Correction of systematic setup errors in prostate radiation therapy: how many images to perform?

The purpose of this study was to develop an evidence-based off-line setup correction protocol for systematic errors in prostate radiation therapy. Daily orthogonal electronic portal images were acquired from 30 patients. Field displacements were measured in the medial-lateral (ML), superior-inferior (SI), and anterior-posterior (AP) directions for each treatment fraction. The off-line protocol corrects the mean field displacement found from n consecutive images, starting at a particular fraction of treatment, with a fixed tolerance level. Simulations were performed with the measured data to determine (1) how many images (n) should be averaged to determine the systematic error; (2) on which treatment fraction should the protocol be initiated; and (3) what tolerance level should be applied to determine whether the patient position should be corrected. Uncorrected systematic errors in the ML, SI, and AP directions were (mean position +/- 1 standard deviation [SD]): -0.7 +/- 2.2 mm, -1.5 +/- 1.3 mm, and 1.4 +/- 2.6 mm, respectively. Random errors (1 SD and range) were 1.9 mm (1.3 - 3.3), 1.5 mm (0. - 4.1), and 1.8 mm (1.0-2.6), respectively. A correction based on a single image taken on the first fraction actually increased the systematic errors in the ML and SI directions compared with no correction. More accurate correction of systematic errors was achieved with increasing number of images averaged, with only small benefit after 5 images. With fewer images averaged, delaying the start of the protocol resulted in more accurate correction because of the influence of unrepresentative positions at early fractions. The number of corrections made on patients with small (< 2 mm) systematic errors was minimized for tolerance values of 2 mm and n > or = 5 images averaged. The optimal off-line setup correction protocol would be to shift the patient by the mean displacement of the first 5 portal images of a radical course of radiation therapy. A small tolerance level should be utilized with 2 mm giving good accuracy with minimal unnecessary shifts.     

Evaluation of inter- and intrafraction organ motion during intensity modulated radiation therapy (IMRT) for localized prostate cancer measured by a newly developed on-board image-guided system

PURPOSE: To investigate prostatic organ motion at both setup and intrafraction using an onboard image-guided system. An intrafraction field-based repositioning method also was evaluated. MATERIALS AND METHODS: A dual fluoroscopy with amorphous-silicon flat panel (DFFP) system was used for the three-dimensional registration of implanted markers in the prostate of eight organ-confined cancer patients planned for treatment with intensity modulated radiation therapy (IMRT). Day-to-day motion errors were quantified and intrafraction displacements of more than +/-1 mm were corrected. RESULTS: Among 214 fractions and 565 system views, day-to-day mean magnitude of marker discrepancy +/- standard deviation (SD) was 1.76 +/- 1.4 mm, 3.14 +/- 1.6 mm, and 3.78 +/- 2.4 mm in the right-left, cranial-caudal, and anterior-posterior directions, respectively. The intrafractional mean magnitude +/- SD of marker displacement was 0.45 +/- 0.7 mm, 1.08 +/- 1.38 mm and 1.45 +/- 1.70 mm in the right-left, cranial-caudal, and anterior-posterior directions, respectively. Intrafraction corrected sessions (84/214) showed a median (range) of motion of 0.1 mm (-1.2 to 0.7 mm), -0.2 mm (-2.1 to 1.1 mm), and -0.2 mm (-1.7 to 2.0 mm) in the right-left, cranial-caudal, and anterior-posterior directions, respectively. CONCLUSION: Motion uncertainty can be considerably decreased with daily use of the DFFP system. Reduced intrafraction organ motion clearly endorsed the value of the repositioning approach, allowing a safer dose escalation protocol.     

Comparison of imaging modalities for image-guided radiation therapy (IGRT)

The use of a new integrated CT/LINAC combination, in which the CT scanner is inside the treatment room, and the same patient couch is used for CT scanning and treatment, should allow for accurate and precise correction of interfractional set-up errors. Comparison of intentional shifts in phantom position as detected by the CT system with shifts detected by an ultrasound localization device commonly used for prostate localization, and by an electronic portal imaging device, showed the standard deviation of set-up uncertainties to be within 1 mm in all directions for each modality. The agreement between the modalities was within 2.6 mm, 1.6 mm and 1.7 mm in the superior-inferior (SI), anterior-posterior (AP) and right-left (RL) directions, respectively. All these modalities are, therefore, suitable for image-guided radiation therapy applications. The choice of modality can be made based of factors such as anatomical site, inter-user variations, and patient set-up.     

