Kilovoltage Cone-beam CT Research Articles

Performance Characteristics and Quality Assurance Aspects of Kilovoltage Cone-Beam CT on Medical Linear Accelerator

A medical linear accelerator equipped with optical position tracking, ultrasound imaging, portal imaging, and radiographic imaging systems was installed at University of Pittsburgh Cancer Institute for the purpose of performing image-guided radiation therapy (IGRT) and image-guided radiosurgery (IGRS) in October 2005. We report the performance characteristics and quality assurance aspects of the kilovoltage cone-beam computed tomography (kV-CBCT) technique. This radiographic imaging system consists of a kilovoltage source and a large-area flat panel amorphous silicon detector mounted on the gantry of the medical linear accelerator via controlled arms. The performance characteristics and quality assurance aspects of this kV-CBCT technique involves alignment of the kilovoltage imaging system to the isocenter of the medical linear accelerator and assessment of (a) image contrast, (b) spatial accuracy of the images, (c) image uniformity, and (d) computed tomography (CT)-to-electron density conversion relationship were investigated. Using the image-guided tools, the alignment of the radiographic imaging system was assessed to be within a millimeter. The low-contrast resolution was found to be a 6-mm diameter hole at 1% contrast level and high-contrast resolution at 9 line pairs per centimeter. The spatial accuracy (50 mm ± 1%), slice thickness (2.5 mm and 5.0 mm ± 5%), and image uniformity (± 20 HU) were found to be within the manufacturer’s specifications. The CT-to-electron density relationship was also determined. By using well-designed procedures and phantom, the number of parameter checks for quality assurance of the IGRT system can be carried out in a relatively short time.

Deep-inspiration breath-hold kilovoltage cone-beam CT for setup of stereotactic body radiation therapy for lung tumors: Initial experience

We report our initial experience with deep-inspiration breath-hold (DIBH) cone-beam CT (CBCT) on the treatment table, using the kilovoltage imager integrated into our linear accelerator, for setting up patients for DIBH stereotactic body radiation therapy (SBRT) for lung tumors. Nine patients with non-small cell lung cancer (seven stage I), were given 60Gy in three fractions. All nine patients could perform a DIBH for 35s. For each patient we used a diagnostic reference CT volume image acquired during a DIBH to design an SBRT plan consisting of 7-10 noncoplanar conformal beams. Four patients were setup by registering DIBH kilovoltage projection radiographs or megavoltage portal images on the treatment table to digitally reconstructed radiographs from the reference CT. Each of the last 14 fractions out of a total of 27 was setup by acquiring a CBCT volume image on the treatment table in three breath-holds. The CBCT and reference CT volume images were directly registered and the shift was calculated from the registration. The CBCT volume images contained excellent detail on soft tissue and bony anatomy for matching to the reference CT. Most importantly, the tumor was always clearly visible in the CBCT images, even when it was difficult or impossible to see in the radiographs or portal images. The accuracy of the CBCT method was confirmed by DIBH megavoltage portal imaging and each treatment beam was delivered during a DIBH. CBCT acquisition typically required five more minutes than radiograph acquisition but the overall setup time was often shorter using CBCT because repeat imaging was minimized. We conclude that for setting up SBRT treatments of lung tumors, DIBH CBCT is feasible, fast and may result in less variation among observers than using bony anatomy in orthogonal radiographs.

Dispositifs de repositionnement prostatique sous l'accélérateur linéaire

The development of sophisticated conformal radiation therapy techniques for prostate cancer, such as intensity-modulated radiotherapy, implies precise and accurate targeting. Inter- and intrafraction prostate motion can be significant and should be characterized, unless the target volume may occasionally be missed. Indeed, bony landmark-based portal imaging does not provide the positional information for soft-tissue targets (prostate and seminal vesicles) or critical organs (rectum and bladder). In this article, we describe various prostate localization systems used before or during the fraction: rectal balloon, intraprostatic fiducials, ultrasound-based localization, integrated CT/linear accelerator system, megavoltage or kilovoltage cone-beam CT, Calypso 4D localization system tomotherapy, Cyberknife and Exactrac X-Ray 6D. The clinical benefit in using such prostate localization tools is not proven by randomized studies and the feasibility has just been established for some of these techniques. Nevertheless, these systems should improve local control by a more accurate delivery of an increased prescribed dose in a reduced planning target volume.

