Automatic localization of the prostate for on-line or off-line image-guided radiotherapy

PURPOSE: With higher radiation dose, higher cure rates have been reported in prostate cancer patients. The extra margin needed to account for prostate motion, however, limits the level of dose escalation, because of the presence of surrounding organs at risk. Knowledge of the precise position of the prostate would allow significant reduction of the treatment field. Better localization of the prostate at the time of treatment is therefore needed, e.g. using a cone-beam computed tomography (CT) system integrated with the linear accelerator. Localization of the prostate relies upon manual delineation of contours in successive axial CT slices or interactive alignment and is fairly time-consuming. A faster method is required for on-line or off-line image-guided radiotherapy, because of prostate motion, for patient throughput and efficiency. Therefore, we developed an automatic method to localize the prostate, based on 3D gray value registration. METHODS AND MATERIALS: A study was performed on conventional repeat CT scans of 19 prostate cancer patients to develop the methodology to localize the prostate. For each patient, 8-13 repeat CT scans were made during the course of treatment. First, the planning CT scan and the repeat CT scan were registered onto the rigid bony structures. Then, the delineated prostate in the planning CT scan was enlarged by an optimum margin of 5 mm to define a region of interest in the planning CT scan that contained enough gray value information for registration. Subsequently, this region was automatically registered to a repeat CT scan using 3D gray value registration to localize the prostate. The performance of automatic prostate localization was compared to prostate localization using contours. Therefore, a reference set was generated by registering the delineated contours of the prostates in all scans of all patients. Gray value registrations that showed large differences with respect to contour registrations were detected with a chi(2) analysis and were removed from the data set before further analysis. RESULTS: Comparing gray value registration to contour registration, we found a success rate of 91%. The accuracy for rotations around the left-right, cranial-caudal, and anterior-posterior axis was 2.4 degrees, 1.6 degrees, and 1.3 degrees (1 SD), respectively, and for translations along these axes 0.7, 1.3, and 1.2 mm (1 SD), respectively. A large part of the error is attributed to uncertainty in the reference contour set. Automatic prostate localization takes about 45 seconds on a 1.7 GHz Pentium IV personal computer. CONCLUSIONS: This newly developed method localizes the prostate quickly, accurately, and with a good success rate, although visual inspection is still needed to detect outliers. With this approach, it will be possible to correct on-line or off-line for prostate movement. Combined with the conformity of intensity-modulated dose distributions, this method might permit dose escalation beyond that of current conformal approaches, because margins can be safely reduced.     

Clinical feasibility of using an EPID in CINE mode for image-guided verification of stereotactic body radiotherapy

PURPOSE: To introduce a novel method for monitoring tumor location during stereotactic body radiotherapy (SBRT) while the treatment beam is on by using a conventional electronic portal imaging device (EPID). METHODS AND MATERIALS: In our clinic, selected patients were treated under a phase I institutional review board-approved SBRT protocol for limited hepatic metastases from solid tumors. Before treatment planning multiple gold fiducial markers were implanted on the periphery of the tumor. During treatment the EPID was used in cine mode to collect the exit radiation and produce a sequence of images for each field. An in-house program was developed for calculating the location of the fiducials and their relative distance to the planned locations. RESULTS: Three case studies illustrate the utility of the technique. Patient A exhibited a systematic shift of 4 mm during one of the treatment beams. Patient B showed an inferior drift of the target of approximately 1 cm from the time of setup to the end of the fraction. Patient C had a poor setup on the first day of treatment that was quantified and accounted for on subsequent treatment days. CONCLUSIONS: Target localization throughout each treatment beam can be quickly assessed with the presented technique. Treatment monitoring with an EPID in cine mode is shown to be a clinically feasible and useful tool.     

