingras L, Beaulieu L. An adaptive approach to metal artifact reduction in helical computed tomography for radiation therapy treatment planning: experimental and clinical studies. Int J Radiat Oncol Biol Phys 2005;62:1224–31.[CrossRef][Medline]
- Pouliot J, Bani-Hashemi A, Chen J, et al. Low-dose megavoltage cone-beam CT for radiation therapy. Int J Radiat Oncol Biol Phys 2005;61:238–46.
- Morin O, Gillis A, Chen J, Aubin M, Bucci MK, Pouliot J. Megavoltage cone-beam CT: system description and IGRT clinical applications. Special issue on image-guided radiation therapy (IGRT). Med Dosim 2006;31:51–61.[CrossRef][Medline]
- Aubin M, Morin O, Bucci K, Chan A, Chen J, Ghelmansarai F, et al. Megavoltage conebeam CT to complement prostate planning CT in presence of hip prosthesis. ESTRO Annual Meeting (abstract), Lisbon, 2005
This article has been cited by other articles:
|
|
|
|
 
J. L. Speight and M. Roach III
Advances in the Treatment of Localized Prostate Cancer: The Role of Anatomic and Functional Imaging in Men Managed With Radiotherapy
J. Clin. Oncol.,
March 10, 2007;
25(8):
987 - 995.
[Abstract]
[Full Text]
[PDF]
|
|
|
Top
Abstract
Introduction
