As the ability to irradiate a tumor more precisely continues to increase, so does the radiation dose delivered to destroy that tumor. Even though multiple radiation-therapy beams can be focused on their target with a high degree of precision, the possibility of radiation-induced damage to the surrounding healthy tissue exists if the tumor is not where it is expected to be. For the same reason, the tumor might not receive the full radiation dose planned for it.

Since it has been apparent for some time that tumors can shrink and expand between sessions of radiation therapy, and that the patient’s soft tissues are constantly shifting in relation to bone, it is no longer sufficient to rely on the radiographic position of bony landmarks for patient positioning prior to radiation therapy. Soft-tissue images obtained after prior treatment sessions are also inadequate to compensate for daily changes. For the highest degree of accuracy in radiation therapy to be achieved, a new set of images of the target tumor’s position should be obtained immediately before each treatment, while the patient is in position for therapy.

Two of the methods used to achieve this real-time correction in the patient’s position are megavolt and kilovolt localization. At the University of California, San Francisco, Jean Pouliot, PhD, is using two megavolt cone-beam CT systems with portal imaging—MVision from Siemens Medical Solutions (Malvern, Pa)—for daily localization. The advantages of this technology, he says, include reliability, speed, and efficiency.

At Duke University (Durham, NC), Fang-Fang Yin, PhD, is using the Duke On-Board Imager for daily localization. This system places a kilovolt generator and kilovolt detector on each side of the patient and is also equipped for portal imaging. Yin describes the advantages of kilovolt localization as low radiation dose, good contrast, lack of metal-induced artifacts, and relatively small data-file size.

Megavoltage Cone Beam CT

Figure 1. The MVCBCT system consists of a standard treatment unit equipped with a flat-panel detector optimized for megavolt photons.

by Jean Pouliot, PhD

The objective of image-guided radiation therapy (IGRT) is to verify the proper positioning of the patient immediately before the delivery of radiation dose at each fraction of radiation therapy. This is becoming essential as dose distributions become complex and dose gradients become even steeper, thanks to the increasing ability to accurately deliver dose to a specific location. For the last 5 years, the University of California, San Francisco (UCSF), in collaboration with Siemens Oncology Care Systems, has been working on the development and the clinical implementation of megavoltage cone beam computerized tomography (MVCBCT).1 Two of these MVCBCT systems—MVision, capable of portal imaging, and cone-beam CT2—are in routine use in our clinic and at an increasing number of centers.


The MVCBCT system consists of a standard treatment unit, either a Primus or a ONCOR linear accelerator from Siemens Medical, equipped with an amorphous silicon flat panel optimized for MV photons. The 41- x 41-cm2 flat panel has 1024 x 1024 pixels. The detector is mounted on a retractable support that deploys in less than 10 seconds with a positional reproducibility of 1 mm in any direction. The entire imaging system, presented in Figure 1, operates under a syngo-based Coherence therapist workspace, which communicates to the control console, the linac, and a local patient database. The workspace contains applications allowing for the automatic acquisition of projection images, image reconstruction, CT to CBCT image registration, couch-position adjustment, and treatment delivery. This provides a 3D patient-anatomy volume in the actual treatment position, relative to the treatment isocenter, moments before the dose delivery, that can be tightly aligned to the planning CT, allowing verification and correction of the patient position, detection of anatomical changes, and dose calculation.

Clinical Workflow

The MVCBCT imaging procedure is well integrated in the clinical workflow for patient alignment. In clinical mode, the cone-beam acquisition protocol is selected at the treatment console. The linac gantry is placed at the starting position, namely 270°. During the acquisition, the gantry rotates 200° until it reaches its final position, 110°. Projection images are continuously acquired at each degree. On completion of the reconstruction image, the cone-beam image is automatically loaded in the Adaptive Targeting Sofware, and the CB to CT image registration is performed automatically in a few seconds using a mutual information algorithm. Further manual adjustment of the registration in any of the typical planes (axial, coronal, and sagittal) is possible. The acquisition and reconstruction take approximately 2 minutes. This means that a patient can be set up, position adjusted based on internal anatomy, and ready for treatment in less than 5 minutes.

The system demonstrates submillimeter localization precision and sufficient soft-tissue resolution to visualize such structures as the prostate. At UCSF, the systems, among other applications, are used to perform patient setup, to detect nonrigid spinal cord distortions, to align prostate patients with or without implanted gold markers, to monitor tumor growth and shrinkage, and to locate and position stationary tumors in the lung. In a clinical study performed on head and neck patients, MVCBCT showed translation shifts, rotations, and anatomy deformations not always appreciated using portal imaging.