Cone-beam CT localization of internal target volumes for stereotactic body radiotherapy of lung lesions

PURPOSE: In this study, we investigate a technique of matching internal target volumes (ITVs) in four-dimensional (4D) simulation computed tomography (CT) to the composite target volume in free-breathing on-board cone-beam (CB) CT. The technique is illustrated by using both phantom and patient cases. METHODS AND MATERIALS: A dynamic phantom with a target ball simulating respiratory motion with various amplitude and cycle times was used to verify localization accuracy. The dynamic phantom was scanned using simulation CT with a phase-based retrospective sorting technique. The ITV was then determined based on 10 sets of sorted images. The size and epicenter of the ITV identified from 4D simulation CT images and the composite target volume identified from on-board CBCT images were compared to assess localization accuracy. Similarly, for two clinical cases of patients with lung cancer, ITVs defined from 4D simulation CT images and CBCT images were compared. RESULTS: For the phantom, localization accuracy between the ITV in 4D simulation CT and the composite target volume in CBCT was within 1 mm, and ITV was within 8.7%. For patient cases, ITVs on simulation CT and CBCT were within 8.0%. CONCLUSION: This study shows that CBCT is a useful tool to localize ITV for targets affected by respiratory motion. Verification of the ITV from 4D simulation CT using on-board free-breathing CBCT is feasible for the target localization of lung tumors.     

Vertebral fractures following stereotactic body radiotherapy for spine metastases

Stereotactic body radiotherapy has emerged as one of the preferred treatments for patients with spine metastases, with the potential for long‐term control from lesion irradiation. Post‐treatment vertebral compression fractures are a known complication of this therapy, contributing to worsening pain and reduced quality of life, sometimes requiring surgical intervention. This review explores the current knowledge of post‐radiotherapy fractures, in terms of the rates and associated predictive factors. A search of databases including Medline, Embase and the Cochrane Library was conducted using keywords such as ‘vertebral compression fracture’, ‘stereotactic body radiotherapy’ and ‘spine metastases’. The search was limited to published studies up to March 2019, reporting clinical outcomes including both the post‐treatment fracture rate and statistical identification of associated risk factors. Rates of post‐treatment fractures ranged from 4 to 39%. A variety of factors were found to increase the risk, including the appearance of lytic vertebral disease, degree of pre‐existing compression, spinal malalignment, increased dose per fraction and a Spinal Instability Neoplastic Score >6. This knowledge can enable clinicians to counsel patients when considering management options for spine metastases, maintaining the balance between local tumour control and the risk of subsequent fracture.     

Target coverage in image-guided stereotactic body radiotherapy of liver tumors

PURPOSE: To determine the effect of image-guided procedures (with computed tomography [CT] and electronic portal images before each treatment fraction) on target coverage in stereotactic body radiotherapy for liver patients using a stereotactic body frame (SBF) and abdominal compression. CT guidance was used to correct for day-to-day variations in the tumor's mean position in the SBF. METHODS AND MATERIALS: By retrospectively evaluating 57 treatment sessions, tumor coverage, as obtained with the clinically applied CT-guided protocol, was compared with that of alternative procedures. The internal target volume-plus (ITV(+)) was introduced to explicitly include uncertainties in tumor delineations resulting from CT-imaging artifacts caused by residual respiratory motion. Tumor coverage was defined as the volume overlap of the ITV(+), derived from a tumor delineated in a treatment CT scan, and the planning target volume. Patient stability in the SBF, after acquisition of the treatment CT scan, was evaluated by measuring the displacement of the bony anatomy in the electronic portal images relative to CT. RESULTS: Application of our clinical protocol (with setup corrections following from manual measurements of the distances between the contours of the planning target volume and the daily clinical target volume in three orthogonal planes, multiple two-dimensional) increased the frequency of nearly full (> or = 99%) ITV(+) coverage to 77% compared with 63% without setup correction. An automated three-dimensional method further improved the frequency to 96%. Patient displacements in the SBF were generally small (< or = 2 mm, 1 standard deviation), but large craniocaudal displacements (maximal 7.2 mm) were occasionally observed. CONCLUSION: Daily, CT-assisted patient setup may substantially improve tumor coverage, especially with the automated three-dimensional procedure. In the present treatment design, patient stability in the SBF should be verified with portal imaging.     

