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Kilovoltage imaging is more suitable than megavoltage imaging for guiding radiation therapy

2007; Wiley; Volume: 34; Issue: 12 Linguagem: Inglês

10.1118/1.2799489

2473-4209

Lei Xing, Jenghwa Chang, Colin G. Orton,

Radiation Therapy and Dosimetry

Medical PhysicsVolume 34, Issue 12 p. 4563-4566 Point/CounterpointFree Access Kilovoltage imaging is more suitable than megavoltage imaging for guiding radiation therapy Lei Xing Ph.D., Lei Xing Ph.D. Department of Radiation Oncology, Stanford University School of Medicine, Stanford, California 94305-5847 (Tel: 650-498-7896, E-mail: lei@reyes.stanford.edu)Search for more papers by this authorJenghwa Chang Ph.D., Jenghwa Chang Ph.D. Medical Physics Department, Memorial Sloan-Kettering Cancer Center, New York, New York 10021 (Tel: 212-639-6036, E-mail: changj@mskcc.org)Search for more papers by this authorColin G. Orton Ph.D., Colin G. Orton Ph.D. ModeratorSearch for more papers by this author Lei Xing Ph.D., Lei Xing Ph.D. Department of Radiation Oncology, Stanford University School of Medicine, Stanford, California 94305-5847 (Tel: 650-498-7896, E-mail: lei@reyes.stanford.edu)Search for more papers by this authorJenghwa Chang Ph.D., Jenghwa Chang Ph.D. Medical Physics Department, Memorial Sloan-Kettering Cancer Center, New York, New York 10021 (Tel: 212-639-6036, E-mail: changj@mskcc.org)Search for more papers by this authorColin G. Orton Ph.D., Colin G. Orton Ph.D. ModeratorSearch for more papers by this author First published: 09 November 2007 https://doi.org/10.1118/1.2799489Citations: 8AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat OVERVIEW Image-guided radiation therapy involves the use of imaging to delineate the structures of interest, to plan appropriate treatment fields, and to ensure that treatments are administered as planned. It is becoming increasingly common to utilize in-room imaging and localization systems for these purposes and, most often, these involve use of either MV x rays directly from the linear accelerators used to treat the patients, or kV x rays from auxiliary x-ray machines. It could be argued that the kV option is superior because kV imaging is bound to provide better quality images. On the other hand, some might argue that MV systems are preferable because, among other things, they use identical geometry to that used for treatment and hence provide more accurate geometrical information. There are many arguments for and against each of these modalities and this is the topic of this month's Point/Counterpoint. Arguing for the Proposition is Lei Xing, Ph.D. Dr. Xing earned his Ph.D. in physics from the Johns Hopkins University and obtained his medical physics training from the University of Chicago. He is currently an Associate Professor and Chief of Research in the Department of Radiation Oncology at Stanford University. His major areas of interest are image-guided and adaptive radiation therapy, treatment planning, and dose optimization, and the application of PET/CT and other emerging biological imaging modalities in radiation oncology. He currently serves on the AAPM workgroups on IMRT and Molecular Imaging in Radiation Oncology, and the ASTRO Collaborative Committee on IGRT, and has served on the Board of Editors of Medical Physics. He is certified by the American Board of Radiology in Therapeutic Radiological Physics. Arguing against the Proposition is Jenghwa Chang, Ph.D. Dr. Chang obtained an MS degree in Computer & Information Science, and MS and Ph.D. degrees in Electrical Engineering, all from the Polytechnic University of New York, Brooklyn, NY. He is board certified by the ABR in Therapeutic Radiological Physics and the ABMP in Radiation Oncology Physics and is currently an Associate Member of the Medical Physics Department, Memorial Sloan-Kettering Cancer Center, New York, NY. His major research interests are intensity modulated radiotherapy, respiratory gating, and various forms of imaging for radiotherapy including cone-beam CT, portal imaging, functional MRI, and magnetic resonance spectroscopy. FOR THE PROPOSITION: Lei Xing, Ph.D. Opening Statement When talking about onboard imaging for therapeutic guidance, one has a number of options: the best imaging plus the best therapy, so-so imaging plus the best therapy, not-so-good imaging plus the best therapy,, so-so imaging plus so-so therapy. The issue here is not which modality is better for IGRT, rather where we should settle in this long list of choices. The first thoughts one may have are what defines the best imaging technology and what is the best therapy machine? Instead of going through a lengthy description, I simply suggest visits to the exhibit halls at the RSNA and AAPM annual meetings. Current state-of-the-art imaging devices are displayed each year at the RSNA meeting, but finding an MV CT scanner there is improbable, much like trying to find a Co-60 machine on the AAPM exhibition floor. If still unconvinced, perhaps comparing MV and kV films on a light box in your clinic would help, whence it should become transparent that, compared to its counterpart, kV x rays are better in