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Radiology


Image-guided Radiotherapy Based on Kilovoltage Cone-beam Computed Tomography – A Review of Technology and Clinical Outcome


Matthias Guckenberger Radiation Oncologist and Vice Chairman, Department of Radiation Oncology, University Hospital Würzburg


Abstract


In this article an overview about the technology and clinical application of gantry-mounted kilovoltage cone-beam computed tomography (CBCT) systems for image-guided high-precision radiotherapy is provided. The emphasis will be on the body sites that are most frequently targeted in daily clinical routine (prostate, lung and head-and-neck region).


Keywords Image-guided radiotherapy (IGRT), cone-beam computed tomography (CBCT), prostate, lung, head-and-neck region


Disclosure: The author has no conflicts of interest to declare. Received: 9 February 2011 Accepted: 10 March 2011 Citation: European Oncology & Haematology, 2011;7(2):121–4 Correspondence: Matthias Guckenberger, Department of Radiation Oncology, University Hospital Würzburg, Josef-Schneider-Str 2, 97080 Würzburg, Germany. E: guckenberger_m@klinik.uni-wurzburg.de


Support: The publication of this article was funded by Elekta. The views and opinions expressed are those of the author and not necessarily those of Elekta.


Rationale and Technology of


Cone-beam Computed Tomography-based Image-guided Radiotherapy


Variation of the target position is a major challenge in the clinical practice of external beam radiotherapy (EBRT). Variability of the tumour position from day to day is caused by breathing, changes of the filling of hollow organs and peristaltic or more complex changes of the anatomy of patients. If not corrected or compensated for, this variation of the target position may cause imprecise delivery of the irradiation dose with increased doses to the normal tissue and decreased doses to the tumour. Consequences are increased rates of toxicity and decreased tumour control probability. Traditionally, these uncertainties have been compensated for using safety margins around the tumour, which ensure dose coverage of the target volume despite these uncertainties.


However, application of large safety margins increases the delivery of radiation to healthy tissue: this increases the risk of toxicity and limits the total irradiation dose. Recently, image-guided radiotherapy (IGRT) became broadly available. IGRT aims to detect the tumour position immediately prior to treatment, and allows for the adaptation of RT in case the target position changes compared with the planned situation. This more precise delivery of radiation will decrease safety margins, reduce doses to normal tissue and decrease the risk of toxicity, and may allow a safe increase in the radiation dose to improve local tumour control and survival (improvement of the therapeutic ratio).


Kilovoltage cone-beam computed tomography (CBCT) is currently state-of-the-art technology for IGRT. The technology has been developed by David Jaffrey and was first commercialised by Elekta as


© TOUCH BRIEFINGS 2011


X-ray volume imaging (XVI).1,2


Major advantages of IGRT technology


are high spatial resolution, sufficient soft-tissue contrast not requiring implanted markers, imaging in treatment position and low imaging doses. CBCT-based IGRT is currently in the process of replacing 2D IGRT methods and frame-based intracranial3,4 stereotactic treatments.5–7


and extracranial Of all IGRT solutions, CBCT is the


technology is being installed at the fastest pace. The principles of CBCT-based IGRT are as follows: a flat-panel detector and a kilovolt radiation source are integrated into a linear accelerator. Via rotation of the linac gantry around the patient, multiple projection radiographs are acquired immediately before a RT fraction with acquisition times of 40 seconds to two minutes. The radiographs are reconstructed with a back-projection algorithm to a volumetric image. This CBCT verification image is registered to the reference planning CT data set, preferably by means of automatic image registration, for calculation of the target position relative to the planned reference position. Changes of the target position exceeding a pre-defined threshold are then corrected online prior to the start of RT. The positioning error is determined in six degrees of freedom while the possibility for correction of rotational errors in addition to translations depends on the specific treatment table.8,9


Imaging, reconstruction and position


correction requires about five minutes with the currently commercially available systems.10


Additionally, more complex


changes of the target and normal tissue such as weight loss and tumour regression are monitored in the verification images, allowing for adaptation of the treatment plan.


In this article, an overview about clinical applications of kilovoltage CBCT-based IGRT is provided, with emphasis on the tumour sites that are most frequently targeted in daily clinical routine (prostate, lung and head-and neck region) (see Figure 1).


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