Cone-beam computed tomography for on-line image guidance of lung stereotactic radiotherapy: localization, verification, and intrafraction tumor position

PURPOSE: Cone-beam computed tomography (CBCT) in-room imaging allows accurate inter- and intrafraction target localization in stereotactic body radiotherapy of lung tumors. METHODS AND MATERIALS: Image-guided stereotactic body radiotherapy was performed in 28 patients (89 fractions) with medically inoperable Stage T1-T2 non-small-cell lung carcinoma. The targets from the CBCT and planning data set (helical or four-dimensional CT) were matched on-line to determine the couch shift required for target localization. Matching based on the bony anatomy was also performed retrospectively. Verification of target localization was done using either megavoltage portal imaging or CBCT imaging; repeat CBCT imaging was used to assess the intrafraction tumor position. RESULTS: The mean three-dimensional tumor motion for patients with upper lesions (n = 21) and mid-lobe or lower lobe lesions (n = 7) was 4.2 and 6.7 mm, respectively. The mean difference between the target and bony anatomy matching using CBCT was 6.8 mm (SD, 4.9, maximum, 30.3); the difference exceeded 13.9 mm in 10% of the treatment fractions. The mean residual error after target localization using CBCT imaging was 1.9 mm (SD, 1.1, maximum, 4.4). The mean intrafraction tumor deviation was significantly greater (5.3 mm vs. 2.2 mm) when the interval between localization and repeat CBCT imaging (n = 8) exceeded 34 min. CONCLUSION: In-room volumetric imaging, such as CBCT, is essential for target localization accuracy in lung stereotactic body radiotherapy. Imaging that relies on bony anatomy as a surrogate of the target may provide erroneous results in both localization and verification.     

Automatic prostate localization on cone-beam CT scans for high precision image-guided radiotherapy

PURPOSE: Previously, we developed an automatic three-dimensional gray-value registration (GR) method for fast prostate localization that could be used during online or offline image-guided radiotherapy. The method was tested on conventional computed tomography (CT) scans. In this study, the performance of the algorithm to localize the prostate on cone-beam CT (CBCT) scans acquired on the treatment machine was evaluated. METHODS AND MATERIALS: Five to 17 CBCT scans of 32 prostate cancer patients (332 scans in total) were used. For 18 patients (190 CBCT scans), the CBCT scans were acquired with a collimated field of view (FOV) (craniocaudal). This procedure improved the image quality considerably. The prostate (i.e., prostate plus seminal vesicles) in each CBCT scan was registered to the prostate in the planning CT scan by automatic 3D gray-value registration (normal GR) starting from a registration on the bony anatomy. When these failed, registrations were repeated with a fixed rotation point locked at the prostate apex (fixed apex GR). Registrations were visually assessed in 3D by one observer with the help of an expansion (by 3.6 mm) of the delineated prostate contours of the planning CT scan. The percentage of successfully registered cases was determined from the combined normal and fixed apex GR assessment results. The error in gray-value registration for both registration methods was determined from the position of one clearly defined calcification in the prostate gland (9 patients, 71 successful registrations). Results: The percentage of successfully registered CBCT scans that were acquired with a collimated FOV was about 10% higher than for CBCT scans that were acquired with an uncollimated FOV. For CBCT scans that were acquired with a collimated FOV, the percentage of successfully registered cases improved from 65%, when only normal GR was applied, to 83% when the results of normal and fixed apex GR were combined. Gray-value registration mainly failed (or registrations were difficult to assess) because of streaks in the CBCT scans caused by moving gas pockets in the rectum during CBCT image acquisition (i.e., intrafraction motion). The error in gray-value registration along the left-right, craniocaudal, and anteroposterior axes was 1.0, 2.4, and 2.3 mm (1 SD) for normal GR, and 1.0, 2.0, and 1.7 mm (1 SD) for fixed apex GR. The systematic and random components of these SDs contributed approximately equally to these SDs, for both registration methods. Conclusions: The feasibility of automatic prostate localization on CBCT scans acquired on the treatment machine using an adaptation of the previously developed three-dimensional gray-value registration algorithm, has been validated in this study. Collimating the FOV during CBCT image acquisition improved the CBCT image quality considerably. Artifacts in the CBCT images caused by large moving gas pockets during CBCT image acquisition were the main cause for unsuccessful registration. From this study, we can conclude that CBCT scans are suitable for online and offline position verification of the prostate, as long as the amount of nonstationary gas is limited.     