Intra-fractional uncertainties in image-guided intensity-modulated radiotherapy (IMRT) of prostate cancer

Purpose

To evaluate intra-fractional uncertainties during intensity-modulated radiotherapy (IMRT) of prostate cancer.

Patients and Methods

During IMRT of 21 consecutive patients, kilovolt (kV) cone-beam computed tomography (CBCT) images were acquired prior to and immediately after treatment: a total of 252 treatment fractions with 504 CBCT studies were basis of this analysis. The prostate position in anterior-posterior (AP) direction was determined using contour matching; patient set-up based on the pelvic bony anatomy was evaluated using automatic image registration. Internal variability of the prostate position was the difference between absolute prostate and patient position errors. Intra-fractional changes of prostate position, patient position, rectal distension in AP direction and bladder volume were analyzed.

Results

With a median treatment time of 16 min, intra-fractional drifts of the prostate were > 5 mm in 12% of all fractions and a margin of 6 mm was calculated for compensation of this uncertainty. Mobility of the prostate was independent from the bony anatomy with poor correlation between absolute prostate motion and motion of the bony anatomy (R² = 0.24). A systematic increase of bladder filling by 41 ccm on average was observed; however, these changes did not influence the prostate position. Small variations of the prostate position occurred independently from intra-fractional changes of the rectal distension; a weak correlation between large internal prostate motion and changes of the rectal volume was observed (R² = 0.55).

Conclusion

Clinically significant intra-fractional changes of the prostate position were observed and margins of 6 mm were calculated for this intra-fractional uncertainty. Repeated or continuous verification of the prostate position may allow further margin reduction.     

Electronic portal imaging vs kilovoltage imaging in fiducial marker image-guided radiotherapy for prostate cancer: an analysis of set-up uncertainties

Objectives

The purpose of this study was to compare interfraction prostate displacement data between electronic portal imaging (EPI) and kilovoltage imaging (KVI) treatment units and discuss the impact of any difference on margin calculations for prostate cancer image-guided radiotherapy (IGRT).

Methods

Prostate interfraction displacement data was collected prospectively for the first 4 fractions in 333 patients treated with IGRT with daily pre-treatment EPI or KVI orthogonal imaging. Displacement was recorded in the anteroposterior (AP), left–right (LR) and superoinferior (SI) directions. The proportion of displacement <3 mm and the difference in median absolute displacements were calculated in all directions.

Results

1088 image pairs were analysed in total, 448 by EPI and 640 by KVI. There were 23% (95% confidence interval [CI] 18–28%) more displacements under 3 mm for EPI than for KVI in the AP direction, 14% (95% CI 10–19%) more in the LR direction and 10% (95% CI 5–15%) more in the SI direction. The differences in absolute median displacement (KVI>EPI) were AP 1 mm, LR 1 mm and SI 0.5 mm. Wilcoxon rank-sum test showed that distributions were significantly different for all three dimensions (p<0.0001 for AP and LR and p=0.02 for SI).