Imaging in radiotherapy treatment planning and delivery

Recent years have seen major developments in radiotherapy techniques and equipment, focusing on conformal external beam techniques with the emphasis on intensity-modulated radiotherapy (IMRT). The progress of IMRT implementation has been summarized in a series of review articles in the British Journal of Radiology during 2003 and 2004. One of the fundamental prerequisites for conformal radiotherapy is the localization of the target, starting with the gross tumour volume (GTV) and moving outwards to the planning target volume (PTV). The judgement of how to define these volumes is crucial. The future value of IMRT and related techniques may depend on the underpinning images of these volumes and how they are interpreted. Fortunately, in parallel with the developments in radiotherapy delivery techniques has come an equally impressive and timely development in associated imaging. Although CT remains the gold standard for radiotherapy planning, MRI has much to offer. Superior soft tissue characterization and new developments in dynamic contrast enhanced, diffusion-weighted and diffusion tensor imaging are making their mark in target delineation. Magnetic resonance spectroscopy (MRS) promises insights into tissue metabolism and, although currently limited by large voxel sizes, may provide information on tumour response and recurrence to complement the structural information of MRI and CT. The traditional role of the simulator is now complemented by the use of the CT scanner as a virtual simulator, thus making 3D image datasets available. Verification on a fraction-by-fraction basis is now possible by several means. CT scanners within the treatment room have been used to compare images at treatment time with planning CT scans and also to measure and correct for interfraction variations in anatomy or positioning. Portal imaging has seen equally impressive developments as a result of the introduction of flat panel imaging systems. As well as their improved image quality for traditional radiographic projection imaging, when used in conjunction with cone beam reconstruction algorithms, they have made megavoltage CT a reality. Alternatively, if used with a kilovoltage X-ray source and cone beam CT algorithm, the CT reconstructions can harness the improved tissue contrast associated with the lower X-ray energies. The development of PET/CT imaging for radiotherapy planning provides, like MRS, another example of the imaging of metabolic function, but in this case the functional information is displayed within the same coordinate system as the familiar structure provided by CT. This is an unusual luxury, however, and the problems of extracting and overlaying data from different imaging studies and modalities has prompted several advances in image registration and fusion algorithms. So these are exciting times in radiotherapy imaging and this special issue reflects both the current practice and future opportunities for exploiting advances in several imaging modalities in order to increase the confidence with which modern conformal radiotherapy treatments may be planned and delivered. The British Journal of Radiology, 79 (2006), S1

Automated 2D‐3D registration of a radiograph and a cone beam CT using line‐segment enhancementa)

The objective of this study was to develop a fully automated two-dimensional (2D)-three-dimensional (3D) registration framework to quantify setup deviations in prostate radiation therapy from cone beam CT (CBCT) data and a single AP radiograph. A kilovoltage CBCT image and kilovoltage AP radiograph of an anthropomorphic phantom of the pelvis were acquired at 14 accurately known positions. The shifts in the phantom position were subsequently estimated by registering digitally reconstructed radiographs (DRRs) from the 3D CBCT scan to the AP radiographs through the correlation of enhanced linear image features mainly representing bony ridges. Linear features were enhanced by filtering the images with "sticks," short line segments which are varied in orientation to achieve the maximum projection value at every pixel in the image. The mean (and standard deviations) of the absolute errors in estimating translations along the three orthogonal axes in millimeters were 0.134 (0.096) AP(out-of-plane), 0.021 (0.023) ML and 0.020 (0.020) SI. The corresponding errors for rotations in degrees were 0.011 (0.009) AP, 0.029 (0.016) ML (out-of-plane), and 0.030 (0.028) SI (out-of-plane). Preliminary results with megavoltage patient data have also been reported. The results suggest that it may be possible to enhance anatomic features that are common to DRRs from a CBCT image and a single AP radiography of the pelvis for use in a completely automated and accurate 2D-3D registration framework for setup verification in prostate radiotherapy. This technique is theoretically applicable to other rigid bony structures such as the cranial vault or skull base and piecewise rigid structures such as the spine.