Residual set-up errors and margins in on-line image-guided prostate localization in radiotherapy.

BACKGROUND AND PURPOSE: Image-guided on-line correction of the target position allows radiotherapy of prostate cancer with narrow set-up margins. The present study investigated the residual set-up error after on-line prostate localization and its impact on margins. MATERIALS AND METHODS: Prostate localization based on two orthogonal X-ray images of gold markers implanted in the prostate was performed with an on-board imager at four treatment sessions for 90 patients. The set-up error in the sagittal plane residual after couch adjustment was evaluated on lateral verification portal images. RESULTS: The set-up error was less than 3.0mm in 92% of the cases in the anterior-posterior (AP) direction and in 95% of the cases in the cranio-caudal (CC) direction. The set-up error was dominated by internal prostate motion taking place during the set-up procedure. Set-up margins were calculated using two formalisms: margins designed to ensure a minimum CTV dose of 95% for 90% of the patient population were 3.6mm (AP) and 3.5mm (CC). Patient-independent normal distributed set-up errors would result in margins of 4.3mm (AP) and 4.0mm (CC) to ensure complete CTV inclusion in the PTV with 90% probability. CONCLUSION: Internal prostate motion during the set-up procedure was the main contributor to residual set-up errors.     

基于图像引导的小动物立体放疗系统介绍

小动物放疗模型的相关理论及其应用,对于放射生物学和临床肿瘤放疗学的研究与发展至关重要。而综合考虑普适性与成本效应等因素,研究人员常运用小鼠模型来研究肿瘤与正常组织的放射生物学机制。近年来,基于不同图像引导的小动物立体放疗设备也取得了巨大发展,它们已能较好模拟人体放疗流程。本文着重介绍了基于不同图像方式(如CBCT与MRI)引导的小动物放疗系统,其次对小动物荧光成像技术进行理解学习,总结对比了不同生物发光与荧光成像设备在小动物成像系统中的应用,最后对未来小动物放疗设备优化方向提出展望。     

Phase I clinical evaluation of near-simultaneous computed tomographic image-guided stereotactic body radiotherapy for spinal metastases

PURPOSE: To evaluate in a Phase I study the safety, feasibility, and patient-positioning accuracy of treating patients with intensity-modulated, near-simultaneous, computed tomographic (CT) image-guided stereotactic body radiotherapy (SBRT). PATIENTS AND METHODS: Fifteen consecutive patients with metastatic spinal disease who met protocol eligibility criteria were entered into a Phase I clinical trial. Each patient received five treatments of intensity-modulated, near-simultaneous CT image-guided SBRT, for a total of 75 treatments with 90 isocenter setups during the course of the study. Patients uniformly received 30 Gy (if possible) of radiotherapy in 5 fractions to the clinical target volume. The total dose was constrained by limiting the spinal cord to a maximum dose of 10 Gy. To verify correct daily patient positioning before each treatment and to determine the daily treatment setup error after radiation delivery, axial CT scans were taken before and immediately after each treatment without moving the patient from the treatment position, for comparison with the planning CT scan. Toxicity was measured using the Common Toxicity Criteria, the Late Effects of Normal Tissue scoring system and a neurologic function scale. Follow-up was conducted 4 weeks after completion of SBRT, and then 2, 3, 6, 9, 12, and every 6 months thereafter. RESULTS: The procedure was technically feasible to perform in all patients. No neurologic toxicity was observed in any patient. The median follow-up time was 9 months (range 6-16). The Clopper-Pearson upper bound on the probability of paralysis with 95% confidence is no greater than 0.181. The positional setup error was determined to be within 1 mm of planning isocenter. CONCLUSIONS: This Phase I study shows that intensity-modulated, near simultaneous, CT image-guided SBRT is a feasible, and highly precise technique for the noninvasive treatment of spinal metastases. Although no paralysis has developed in the 15 patients treated, continued monitoring for spinal cord toxicity is warranted, as larger numbers of patients will be needed to more precisely define the upper bound on the probability of spinal cord myelopathy.     