Conclusion

EPI has a statistically significant smaller set-up error distribution than KVI. We would expect that, because fiducial marker imaging is less clear for EPI, the clinical target volume to planning target volume margin would be greater when using IGRT; however, relying wholly on displacement data gives the opposite result. We postulate that this is owing to observer bias, which is not accounted for in margin calculation formulas.The integration of medical imaging and radiotherapy treatment has the potential to improve pre-treatment localisation of the target and therefore the accuracy of delivery of radiotherapy. Image-guided radiotherapy (IGRT) using implanted gold seeds and pre-treatment orthogonal imaging is increasingly being used as a treatment option for prostate cancer [1]. Orthogonal imaging can be conducted either with megavoltage or with kilovoltage imaging (KVI). Electronic portal imaging (EPI) is a system that uses a few monitor units from the megavoltage treatment beam to capture the pre-treatment position of fiducial markers implanted in the prostate, thus correcting for day-to-day variations in set-up and organ displacement. The set-up error is accounted for by a margin around the target called the planning target volume (PTV) [2]. By reducing set-up error, EPI-IGRT for prostate cancer has been studied in target (or clinical target volume [CTV]) to PTV margin reduction [3-10]. Recently, linear accelerators with gantry-mounted KVI have also become available [11]. Investigators are beginning to publish on margin reduction in prostate cancer IGRT using KVI [12].Compared with EPI, contrast for fiducial markers and bone is better using KVI (Figure 1). Kilovoltage and megavoltage photons interact with matter predominantly through photoelectric and Compton effects, respectively. This difference in atomic interaction results in a greater absorption of kilovoltage energy photons by high atomic number materials such as gold compared with megavoltage energy photons. The result is better visibility of gold seeds on radiographs taken with kilovoltage energy compared with megavoltage energy.Open in a separate windowFigure 1Example of anteroposterior and lateral electronic portal imaging (above) and kilovoltage imaging (below) of fiducial markers on the same patient.The primary aim of this study is to quantify interfraction displacement differences between megavoltage and kilovoltage imaging used in prostate cancer IGRT. Because both groups come from the same population of prostate cancer patients, the average amount of day-to-day prostate displacement should be the same. Any difference in displacement is probably attributable to the difference in observer assessment. Observer inaccuracy in assessing displacement using EPI in IGRT has implications on margin calculations for future studies.     

Tracking target position variability using intraprostatic fiducial markers and electronic portal imaging in prostate cancer radiotherapy

PURPOSE: Modern radiotherapy has achieved substantial improvement in tumour control and toxicity rates by escalating the total dose to the target volume while sparing surrounding normal tissues. It has therefore become necessary to precisely track tumour position in order to minimise geometrical uncertainties due to setup errors and organ motion. We conducted this prospective evaluation of prostate cancer patients treated with image-guided conformal radiation therapy at our institution. We implanted three fiducial markers (gold seeds) within the prostatic gland in order to quantify daily target displacements and to generate specific margins around the clinical target volume (CTV) to create an appropriate planned target volume (PTV). MATERIALS AND METHODS: Between April and December 2009, ten patients affected with localised prostate cancer were transrectally implanted with three radio-opaque markers. Each patient underwent a computed tomography (CT) scan for planning purposes following proper bladder and rectum preparation. During treatment two orthogonal images were acquired daily and compared with previously generated digitally reconstructed radiographs. After manual localisation, comparison between the position of the gold seeds on the portal and reference images was carried out, and a set of extrapolated lateral-lateral (LL), anterior-posterior (AP) and cranial-caudal (CC) shift corrections was calculated and recorded. Couch corrections were applied with a threshold of 3 mm displacement. RESULTS: Systematic and random errors for each direction were calculated either as measured according to displacement of the gold seeds prior to any couch movement and after couch position correction according to the radio-opaque markers. For skin marks, mean systematic and random errors were 0.12+2.94 mm for LL, 1.04+3.37 mm for AP, -1.14+2.71 mm for CC, whereas for seed markers, mean and systematic errors were 0.6+1.5 mm for LL, 0.51+2.45 mm for AP and -0.25+2.51 mm for CC. A scatter plot generated on all measurements after couch repositioning according to gold-seed displacement suggested a confidence range of shift distributions within 5 mm for LL, 8 mm for CC, and 7 mm for AP. The total systematic and random components were then used to calculate proper PTV in patients receiving conventional treatment (7 mm for LL and 9 mm for both AP and CC). CONCLUSIONS: Prostate positional variability during a course of radiation treatment is strongly influenced by setup and organ motion. Organ tracking through fiducial markers and electronic portal imaging is able to reduce the spread of displacements, significantly contributing to improve the ballistic precision of radiation delivery.     