Image-guided radiotherapy using a mobile kilovoltage x-ray device

A mobile isocentric C-arm kilovoltage imager has been evaluated as a potential tool for image-guided radiotherapy. The C-arm is equipped with an amorphous silicon flat panel for high-quality image acquisition. Additionally, the device is capable of cone beam computed tomography (CT) and volumetric reconstruction. This is achieved through the application of a modified Feldkamp algorithm with acquisition over a 180 degrees scan arc. The number of projections can be varied from 100 to 1000, resulting in a reconstructed volume 20 cm in diameter by 15-cm long. While acquisition time depends upon number of projections, acceptable quality images can be obtained in less than 60 seconds. Image resolution and contrast of cone-beam phantom images have been compared with images from a conventional CT scanner. The system has a spatial resolution of > or = 10 lp/cm and resolution is approximately equal in all 3 dimensions. Conversely, subject contrast is poorer than conventional CT, compromised by the increased scatter and underlying noise inherent in cone beam reconstruction, as well as the absence of filtering prior to reconstruction. The mobility of the C-arm makes it necessary to determine the C-arm position relative to the linear accelerator isocenter. Two solutions have been investigated: (1) the use of fiducial markers, embedded in the linac couch, that can subsequently be registered in the image sets; and (2), a navigation approach for infrared tracking of the C-arm relative to the linac isocenter. Observed accuracy in phantom positioning ranged from 1.0 to 1.5 mm using the navigation approach and 1.5 to 2.5 mm using the fiducial-based approach. As part of this work, the impact of respiratory motion on cone-beam image quality was evaluated, and a scheme for retrospective gating was devised. Results demonstrated that kilovoltage cone beam CT provides spatial integrity and resolution comparable to conventional CT. Cone-beam CT studies of patients undergoing radiotherapy have demonstrated acceptable soft tissue contrast, allowing assessment of daily changes in target anatomy. Of the 2 approaches developed to register images to the linac isocenter, the navigation method demonstrated superior accuracy for daily patient positioning relative to the fiducial-based method. Finally, significant image degradation due to respiratory motion was observed. It was demonstrated that this could be improved by correlating the acquisition of individual 2D projections with respiration for retrospective reconstruction of phase-based volumetric datasets.

8 Kilovoltage cone beam CT guided radiotherapy

8 Kilovoltage cone beam CT guided radiotherapy

134 Varian Physics Award Kilo-voltage cone-beam CT for image-guided radiation therapy

134 Varian Physics Award Kilo-voltage cone-beam CT for image-guided radiation therapy

SU‐DD‐A4‐06: Registration of Volumetric KV Or MV Cone Beam CT with Fan Beam CT

Purpose: Kilovoltage (kV) cone beam CT (CBCT) provides the patient's volumetric information and is valuable for patient positioning verification. An important step in this application is the registration of CBCT acquired at simulation or before treatment with the planning CT (PCT). The purpose of this work is to investigate several specific issues involved in CBCT‐TPCT registration and develop an effective algorithm to effectively utilize the volumetric imaging system for IGRT. Method and Materials: The PCT data were acquired using Picker PQ5000 scanner, and the CBCT images were obtained using Varian Trilogy OBI system. The image quality of CBCT is generally poor as compared with conventional CT: Low soft tissue contrast, artifacts/distortions, and limited width/length present practical problems for its clinical application. A BSpline based non‐rigid image fusion software was implemented to provide voxel‐to‐voxel matching between PCT and CBCT. Results: For the phantom study, the quality of CBCT has little effect on the registration and the CBCT‐PCT mapping was relatively straightforward because of the absence of motion artifacts. Examination of “bony” landmarks indicated that an accuracy less than 2mm was achievable. The patient registrations were complicated by artifacts and, in the cervix case, the limited field size of CBCT. While the bony landmarks can be registered to within 2mm, the motion artifacts complicated registration and caused a large uncertainty in the PCT‐CBCT mapping. Conclusion: Low soft tissue contrast and artifacts of CBCT posed a challenge for CBCT‐PCT registration. While continuous effect is needed to improve the CBCT, a robust registration tool that can cope with specific issues of CBCT is highly desirable to fully exploit the volumetric CBCT technology.

Assessment of residual error for online cone-beam CT-guided treatment of prostate cancer patients