Dosimetric effect of translational and rotational errors for patients undergoing image-guided stereotactic body radiotherapy for spinal metastases

PURPOSE: To investigate the dosimetric effects of translational and rotational patient positioning errors on the treatment of spinal and paraspinal metastases using computed tomography image-guided stereotactic body radiotherapy. The results of this study provide guidance for the treatment planning process and recognition of the dosimetric consequences of daily patient treatment setup errors. METHODS AND MATERIALS: The data from 20 patients treated for metastatic spinal cancer using image-guided stereotactic body radiotherapy were investigated in this study. To simulate the dosimetric effects of residual setup uncertainties, 36 additional plans (total, 756 plans) were generated for each isocenter (total, 21 isocenters) on the planning computed tomography images, which included isocenter lateral, anteroposterior, superoinferior shifts, and patient roll, yaw, and pitch rotations. Tumor volume coverage and the maximal dose to the organs at risk were compared with those of the original plan. Six daily treatments were also investigated to determine the dosimetric effect with or without the translational and rotational corrections. RESULTS: A 2-mm error in translational patient positioning error in any direction can result in >5% tumor coverage loss and >25% maximal dose increase to the organs at risk. Rotational correction is very important for patients with multiple targets and for the setup of paraspinal patients when the isocenter is away from bony structures. Compared with the original plans, the daily treatment data indicated that translational adjustments could correct most of the setup errors to mean divergences of -1.4% for tumor volume coverage and -0.3% for the maximal dose to the organs at risk. CONCLUSION: For the best dosimetric results, spinal stereotactic treatments should have setup translational errors of < or =1 mm and rotational errors of < or =2 degrees .     

Commissioning an image-guided localization system for radiotherapy

PURPOSE: To describe the design and commissioning of a system for the treatment of classes of tumors that require highly accurate target localization during a course of fractionated external-beam therapy. This system uses image-guided localization techniques in the linac vault to position patients being treated for cranial tumors using stereotactic radiotherapy, conformal radiotherapy, and intensity-modulated radiation therapy techniques. Design constraints included flexibility in the use of treatment-planning software, accuracy and precision of repeat localization, limits on the time and human resources needed to use the system, and ease of use. METHODS AND MATERIALS: A commercially marketed, stereotactic radiotherapy system, based on a system designed at the University of Florida, Gainesville, was adapted for use at the University of Washington Medical Center. A stereo pair of cameras in the linac vault were used to detect the position and orientation of an array of fiducial markers that are attached to a patient's biteblock. The system was modified to allow the use of either a treatment-planning system designed for stereotactic treatments, or a general, three-dimensional radiation therapy planning program. Measurements of the precision and accuracy of the target localization, dose delivery, and patient positioning were made using a number of different jigs and devices. Procedures were developed for the safe and accurate clinical use of the system. RESULTS: The accuracy of the target localization is comparable to that of other treatment-planning systems. Gantry sag, which cannot be improved, was measured to be 1.7 mm, which had the effect of broadening the dose distribution, as confirmed by a comparison of measurement and calculation. The accuracy of positioning a target point in the radiation field was 1.0 +/- 0.2 mm. The calibration procedure using the room-based lasers had an accuracy of 0.76 mm, and using a floor-based radiosurgery system it was 0.73 mm. Target localization error in a phantom was 0.64 +/- 0.77 mm. Errors in positioning due to couch rotation error were reduced using the system. CONCLUSION: The system described has proven to have acceptable accuracy and precision for the clinical goals for which it was designed. It is robust in detecting errors, and it requires only a nominal increase in setup time and effort. Future work will focus on evaluating its suitability for use in the treatment of head-and-neck cancers not contained within the cranial vault.     