Erste Erfahrungen mit einem nichtinvasiven Patientenfixieungssystem für die stereotaktische Strahlentherapie der Prostata

PURPOSE: Highly conformal radiotherapy techniques require precise patient positioning. We report our first experience with a new cast system for fixation of the pelvis during stereotactically guided intensity modulated radiotherapy (IMRT) of the prostate with respect to positioning accuracy of the prostate. MATERIAL AND METHODS: The immobilization device consists of a custom-made wrap-around body cast that extends from the abdomen to the thighs and a separate head mask, both made from Scotchcast, and attaches to a frame for extracranial stereotaxy. Sixteen CT-studies (> or = 25 slices, thickness: 3 mm) of 2 patients who were immobilized for IMRT of prostate tumors were evaluated with respect to set-up accuracy of bony structures and the prostate itself. CT-studies were performed immediately before or after a treatment fraction. Deviations of bony landmarks and anatomical landmarks inside the planning target volume were measured in all 3 dimensions. RESULTS: Mean patient movements of 0.15 +/- 0.3 mm (latero-lateral), 0.9 +/- 1 mm (anterior-posterior), 1 +/- 1 mm (tranversal vectorial error) and < 3 mm slice thickness (craniocaudal) were recorded using bony landmarks and 0.9 +/- 0.9 mm (latero-lateral), 1.8 +/- 1.5 mm (anterior-posterior), 2.2 +/- 1.5 mm (transversal vectorial error) and < 3 mm (craniocaudal) using the confines of, or landmarks within the prostate. Standard deviations of absolute positioning error as an often used metric for positioning accuracy ranged between 0.3 and 1.7 mm in the transversal plane. The worst case transversal vectorial deviation for the prostate was 4.4 mm. Figure 4 summarizes the set-up accuracy of bony landmarks and the prostate. CONCLUSION: The presented combination of a body cast and head mask system in a rigid stereotactic body frame ensures reliable noninvasive patient fixation for fractionated extracranial stereotactic radiotherapy. It provides precise and reliable positioning of the prostate and meets the requirements for highly conformal radiotherapy such as IMRT. No further improvement of repositioning can be achieved with external immobilization devices since the positioning error of the target relative to the skeleton exceeds the accuracy of the positioning of the skeleton itself.     

Using kV-kV and CBCT imaging to evaluate rectal cancer patient position when treated prone on a newly available belly board

The goal of this work was to use daily kV-kV imaging and weekly cone-beam CT (CBCT) to evaluate rectal cancer patient position when treated on a new couch top belly board (BB). Quality assurance (QA) of the imaging system was conducted weekly to ensure proper performance. The positional uncertainty of the combined kV-kV image match and subsequent couch move was found to be no more than ± 1.0 mm. The average (1 SD) CBCT QA phantom match was anterior-posterior (AP) = ?0.8 ± 0.2 mm, superior-inferior (SI) = 0.9 ± 0.2 mm, and left-right (LR) = ?0.1 ± 0.1 mm. For treatment, a set of orthogonal kV-kV images were taken and a bony anatomy match performed online. Moves were made along each axis (AP, SI, and LR) and recorded for analysis. CBCT data were acquired once every 5 fractions for a total of 5 images per patient. The images were all taken after the couch move but before treatment. A 3-dimensional (3D-3D) bony anatomy auto-match was performed offline and the residual difference in position recorded for analysis. The average (± 1 SD) move required from skin marks, calculated over all 375 fractions (15 patients × 25 fractions/patient), were AP = ?2.6 ± 3.7 mm, SI = ?0.3 ± 4.9 mm, and LR = 1.8 ± 4.5 mm. The average residual difference in patient position calculated from the weekly CBCT data (75 total) were AP = ?1.7 ± 0.4 mm, SI = 1.1 ± 0.6 mm, and LR = ?0.5 ± 0.2 mm. These results show that the BB does provide simple patient positioning that is accurate to within ± 2.0 mm when using online orthogonal kV-kV image matching of the pelvic bony anatomy.     

Short communication: Setup variations in locoregional radiotherapy for breast cancer: an electronic portal imaging study

Recent trials demonstrating a survival benefit with locoregional radiotherapy (LRRT) to the chest wall and regional nodes in women with node-positive breast cancer have led to increased use of complex techniques to match three or more radiation fields, but information on setup reproducibility with LRRT for breast cancer is scarce. This study reports the magnitude and directions of random and systematic deviations in LRRT for breast cancer using an offline electronic portal imaging verification protocol. Electronic portal images (EPIs) of 46 consecutive women treated with LRRT for breast cancer from March 2001 to February 2002 with LRRT were analysed. Comparisons of EPIs to the corresponding digitally reconstructed radiographs were performed offline with anatomy matching. Displacements in mm were recorded in the superior-inferior (SI), medial-lateral (ML), and anterior-posterior (AP) directions. Random errors ranged from 2.0 mm to 2.5 mm for the breast/chest wall tangential treatments and 2.3 mm to 3.9 mm for the supraclavicular nodal treatments. Systematic errors occurred to a greater degree in the AP direction for the tangential fields and in the ML direction for the supraclavicular field. Displacements of > or =10 mm were found in 1.2% of breast/chest wall tangential treatments and in 6.2% of supraclavicular nodal treatments. These data demonstrate that EPI is a useful tool to verify setup reproducibility in LRRT for breast cancer.     