Kilovoltage cone-beam CT (CBCT) implemented on board a medical accelerator is available for image-guidance applications in our clinic. The objective of this work was to assess the magnitude and stability of the residual setup error associated with CBCT online-guided prostate cancer patient setup. Residual error pertains to the uncertainty in image registration, the limited mechanical accuracy, and the intrafraction motion during imaging and treatment. The residual error for CBCT online-guided correction was first determined in a phantom study. After online correction, the phantom residual error was determined by comparing megavoltage portal images acquired every 90 degrees to the corresponding digitally reconstructed radiographs. In the clinical study, 8 prostate cancer patients were implanted with three radiopaque markers made of high-winding coils. After positioning the patient using the skin marks, a CBCT scan was acquired and the setup error determined by fusing the coils on the CBCT and planning CT scans. The patient setup was then corrected by moving the couch accordingly. A second CBCT scan was acquired immediately after the correction to evaluate the residual target setup error. Intrafraction motion was evaluated by tracking the coils and the bony landmarks on kilovoltage radiographs acquired every 30 s between the two CBCT scans. Corrections based on soft-tissue registration were evaluated offline by aligning the prostate contours defined on both planning CT and CBCT images. For ideal rigid phantoms, CBCT image-guided treatment can usually achieve setup accuracy of 1 mm or better. For the patients, after CBCT correction, the target setup error was reduced in almost all cases and was generally within +/-1.5 mm. The image guidance process took 23-35 min, dictated by the computer speed and network configuration. The contribution of the intrafraction motion to the residual setup error was small, with a standard deviation of +/-0.9 mm. The average difference between the setup corrections obtained with coil and soft-tissue registration was greatest in the superoinferior direction and was equal to -1.1 +/- 2.9 mm. On the basis of the residual setup error measurements, the margin required after online CBCT correction for the patients enrolled in this study would be approximatively 3 mm and is considered to be a lower limit owing to the small intrafraction motion observed. The discrepancy between setup corrections derived from registration using coils or soft tissue can be due in part to the lack of complete three-dimensional information with the coils or to the difficulty in prostate delineation and requires further study.

Second cancer risk, concomitant exposures and IRMER(2000)

In this issue we publish a Commentary by Dr Roger Harrison [1] on second cancer risk following radiotherapy. Harrison looks at the evidence from various sources, including an NRPB summary, and the complexity of comparing risk in the radiotherapy patient population with that in the normal population. He also addresses the particular problem of ‘‘concomitant dose’’ in radiotherapy planning and verification that has been highlighted by other authors in articles in BJR, particularly Waddington and Mackenzie [2] and Munro [3]. The Ionising Radiation (Medical Exposures) Regulations (IRMER(2000)) [4] require justification of all medical exposures [Reg:6(1)]. Under the optimization regulation [Reg:7], the Practitioner is also required to ‘‘ensure that exposures of target volumes are individually planned, taking into account that doses to non-target volumes and tissues shall be as low as reasonably practicable (ALARP) and consistent with the intended radiotherapeutic purposes of the exposure’’. Neither of these regulations distinguishes between the radiotherapeutic dose and the ‘‘planning and verification doses’’ required by the total process. Also, ALARP has such a different meaning for radiotherapy exposures from that for diagnostic exposures that it is almost inappropriate as a concept. (We may need to develop our own language specifically for protection in radiotherapy, but more of that later.) A specific interpretation is put on these regulations in the Medical and Dental Guidance Notes [5]. In situations where the clinician is required to consider the extra dose from planning and verification exposures independently, ‘‘The IRMER Practitioner responsible for the treatment exposure (the Clinical Oncologist) can justify the concomitant exposures at the outset or during the radiotherapy course, but in doing so must be aware of the likely exposures and the resulting dose so that the benefit and detriment can be assessed. This can be achieved by including likely concomitant (extra-target) exposures within site-specific protocols with a total effective dose agreed’’. Waddington and McKenzie [2] chose to follow the Guidance Notes to demonstrate the possibility of expressing concomitant exposures using ‘‘effective dose’’, and have presented an excellent paper analysing that part of the ‘‘dose’’ from verification images; and at the same time justifying the need to record these doses by looking at the potential for cancer induction. Harrison [1] has taken this a stage further to analyse in more depth the radiation induced cancer issue for radiotherapy patients. One of his conclusions is that to specify and document individual concomitant exposures separately would seem to be unnecessary. This Editorial aims to stimulate further debate on these subjects, which are partly about the need for radiotherapy departments to comply with IRMER(2000) [4], but also about the need for all staff to gain a better understanding on cost-benefits of radiotherapy. It is extremely important that the radiotherapy community addresses this subject and arrives at a consensus that will benefit the patient. In the summary of the current position, which follows, absorbed dose (shortened to ‘‘dose’’) has been used throughout. The use of ‘‘effective dose’’ is questionable in radiotherapy for several reasons: the radiotherapy patient population is a highly selected subgroup of the general population with different longevity; also tissue weighting factors will be different because dose gradients and doses are much greater in radiotherapy than in diagnostic radiography. Therefore it is avoided here.