Image localization for frameless stereotactic radiotherapy

Purpose: Infrared light-emitting diodes (IRLEDs) have been used for optic-guided stereotactic radiotherapy localization at the University of Florida since 1995. The current paradigm requires stereotactic head ring placement for the patient’s first fraction. The stereotactic coordinates and treatment plan are determined relative to this head ring. The IRLEDs are attached to the patient via a maxillary bite plate, and the position of the IRLEDs relative to linac isocenter is saved to file. These positions are then recalled for each subsequent treatment to position the patient for fractionated therapy. The purpose of this article was to report a method of predicting the desired IRLED locations without need for the invasive head ring.

Methods and Materials: To achieve the goal of frameless optic-guided radiotherapy, a method is required for direct localization of the IRLED positions from a CT scan. Because it is difficult to localize the exact point of light emission from a CT scan of an IRLED, a new bite plate was designed that contains eight aluminum fiducial markers along with the six IRLEDs. After a calibration procedure to establish the spatial relationship of the IRLEDs to the aluminum fiducial markers, the stereotactic coordinates of the IRLED light emission points are determined by localizing the aluminum fiducial markers in a stereotactic CT scan.

Results: To test the accuracy of direct CT determination of the IRLED positions, phantom tests were performed. The average accuracy of isocenter localization using the IRLED bite plate was 0.65 ± 0.17 mm for these phantom tests. In addition, the optic-guided system has a unique compatibility with the stereotactic head ring. Therefore, the isocentric localization capability was clinically tested using the stereotactic head ring as the absolute standard. The ongoing clinical trial has shown the frameless system to provide a patient localization accuracy of 1.11 ± 0.3 mm compared with the head ring.

Conclusion: Optic-guided radiotherapy using IRLEDs provides a mechanism through which setup accuracy may be improved over conventional techniques. To date, this optic-guided therapy has been used only as a hybrid system that requires use of the stereotactic head ring for the first fraction. This has limited its use in the routine clinical setting. Computation of the desired IRLED positions eliminates the need for the invasive head ring for the first fraction. This allows application of optic-guided therapy to a larger cohort of patients, and also facilitates the initiation of extracranial optic-guided radiotherapy.     

头颈肿瘤立体定向分次照射靶区定位的误差分析

背景与目的:明确靶区定位的精确度是立体定向分次照射质量保证的基本要求。本文主要分析头颈肿瘤立体定向分次照射(fractionatedstereotacticradiotherapy,FSRT)中机械等中心、CT定位、治疗摆位以及CT图像误差等可能引起的靶区定位误差。方法:使用立体定向治疗计划系统、靶点模拟器、头部定位框架检查各个治疗阶段靶区定位的误差。设置任意5个参考点,使用靶点模拟器检查CT定位误差;选取7个不同机器臂架/治疗床角度,定期用胶片检验使用的PhilipsSL-18直线加速器等中心误差大小;用验证片检查治疗摆位误差;对自制模体行CT扫描,分析CT图像伪影可能引起的图像误差。结果:CT定位误差约为(1.5±0.4)mm;在检查的不同机器臂架/治疗床角度中机械等中心最大误差为(1.0±0.6)mm;患者摆位的距离误差为(1.0±0.3)mm;整个治疗过程中靶区定位误差约为(2.1±0.8)mm。结论:立体定向分次照射中需要综合考虑各个阶段中可能对治疗靶区定位产生的影响,误差分析结果可用来确定治疗的计划靶区。     