MVCBCT for IGRT in Presence on “Noncompatible CT” Objects

Metallic implants create artifacts and image distortion with standard kilovoltage (kV) CT and MRI, which can impede image fusion and accurate delineation of hardware and anatomy, thereby limiting subsequent electron-density corrections and dose calculations. The image degradation associated with kV CT imaging in the presence of high-atomic-number material is greatly reduced with MVCBCT. This is due to the predominantly Compton scattering of high-energy photons delivered in MVCBCT systems. Therefore, MVCBCT improves the delineation of structures in CT images that suffer from metal artifacts and provides a new alternative IGRT approach for patients with implanted metallic objects, such as surgical clips, tooth fillings, dental implants, hip prostheses, and orthopedic hardware. The first clinical use of MVCBCT to aid in treatment of a patient with implanted hardware after surgery for a paraspinous high-grade sarcoma was recently reported.3 MVCBCT played a key role in this example of IGRT, where optimal setup was achieved for a patient who might otherwise be treated with doses unlikely to provide long-term local control or not be treated at all. MVCBCT also is used at UCSF to complement the regular CT for target definition of prostate-cancer treatment of patients with hip replacements.4 This is important, because in Europe and the United States combined, more than half a million people had a hip joint replaced in 2005, contributing to the increasing number of radiotherapy patients with metallic hip prostheses.

An Efficient Use of Technology

The first electronic portal imaging devices (EPID) became available about 15 years ago. It would, at that time, have been difficult to foresee that an EPID and the linac treatment beam would eventually provide all the tools required to obtain a 3D image of the patient in treatment position immediately before dose delivery. This is what MVCBCT offers. The dual use of the linac beam for treatment and imaging offers several advantages. In addition to being cost-efficient and convenient, the fact that the imaging system and the treatment beam share the same isocenter ensures that the image is always precisely in alignment with the beam.

Toward DGRT

The ultimate strategy of IGRT extends beyond the geometrical verification of the patient anatomy. It would allow the verification and correction of the position of the patient’s 3D anatomy as well as the comparison of the dose delivered with the intended dose distribution.5 The MVCBCT image is well suited for dose recalculation,6 allowing to assess the dosimetric impact of anatomical changes. The knowledge of the dosimetric consequences will provide a quantitative answer about when it is time to replan.


In summary, MVision is a reliable, fast, and efficient IGRT tool. It already has been used to guide the treatment of several hundred patients. The availability, moments before dose delivery, of an image showing the 3D anatomy of the patient in treatment position allows accurate patient setup, even in the presence of implanted high-Z material. It also allows monitoring of anatomical changes or tumor evolution as the treatment progresses.

MVCBCT has undergone significant development in the past few years. Current image quality already has proven sufficient for many IGRT applications. And one can expect the range of clinical applications for MVCBCT to grow as imaging technology continues to improve. Finally, it promises to be a key player in dose-guided radiation therapy, an extension of adaptive radiation therapy where dosimetric considerations constitute the basis of treatment modification.

Jean Pouliot, PhD, is professor of radiation oncology at the University of California, San Francisco, UCSF Comprehensive Cancer Center.


  1. 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(2):238?246.
  2. Aubin M, Morin O, Chen J, et al. The use of megavoltage cone-beam CT to complement CT for target definition in pelvic radiotherapy in the presence of hip replacement. Brit J Radiol. August 17, 2006. Available at: Accessed September 14, 2006.
  3. Hansen EK, Larson DA, Morin O, et al. Image-guided radiotherapy using megavoltage cone-beam CT for treatment of paraspinous tumors in the presence of orthopedic hardware. Int J Radiat Oncol Biol Phys. 2006. In press.
  4. 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 Dosi. 2006;31(1):51?61.
  5. Chen J, Morin O, Aubin M, Bucci MK, Chuang CF, Pouliot J. Dose-guided radiation therapy with megavoltage cone-beam CT. [invited paper] Br J Radiol. 2006. In press.
  6. Morin O, Chen J, Aubin M, et al. Dose calculation using megavoltage cone-beam CT. Proceedings: American Society for Therapeutic Radiology and Oncology, 47th Annual Meeting. Denver, CO: Int J of Radiat Oncol Biol Phys; 2005(63):S62.

Kilovolt Daily Localization

by Fang-Fang Yin, PhD

As in-room imaging for daily localization before radiation therapy improves, new challenges emerge; consideration of image quality alone is supplemented by additional concerns involving radiation doses, data-file sizes, and quality assurance.

Image-guided radiation therapy (IGRT) involves an ongoing interaction among five components. The first three are accumulations of acquired information: patient data, image data, and clinical data. These are combined within the fourth component, the information system, and then subjected to the fifth (human reasoning) in order to create IGRT. Information flows in both directions throughout this process, with any new data or changes in reasoning leading to adjustments in therapy.

Accurate targeting of treatment volume is critical for conformal radiation therapy. Different imaging methods are adopted to verify treatment ports, isocenters and correct patient positioning. Offline correction uses a comparison of reference images and post-treatment on-board images to position the patient for treatment. Because soft tissues can shift in position and tumor borders can change between treatments, this approach is obviously a less-accurate method. On-board imaging using kilovoltage x-ray sources allows performance of online correction prior to patient treatment.