providing image guidance. All in all, it is written on the wall as well as in physics books that the tissue needs to absorb a sufficient number of photons to be seen, and a structure needs to absorb a significantly different number of photons from its neighbors to be visualized. Compton scattering simply cannot beat the photoelectric effect in this regard. Poor soft tissue contrast alone is sufficient for the MV imaging system to shy away. But there is more. MV imaging delivers more radiation dose to the patient. IGRT is rapidly moving toward adaptive replanning1–3 and/or real-time (or at least reasonably frequent) feedback of anatomical information during the beam delivery process,4,5 I simply cannot see how MV alone can meet the increased demand for frequent imaging. Image truncation in current MV volumetric imaging may present another problem since volumetric data are required for dose reconstruction6,7 and adaptive replanning. Not every system in the real world is made of a combination of the greatest things for various reasons. Each choice often comes with a different price tag and other pros and cons, and the optimal choice is a result of balancing different competing factors. In the issue debated here, the equation in front of us is actually not that complicated. On one hand, one has a kV imaging system mounted on the LINAC gantry, which is reasonably affordable and provides superior soft tissue contrast and full 3D anatomic information. On the other hand, there is an MV-based approach, which uses the treatment beam for volumetric imaging and has a long list of serious compromises. While the configuration of the MV system is simpler, the sacrifice we have to make in image quality and patient radiation dose is simply too large to justify the routine use of this modality in the clinic. Let me conclude by saying that an ideal IGRT solution should be composed of the best imaging plus the best therapy machine, and an onboard kV imaging system fits this philosophy well. No seeing, no hitting. Futuristically, I also see more hope for kV image-guidance since much more research is being devoted to kV x-ray imaging, which may further enhance the performance of kV devices. Some day, we may see onboard phase-contrast CT, or inverse-geometry CT, or even multiplexing nanotube-based CT in radiation oncology clinics. In the spirit of "the best plus the best," I am also glad to see that the hybrid of MRI and linear accelerator is on the horizon. AGAINST THE PROPOSITION: Jenghwa Chang, Ph.D. Opening Statement Multiple factors, including clinical usefulness, technical complexity, and cost, must be considered to determine the best combination of technologies for IGRT. It is a general conception (or misconception) that megavoltage MV imaging can never compete with kV imaging because image quality and dose are orders-of-magnitude worse. Recent advances in MV cone-beam computed tomography (CBCT), however, suggest that MV imaging may in fact be more suitable than kV imaging for many IGRT applications. Recent improvement in MV imagers has enabled them to produce clinically useful images with acceptable imaging dose.8–10 Although the quality of MV images is generally inferior to kV images, the difference narrows as the patient thickness increases,10 and almost vanishes for CBCT due to the much higher scatter for kV CBCT compared to MV CBCT.11 The imaging dose of MV CT/CBCT is slightly higher but the difference is now less than a factor of 2 and MV imaging dose can be readily included in the treatment planning process to minimize its adverse effects.12 MV imaging has been successfully used for two-dimensional IGRT applications including tracking of implanted markers or bony anatomy for target localization and setup correction13 with an accuracy comparable to kV imaging.14 Observing intrafractional motion in the beam direction on an MV imager makes more clinical sense than on a kV imager perpendicular to the beam. MV CT/CBCT systems have also been successfully applied to three-dimensional setup and verification with excellent accuracy.8–10 MV CT/CBCT imaging is superior in its linear relationship between relative electron density and CT number for dose calculation.9 Because artifacts due to metal objects and beam hardening are less critical for MV sources, MV CBCT scans have been acquired to complement diagnostic CT scans when these artifacts are severe.9 Technical complexity is a major concern for clinical implementation of IGRT because more complex systems demand more quality assurance (QA) and are more susceptible to errors. MV imaging using the MV source and imager on a linear accelerator is technically simple and robust, and has a lower cost for hardware than kV imaging that requires an extra detector and x-ray source. The QA for MV CBCT systems is basically the same as that for an MV imager, while the more complex QA for kV CBCT systems15 demands additional manpower and therefore costs more. Yoo et al.15 pointed out that the most critical QA of an IGRT system involves maintaining geometric accuracy for patient repositioning. In this regard, kV IGRT systems are overly complex because three isocenters (MV