Transabdominal Ultrasonography, Computed Tomography and Electronic Portal Imaging for 3-Dimensional Conformal Radiotherapy for Prostate Cancer

PURPOSE: To evaluate the feasibility and accuracy of daily B-mode acquisition and targeting ultrasound-based prostate localization (BAT) and to compare it with computed tomography (CT) and electronic portal imaging (EPI) in 3-dimensional conformal radiotherapy (3-D CRT) for prostate cancer. PATIENTS AND METHODS: Ten patients were treated with 3-D CRT (72 Gy/30 fractions, 2.4 Gy/fraction, equivalent to 80 Gy/40 fractions, for alpha/beta ratio of 1.5 Gy) and daily BAT-based prostate localization. For the first 5 fractions, CT and EPI were also performed in order to compare organ-motion and set-up error, respectively. RESULTS: 287 BAT-, 50 CT- and 46 EPI-alignments were performed. The average BAT-determined misalignments in latero-lateral, antero-posterior and cranio-caudal directions were -0.9 mm+/-3.3 mm, 1.0 mm+/-4.0 mm and -0.9 mm+/-3.8 mm, respectively. The differences between BAT- and CT-determined organ-motion in latero-lateral, antero-posterior and cranio-caudal directions were 2.7 mm+/-1.9 mm, 3.9+/-2.8 mm and 3.4 +/- 3.0 mm, respectively. Weak correlation was found between BAT- and CT-determined misalignments in antero-posterior direction, while no correlation was observed in latero-lateral and cranio-caudal directions. The correlation was more significant when only data of good image-quality patients were analyzed (8 patients). CONCLUSION: BAT ensures the relative positions of target are the same during treatment and in treatment plan, however, the reliability of alignment is patient-dependent. The average BAT-determined misalignments were small, confirming the prevalence of random errors in 3-D CRT. Further study is warranted in order to establish the clinical value of BAT.     

A strategy for the use of image-guided radiotherapy (IGRT) on linear accelerators and its impact on treatment margins for prostate cancer patients

Background and Purpose

In external beam radiotherapy of prostate cancer, the consideration of various systematic error types leads to wide treatment margins compromising normal tissue tolerance. We investigated if systematic set-up errors can be reduced by a set of initial image-guided radiotherapy (IGRT) sessions.

Patients and Methods

27 patients received daily IGRT resulting in a set of 882 cone-beam computed tomographies (CBCTs). After matching to bony structures, we analyzed the dimensions of remaining systematic errors from zero up to six initial IGRT sessions and aimed at a restriction of daily IGRT for 10% of all patients. For threshold definition, we determined the standard deviations (SD) of the shift corrections and selected patients out of this range for daily image guidance. To calculate total treatment margins, we demanded for a cumulative clinical target volume (CTV) coverage of at least 95% of the specified dose in 90% of all patients.

Results

The gain of accuracy was largest during the first three IGRTs. In order to match precision and workload criteria, thresholds for the SD of the corrections of 3.5 mm, 2.0 mm and 4.5 mm in the left-right (L-R), cranial-caudal (C-C), and anterior-posterior (A-P) direction, respectively, were identified. Including all other error types, the total margins added to the CTV amounted to 8.6 mm in L-R, 10.4 mm in C-C, and 14.4 mm in A-P direction.

Conclusion

Only initially performed IGRT might be helpful for eliminating gross systematic errors especially after virtual simulation. However, even with daily IGRT performance, a substantial PTV margin reduction is only achievable by matching internal markers instead of bony anatomical structures.     