Accuracy of image guidance for stereotactic liver radiotherapy using active breathing control and daily soft tissue imaging

External skin marks or radiological bone structures have traditionally been used as positioning references in conventional radiotherapy (RT). However, these are poor surrogates for liver tumor position which is not fixed to the skeleton but is better correlated with diaphragm position. PTV margins must be large enough to account for uncertainty in tumour position using such localization methods. In addition, the liver moves due to breathing, requiring increased PTV margins and normal tissue irradiation. We have developed a hypofractionated 6 fraction stereotactic RT trial for liver cancer, using active breathing control (ABC) to immobilize the liver and on-line orthogonal imaging with repositioning prior to each radiation treatment in an effort to reduce geometric uncertainties and breathing motion. The purpose of this study is to report feasibility of this localization strategy for patients with liver cancer and to measure the accuracy of liver localization using ABC and on-line imaging compared to traditional setup based on skin marks. Eleven patients with liver cancer were treated with RT on study from August 2003 to February 2004. ABC was not used in three patients due to poor reproducibility with ABC (2) or patient intolerance (1). In these patients, the skeleton was used for localization with PTV margins accounting for breathing motion, leaving 8 patients treated with ABC and daily imaging. For the patients treated with ABC, following daily setup using skin marks, orthogonal megavoltage imaging was acquired at breath hold using ABC. A graticule provided a reference point at the beam central axis. The images were aligned to the planning CT digitally reconstructed radiographs using the diaphragm for caudal-cranial (CC) alignment and the skeleton for anterior-posterior (AP) and left-right (LR) alignment (see figure). Adjustments were made for positioning errors greater than a threshold of 3 mm and images were repeated. Off-line image alignments were repeated twice by 2 investigators to determine the final setup accuracy and the error in initial setup measurement prior to repositioning. Liver position during treatment was also measured using megavoltage movie loops acquired during delivery of each beam. Orthogonal imaging from 48 treatment fractions delivered using ABC in 8 patients were evaluated. Repositioning occurred in 45 of the 48 fractions (due primarily to offsets in the CC direction). Treatment was delivered using repeat breath holds from 10 to 25 seconds. The average treatment time was 25 minutes (range: 19 - 40 minutes). The use of on-line imaging and repositioning reduced random setup errors (σ) from 4.1 mm (CC), 2.8 mm (AP), and 2.5 mm (ML) to 2.6 mm (CC), 2.1 mm (AP) and 1.8 mm (ML). Systemic errors (μ) were reduced from -3.4 mm (CC), 2.8 mm (AP) and -1.3 mm (ML) to -0.3 mm (CC), 0.1 mm (AP) and 0.6 mm (ML). The average absolute offset in position was reduced from 5.1 mm (CC), 3.0 mm (AP) and 2.7 mm (ML) to 2.6 mm (CC), 2.0 mm (AP) and 1.9 mm (ML). Analysis of movie loops acquired during treatment is ongoing. Daily targeting of liver cancers using orthogonal megavoltage imaging to localize the liver for alignment is feasible and improves accuracy compared to conventional localization without daily imaging and repositioning. As the liver is a surrogate for actual tumor position, we are investigating kilovoltage cone beam CT for direct tumor targeting.

Accuracy of image guidance for stereotactic liver radiotherapy using active breathing control and daily soft tissue imaging