Evaluation of ultrasound-based prostate localization for image-guided radiotherapy

To evaluate the use of the ultrasound-based BAT system for daily prostate alignment.Prostate alignments using the BAT system were compared with alignments using radiographic images of implanted radiopaque markers. The latter alignments were used as a reference. The difference between the BAT and marker alignments represents the displacements that would remain if the alignments were done using ultrasonography. The inter-user variability of the contour alignment process was assessed.On the basis of the marker alignments, the initial displacement of the prostate in the AP, superoinferior, and lateral direction was -0.9 +/- 3.9, 0.1 +/- 3.9, and 0.2 +/- 3.4 mm respectively. The directed differences between the BAT and marker alignments in the respective directions were 0.2 +/- 3.7, 2.7 +/- 3.9, and 1.6 +/- 3.1 mm. The occurrence of displacements >/=5 mm was reduced by a factor of two in the AP direction after the BAT system was used. Among eight users, the average range of couch shifts due to contour alignment variability was 7, 7, and 5 mm in the antero-posterior (AP), superoinferior, and lateral direction, respectively.In our study, the BAT alignments were systematically different from the marker alignments in the superoinferior, and lateral directions. The remaining random variability of the prostate position after the ultrasound-based alignment was similar to the initial variability. However, the occurrence of displacements >/=5 mm was reduced in the AP direction. The inter-user variation of the contour alignment process was significant.     

体部肿瘤的立体定向放疗

目的：体部肿瘤立体定向放疗的研究。方法：１９９７年５月－１９９８年１２月期间治疗３２例共３５个病灶，根据病变大小进行适形立体定向放疗，射野一般５－７个，治疗次数３－７次，治疗间隔１－３天，每次治疗周边剂量４５０－１０００ｃＧｙ，总量１８９０－３４２０Ｇｙ。结果：３２例３５个病灶根据ＣＴ复查肿瘤退缩情况确定ＣＲ为２８．６％，ＰＲ为３１．４％，总有效率为６０％。１年以上生存率为５８．３％。结论：立体定     

Comparison of localization performance with implanted fiducial markers and cone-beam computed tomography for on-line image-guided radiotherapy of the prostate

PURPOSE: The aim of this work was to assess the accuracy of kilovoltage (kV) cone-beam computed tomography (CBCT)-based setup corrections as compared with orthogonal megavoltage (MV) portal image-based corrections for patients undergoing external-beam radiotherapy of the prostate. METHODS AND MATERIALS: Daily cone-beam CT volumetric images were acquired after setup for patients with three intraprostatic fiducial markers. The estimated couch shifts were compared retrospectively to patient adjustments based on two orthogonal MV portal images (the current clinical standard of care in our institution). The CBCT soft-tissue based shifts were also estimated by digitally removing the gold markers in each projection to suppress the artifacts in the reconstructed volumes. A total of 256 volumetric images for 15 patients were analyzed. RESULTS: The Pearson coefficient of correlation for the patient position shifts using fiducial markers in MV vs. kV was (R2 = 0.95, 0.84, 0.81) in the left-right (LR), anterior-posterior (AP), and superior-inferior (SI) directions, respectively. The correlation using soft-tissue matching was as follows: R2 = 0.90, 0.49, 0.51 in the LR, AP and SI directions. A Bland-Altman analysis showed no significant trends in the data. The percentage of shifts within a +/-3-mm tolerance (the clinical action level) was 99.7%, 95.5%, 91.3% for fiducial marker matching and 99.5%, 70.3%, 78.4% for soft-tissue matching. CONCLUSIONS: Cone-beam CT is an accurate and precise tool for image guidance. It provides an equivalent means of patient setup correction for prostate patients with implanted gold fiducial markers. Use of the additional information provided by the visualization of soft-tissue structures is an active area of research.