前列腺癌放疗定位中膀胱体积的变化对摆位重复性的影响

目的 研究前列腺癌放疗定位膀胱体积对放疗中膀胱体积的一致性和摆位精度的影响,为临床实践提供参考。方法 回顾性选取2015年8月至2020年11月在中山大学肿瘤防治中心进行调强放疗的66例前列腺癌患者,患者在CT定位及治疗前自主憋尿后进行定位扫描或执行放疗,每次放疗前行锥形束计算机体层(CBCT)扫描获得左右、头脚和前后平移方向误差。在CT模拟定位影像和CBCT影像上勾画膀胱轮廓并计算体积,根据CT定位影像上膀胱体积进行分组,200~300 ml组18例、300~400 ml组24例、>400 ml组24例,分析CT定位膀胱体积对放疗过程中CBCT膀胱体积相对计划体积的变化百分比和摆位误差的影响。结果 200~300 ml组放疗中膀胱体积减少15%,300~400 ml组放疗中膀胱体积减少26%,>400 ml组放疗中膀胱体积减少32%,3组膀胱体积变化百分比两两比较差异均有统计学意义(Z=3.43、7.97、4.83,P<0.05)。三维平移方向摆位误差比较:头脚方向差异有统计学意义(H=26.72,P<0.05),左右、前后方向无统计学意义(P>0.05)。头脚方向摆位误差分别为200~300 ml组:0.00(-0.20,0.20)cm; 300~400 ml组:0.00(-0.20,0.30)cm;>400 ml组: -0.10(-0.30,0.20)cm。>400 ml组在头脚方向摆位误差大于其余两组,差异有统计学意义(Z=4.17、4.66,P<0.05),其余差异无统计学意义(P>0.05)。结论 模拟定位时膀胱充盈容积控制在200~300 ml,有利于患者在放疗中保持膀胱体积一致性及减少放疗时的摆位误差。     

Comparison between manual and automatic image registration in image-guided radiation therapy using megavoltage cone-beam computed tomography with an imaging beam line for prostate cancer

This study aimed to compare and assess the compatibility of the bone-structure-based manual and maximization of mutual information (MMI)-algorithm-based automatic image registration using megavoltage cone-beam computed tomography (MV-CBCT) images acquired with an imaging beam line. A total of 1163 MV-CBCT images from 30 prostate cancer patients were retrospectively analyzed. The differences between setup errors in three directions (left–right, LR; superior–inferior, SI; anterior–posterior, AP) of both registration methods were investigated. Pearson’s correlation coefficients (r) and Bland–Altman agreements were evaluated. Agreements were defined by a bias close to zero and 95% limits of agreement (LoA) less than ±?3 mm. The cumulative frequencies of the absolute differences between the two registration methods were calculated to assess the distributions of the setup error differences. There were significant differences (p?<?0.001) in the setup errors between both registration methods. There were moderate (SI, r?=?0.45) and strong positive correlation coefficients (LR, r?=?0.74; AP, r?=?0.72), whereas the 95% LoA (bias?±?1.96?×?standard deviation of the setup error differences) were ??1.61?±?4.29 mm (LR), ??0.41?±?5.45 mm (SI), and 0.67?±?4.29 mm (AP), revealing no agreements in all directions. The cumulative frequencies (%) of the cases with absolute setup error differences within 3 mm in each direction were 80.83% (LR), 81.86% (SI), and 90.71% (AP), with all directions having large proportions of >?3-mm differences. The MMI-algorithm-based automatic registration is not compatible with the bone-structure-based manual registration and should not be used alone for prostate cancer.     

Impact of inter- and intrafraction deviations and residual set-up errors on PTV margins

Purpose

The aim of this work was to analyze interfraction and intrafraction deviations and residual set-up errors (RSE) after online repositioning to determine PTV margins for 3 different alignment techniques in prostate cancer radiotherapy.

Methods

The present prospective study included 44 prostate cancer patients with implanted fiducials treated with three-dimensional (3D) conformal radiotherapy. Daily localization was based on skin marks followed by marker detection using kilovoltage (kV) imaging and subsequent patient repositioning. Additionally, in-treatment megavoltage (MV) images were obtained for each treatment field. In an off-line analysis of 7,273 images, interfraction prostate motion, RSE after marker-based prostate localization, prostate position during each treatment session, and the effect of treatment time on intrafraction deviations were analyzed to evaluate PTV margins.