Online correction begins with patient setup and the acquisition of on-board images. If comparison with reference images shows that correction of the patient’s position is needed, the couch is shifted accordingly. Another set of on-board images is then acquired to determine whether the correction was adequate. If so, treatment proceeds; afterward, whether or not a position correction took place, another set of on-board images may be obtained to provide feedback for further position evaluation.

By detecting both random and systematic errors, online portal verification increases precision and accuracy. (Offline correction is intended to detect systemic errors only.) Online correction may consist of image fusion or any of four main forms of localization: cone-beam CT (CBCT) guidance or kilovolt-kilovolt, kilovolt-megavolt, and megavolt-megavolt comparisons. (See articles IGRT: In-Room Technologies and Megavoltage Cone Beam CT)

Kilovolt Systems

Figure 2. The Duke On-board Imager, features kilovolt imager and detector on opposite sides of the gantry.

Digital kilovolt radiography systems may be mobile, gantry mounted, or room mounted. They use flat-panel detectors of amorphous silicon. The configuration of the detector differs in one particular from that of the amorphous-silicon detector used in megavolt imaging; It contains the same support base topped by an amorphous silicon photodiode under a phosphor layer, but the next layer, just beneath the protection layer on the surface, is an antiscatter grid instead of a metal-buildup layer.

On-board kilovolt imaging compares favorably with megavolt imaging in two main areas. Kilovolt imaging provides better contrast between bone and soft tissue, and the radiation dose delivered is smaller. However, metals typically do not produce artifacts on kilovolt images. The Duke On-board Imager has a kilovolt source to one side of the patient and a kilovolt detector on the other; it also incorporates a portal imager (for megavoltage sources).


In-room imaging improves both precision and accuracy. Treatment time for IGRT is extended only 5 to 10 minutes through the use of daily localization. Patient setup accounts for 2 to 5 minutes of treatment time, and Duke On-Board Imager kilovolt or megavolt imaging takes about a minute. Two-dimensional matching analysis requires another 2 to 5 minutes. CBCT imaging takes 3 minutes, and 3D matching analysis takes 2 to 5 minutes. Reposi-tioning based on the localization information takes about a minute and is followed by 10 to 15 minutes of treatment. The total case time, with CBCT, is 20 to 35 minutes; without CBCT, 15 to 25 minutes.

The imaging modality chosen for localization will naturally affect the patient’s total radiation exposure. The lowest dose (less than 0.1 cGy) is associated with obtaining an orthogonal set of kilovolt images, and the highest, at 8 cGy, is for a good quality CBCT head scan. An orthogonal set of megavolt digital images can approach the same dose, at 5 to 8 cGy; megavolt CT tomotherapy, as reported, 1 to 2 cGy. A CBCT body scan delivers a dose of 3 to 4 cGy.

Quality Assurance

System performance, user experience, and facility expectations can all change the frequency with which quality-assurance procedures must be performed. Nonetheless, some general recommendations can be made. Detector stability and system performance should be evaluated daily or before each use. Dark image calibration is needed before each use as well. In geometric quality assurance, a tolerance of less than 1 mm should be expected for the localizing lasers, a simple test object, and the motion and deployment of the system’s components.

Safety testing should ensure that the warning lights and the interlocks that prevent or interrupt irradiation are functioning. Warm-up evaluation should cover functional status of the generator, detector, and collimator operation, as well as whether the detector’s signal is within the expected range. Elements of the clinical process, such as database integrity and the availability of storage space, should be checked at the same time. More detailed quality assurance should usually be conducted monthly.

Data Management and System Costs

Management of patient information is of critical importance in modern radiation therapy. File sizes for tumor localization vary immensely, from 0.4MB for a megavolt image to 28MB for CBCT. A kilovolt image is of moderate size (at 1.5MB). Overall, a CT-based plan may require 50MB to 200MB of data storage, but an IGRT patient’s folder is three or more times larger, needing 600MB to 2 gigabytes of storage.

There are, of course, other factors to be considered in determining the best daily localization method. No matter which technology is chosen, hardware and software will be required, along with an information network to support data movement.

Because financial considerations can become important if there are large differences in hardware, software, and network costs, the planning process should include more than the hardware, software, and network components. The actual cost of daily-localization technology will also be affected by its power requirements, reimbursability, training needs, and staffing expenses, all of which need careful evaluation.

Fang-Fang Yin, PhD, is Professor and Director of Radiation Physics, Department of Radiation Oncology, Duke University Medical Center, Durham, NC. This article has been adapted from Dr Yin’s presentation at the annual meeting of the American Association of Physicists in Medicine, July 30, 2006, in Miami.