source, kV source, and laser) need to be identified and constantly checked.15 MV IGRT systems with coincident treatment and imaging isocenters, on the other hand, are true "what you see is what you treat" systems that can do without the complex, error-prone calculation of systematic shifts between isocenters. In conclusion, clinical usefulness of MV imaging is comparable or in some cases even superior to kV imaging for IGRT. There are no reasons to buy a more expensive and error-prone system if a significantly cheaper and notably less technically challenging alternative is available. Rebuttal: Lei Xing, Ph.D. In order to debate whether kV or MV imaging is more suitable for IGRT, the clinical goals must be clear. If IGRT is all about locating static metallic fiducials and bony structures, nothing discussed here matters, since even MV projection images will do the job. In reality, the drive to "see" the soft tissue and tumor target, mostly in real time, promotes the development of on-treatment imaging technology. MV image quality is limited by the physics, whereas the problems with kV imaging pointed out by Dr. Chang can be addressed. For example, metal artifacts with kV CT imaging can be removed with appropriate reconstruction algorithms. With the ongoing research in the field, one has every reason to believe that the quality of kV imaging, which is already superior to the competing MV technology, will be further improved. Indeed, with the use of primary modulation with spatial variant attenuation materials for scatter removal, a significant improvement in kV CBCT image quality has been demonstrated.16 To be fair, in-line MV imaging is useful in the localization of implanted metallic fiducials. However, it generally requires a larger marker size and, in a realistic clinical situation, the auto-detection of the fiducials is more difficult, especially when a high temporal resolution is required. MV imaging alone is hardly a solution for real-time fiducial tracking and a simultaneous kV beam seems to be desirable.17 Yes, in principle the large MV imaging dose can be accounted for during treatment planning. But, "accounting for" does not mean that no extra dose, which can otherwise be avoided by shaping the treatment beams, will be delivered to the adjacent sensitive structures. For a breast cancer patient, for example, the imaging dose is delivered not only to the ipsilateral breast, but also the contralateral breast, the heart, and the lungs. For patients with a long life expectancy after radiation therapy, radiation dose resulting from real-time and/or routine adaptive imaging is of particular concern. MV imaging falls short in this aspect and seriously compromises the value of image guidance. QA of the kV imaging device entails additional effort, but the task is quite manageable and can be automated by well-designed phantom and analysis software tools.18 Given the potential impact resulting from image-guided 4D and adaptive therapy, the minimal extra QA effort is clearly worthwhile. After all, 2D/3D kV imaging is providing us with better, and often additional, information for therapeutic guidance. Rebuttal: Jenghwa Chang, Ph.D. What is the best IGRT system? Should it be the best imaging plus the best therapy device as Dr. Xing proposed, or, as I pointed out, the system that best meets overall clinical needs? For conventional radiotherapy, the imaging devices for simulation and the therapy devices for treatment can be separately optimized because the simulation and treatment delivery processes are independent. For IGRT, however, optimizing each device on its own may not lead to the best solution because the imaging device is used to guide the therapy device. Dose, time, and personnel constraints will limit routine use of lengthy, repeated on-board CT scans for adaptive replanning or real-time feedback of anatomic information. Instead, tracking implanted markers is often sufficient for monitoring intrafractional tumor motion. Although both kV and MV imaging can be used,13 tracking marker motion relative to the beam direction with an MV imager is more intuitive and makes more clinical sense. In cases where the benefits of replanning outweigh dose and practical time concerns, MV CBCT provides more accurate CT number information and is less sensitive to metal artifacts.9 Dr. Xing is correct in stating that kV imaging is superior to MV imaging in terms of image quality and dose. However, the difference is probably clinically insignificant. Major research efforts have made the soft tissue contrast and imaging dose of MV imaging comparable to those for kV imaging.8–10 In fact, neither kV nor MV on-board imaging devices for IGRT can compete with diagnostic imaging devices in image quality, but both can be registered with simulation images for setup and verification. Imaging dose can also be easily included in treatment planning for both modalities,12 though adding the kV dose requires new beam data that are not collected as part of a normal commissioning process. In conclusion, we should focus on the overall clinical needs when evaluating IGRT devices. 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