Results

Margins accounting for interfraction deviation, RSE and intrafraction motion were 14.1, 12.9, and 15.1 mm in anterior–posterior (AP), superior–inferior (SI), and left–right (LR) direction for skin mark alignment and 9.6, 8.7, and 2.6 mm for bony structure alignment, respectively. Alignment to implanted markers required margins of 4.6, 2.8, and 2.5 mm. As margins to account for intrafraction motion increased with treatment prolongation PTV margins could be reduced to 3.9, 2.6, and 2.4 mm if treatment time was ≤?4 min.

Conclusion

With daily online correction and repositioning based on implanted fiducials, a significant reduction of PTV margins can be achieved. The use of an optimized workflow with faster treatment techniques such as volumetric modulated arc techniques (VMAT) could allow for a further decrease.     

电子射野影像仪与锥形束CT用于胸部肿瘤 影像引导放疗的比较研究

目的 比较电子射野影像仪(EPID)和锥形束CT(CBCT)用于胸部肿瘤影像引导放疗,在工作流程和发现患者摆位误差两个方面为临床选择不同影像引导放疗工具提供依据。方法 选择2007年3月至2008年1月在我院接受根治性放疗的17例胸部恶性肿瘤患者(包括肺癌、食管癌和胸腺瘤),每位患者每周分别行千伏锥形束CT(KVCBCT)和EPID影像引导分析各1次。1例患者(肺癌)在完成2次KVCBCT在线引导放疗后自动退出研究,共有16例患者进入最终研究分析。结果 16例患者共获取81对EPI和CBCT影像。采用CBCT引导放疗系统时,患者的治疗总时间较采用EPID引导放疗系统时增加1.2 min。采用EPID引导放疗技术分析胸部肿瘤患者的摆位误差,患者在左右(LR)、头脚(SI)和前后(AP)3个方向上的摆位误差分别为:(-0.1±3.2)mm、(1.3±3.7)mm和(-0.2±3.1)mm。计算临床靶体积(CTV)到计划靶体积(PTV)的预留边界,CTV到PTV的预留边界应设定为10mm。采用KVCBCT引导放疗技术分析这部分患者的摆位误差,LR、SI和AP 3个方向上的摆位误差分别为:(0.1±4.6)mm、(0.6±4.0)mm和(-0.9±4.6)mm,CTV到PTV的预留边界应设定为12mm。结论 与EPID相比,采用CBCT引导放疗系统没有明显延长治疗时间,但增加了发现摆位误差的能力,建议有条件的单位选择CBCT进行胸部肿瘤患者的影像引导放疗或摆位误差分析。     

Stereotactic radiotherapy for lung and liver tumors using a body cast system: setup accuracy and preliminary clinical outcome

PURPOSE: To evaluate the feasibility and treatment outcomes of stereotactic radiotherapy (SRT) using a newly developed simple body cast system for lung and liver tumors. MATERIALS AND METHODS: From April 2003 to July 2004, 20 patients were treated with SRT at the Kyushu University Hospital. Thirteen patients had primary lung cancer, 5 had metastatic lung cancer, and 2 had hepatocellular carcinoma. All patients were fixed with a thermoplastic body cast combined with a vacuum pillow, arm and leg support, and a carbon plate. SRT was given in 5-8 fields with an isocenter dose of 48-60 Gy in 4-10 fractions. Target verification was performed by computed tomography (CT) during the first session, and by anterior-posterior (A-P) and lateral portal images during the second and subsequent sessions. RESULTS: The average setup errors and deviation in the first treatment session were 1.4 +/- 1.2, 1.1 +/- 1.0, and 3.3 +/- 2.8 mm in the lateral, A-P, and cranio-caudal (C-C) directions, respectively. The setup errors in the second and subsequent sessions were 2.4 +/- 0.5, 1.4 +/- 1.8, and 3.7 +/- 2.6 mm in the lateral, A-P, and C-C directions, respectively. The patient's movement during a treatment session was within 5 mm in any direction. Despite the short follow-up periods (1-15 months), complete response was shown in 4 lesions, and partial response was shown in 15 lesions. Neither local progression nor serious complication was observed in any patient. CONCLUSION: SRT using our body cast system was a safe and reliable treatment method for extracranial tumors.     