External skin marks or radiological bone structures have traditionally been used as positioning references in conventional radiotherapy (RT). However, these are poor surrogates for liver tumor position which is not fixed to the skeleton but is better correlated with diaphragm position. PTV margins must be large enough to account for uncertainty in tumour position using such localization methods. In addition, the liver moves due to breathing, requiring increased PTV margins and normal tissue irradiation. We have developed a hypofractionated 6 fraction stereotactic RT trial for liver cancer, using active breathing control (ABC) to immobilize the liver and on-line orthogonal imaging with repositioning prior to each radiation treatment in an effort to reduce geometric uncertainties and breathing motion. The purpose of this study is to report feasibility of this localization strategy for patients with liver cancer and to measure the accuracy of liver localization using ABC and on-line imaging compared to traditional setup based on skin marks. Eleven patients with liver cancer were treated with RT on study from August 2003 to February 2004. ABC was not used in three patients due to poor reproducibility with ABC (2) or patient intolerance (1). In these patients, the skeleton was used for localization with PTV margins accounting for breathing motion, leaving 8 patients treated with ABC and daily imaging. For the patients treated with ABC, following daily setup using skin marks, orthogonal megavoltage imaging was acquired at breath hold using ABC. A graticule provided a reference point at the beam central axis. The images were aligned to the planning CT digitally reconstructed radiographs using the diaphragm for caudal-cranial (CC) alignment and the skeleton for anterior-posterior (AP) and left-right (LR) alignment (see figure). Adjustments were made for positioning errors greater than a threshold of 3 mm and images were repeated. Off-line image alignments were repeated twice by 2 investigators to determine the final setup accuracy and the error in initial setup measurement prior to repositioning. Liver position during treatment was also measured using megavoltage movie loops acquired during delivery of each beam. Orthogonal imaging from 48 treatment fractions delivered using ABC in 8 patients were evaluated. Repositioning occurred in 45 of the 48 fractions (due primarily to offsets in the CC direction). Treatment was delivered using repeat breath holds from 10 to 25 seconds. The average treatment time was 25 minutes (range: 19 - 40 minutes). The use of on-line imaging and repositioning reduced random setup errors (σ) from 4.1 mm (CC), 2.8 mm (AP), and 2.5 mm (ML) to 2.6 mm (CC), 2.1 mm (AP) and 1.8 mm (ML). Systemic errors (μ) were reduced from -3.4 mm (CC), 2.8 mm (AP) and -1.3 mm (ML) to -0.3 mm (CC), 0.1 mm (AP) and 0.6 mm (ML). The average absolute offset in position was reduced from 5.1 mm (CC), 3.0 mm (AP) and 2.7 mm (ML) to 2.6 mm (CC), 2.0 mm (AP) and 1.9 mm (ML). Analysis of movie loops acquired during treatment is ongoing. Daily targeting of liver cancers using orthogonal megavoltage imaging to localize the liver for alignment is feasible and improves accuracy compared to conventional localization without daily imaging and repositioning. As the liver is a surrogate for actual tumor position, we are investigating kilovoltage cone beam CT for direct tumor targeting.

Automatic registration of the prostate for computed-tomography-guided radiotherapy.

The recent development of integrated computed tomography (CT)/linear accelerator (LINAC) combinations, where the CT scanner and the LINAC use the same patient couch, and of kilovoltage cone-beam CT systems attached to the LINAC gantry, means that suitable hardware is now available for CT-guided localization of the prostate. Clinical implementation is, however, currently impeded by the lack of robust and accurate software tools to compare the position of the prostate in the CT images used for the treatment plan with its position in the daily CT images. Manual registration of the planning CT images with the daily CT images can be slow and can introduce significant inter-user variations. We have developed an automatic registration technique that is not adversely influenced by changes in prostate shape, size or orientation, presence of rectal gas, or bladder filling. The cost function used in the registration is the mean absolute difference in CT numbers voxel-by-voxel between the daily CT image and the planning CT image for a volume extracted from the planning CT images using the original physician-drawn gross tumor volume contours. To enhance soft tissue contrast in the prostate region and to reduce the impact of rectal gas calcifications and bone on the registration, voxels with CT numbers that represent gas or bone are filtered out from the calculation. The results of the automatic registration agreed with the mean results of seven human observers, with standard deviations of 0.5 mm, 0.5 mm, and 1.0 mm in the left/right (RL), anterior/posterior (AP), and superior/inferior (SI) directions, respectively, for a patient that was relatively easy to localize. Agreement (one standard deviation) for a patient that was difficult to localize was 0.6 mm, 1.4 mm, and 1.9 mm in the RL, AP, and SI directions, respectively. These results are better than the interuser uncertainties reported for a manual alignment technique and are close to the reported intrauser uncertainties. The results are independent of the shape of contours in the original treatment plan, reducing the impact of interobserver variations in contouring the prostate. The algorithm is fast and reliable, allowing the entire CT localization process to take place in 5-9 minutes. In 120 CT image sets from seven patients, the failure rate was found to be less than 1%. The use of this algorithm will facilitate the clinical implementation of CT-guided localization of the prostate.

Inter- and intra-user variability in CT-guided prostate localization

Purpose/Objective: The introduction of CT/LINAC combinations, where the CT scanner and LINAC share the same patient couch, and of kilovoltage cone-beam CT systems attached to the LINAC gantry, means that CT-guided localization of the prostate may soon become a clinical reality. The daily setup error achieved with these new systems will probably be dominated by the precision with which the prostate position in the daily CT images can be compared with its position in the treatment plan. The aim of this study was to investigate the intra- and inter-user variations that can be expected for a manual alignment technique similar to that used in a commercial ultrasound-based system (BAT, NOMOS Corp), where the user manually aligns GTV contours with the daily images.

Implementation of an on-board kilovoltage cone-beam CT imaging system for clinical applications

Implementation of an on-board kilovoltage cone-beam CT imaging system for clinical applications