An assessment of clinically optimal gold marker length and diameter for pelvic radiotherapy verification using an amorphous silicon flat panel electronic portal imaging device

Verification of target organ position is essential for the accurate delivery of conformal radiotherapy. Megavoltage electronic portal imaging with flat panel amorphous silicon detectors delivers high quality images that can be used for verification of bony landmark position. Gold markers implanted into the target organ can be visualized and used as a surrogate of actual organ position. On-line compensation for marker displacement, by adjusting patient position, can reduce geometric errors associated with radiation delivery. This study assesses the optimal marker length and diameter to be used with an amorphous silicon (a-Si) flat panel detector and electronic portal images (EPIs), prior to implementation of a clinical programme of gold marker insertion in prostate cancer patients. Seven marker sizes varying from 3 mm to 8 mm in length and 0.8 mm to 1.1 mm in diameter were investigated in a group of patients undergoing pelvic radiotherapy using an 8 MV Elekta SL20 linear accelerator. Markers were placed on the skin entry and exit sites of the treatment beam and EPIs in both lateral and anterior pelvic views were acquired. Three observers independently assessed visibility success and failure using a subjective scoring system. Markers less than 5 mm in length or 0.9 mm in diameter were poorly visualized (<70% visualization success in lateral EPIs). The marker measuring 0.9 mm x 5 mm appears to be clinically optimal in pelvic radiotherapy patients (80% visualization success in lateral EPIs) and will be used for actual organ implantation.     

胸部肿瘤常规放疗摆位偏差的测量与分析

目的　确定胸部肿瘤常规放射治疗时的摆位偏差。方法　使用电子射野影像装置(EPID)对 2 1例胸部肿瘤病人常规放射治疗时所拍摄的 40 8幅射野图像 ,与计划系统生成的标准射野数字重建 (DRR)图像进行了比较 ,并对病人摆位的横向 (RL)和纵向 (SI)及前后 (AP)轴旋转角度的偏差进行了测量。结果　没有采用任何固定装置治疗时 ,RL和SI方向摆位的平均平移偏差分别为(0 7± 3 1 )mm和 (1 5± 4 1 )mm ;平均旋转偏差为 (0 3± 2 4)°。当使用体膜固定治疗时 ,RL和SI方向摆位的平均平移偏差分别为 (0 5± 2 4)mm和 (0 8± 2 7)mm ;平均旋转偏差为 (0 2± 1 6)°。结论　胸部肿瘤放射治疗时采用体部固定装置可降低摆位偏差。SI方向的摆位偏差大于RL方向的摆位偏差。摆位偏差的主要来源是随机误差     

An IGRT margin concept for pelvic lymph nodes in high-risk prostate cancer

Purpose

Gold-marker-based image-guided radiation therapy (IGRT) of the prostate allows to correct for inter- and intrafraction motion and therefore to safely reduce margins for the prostate planning target volume (PTV). However, pelvic PTVs, when coadministered in a single plan (registered to gold markers [GM]), require reassessment of the margin concept since prostate movement is independent from the pelvic bony anatomy to which the lymphatics are usually referenced to.

Methods

We have therefore revisited prostate translational movement relative to the bony anatomy to obtain adequate margins for the pelvic PTVs compensating mismatch resulting from referencing pelvic target volumes to GMs in the prostate. Prostate movement was analyzed in a set of 28 patients (25 fractions each, totaling in 684 fractions) and the required margins calculated for the pelvic PTVs according to Van Herk’s margin formula \(M=2.5\Upsigma +1.64\left (\sigma^{\prime}-\sigma _{p}\right )\).

Results

The overall mean prostate movement relative to bony anatomy was 0.9 ± 3.1, 0.6 ± 3.4, and 0.0 ± 0.7?mm in anterior/posterior (A/P), inferior/superior (I/S) and left/right (L/R) direction, respectively. Calculated margins to compensate for the resulting mismatch to bony anatomy were 9/9/2?mm in A/P, I/S, and L/R direction and 10/11/6?mm if an additional residual error of 2?mm was assumed.

Conclusion

GM-based IGRT for pelvic PTVs is feasible if margins are adapted accordingly. Margins could be reduced further if systematic errors which are introduced during the planning CT were eliminated.

     

Patient motion correction for multiplanar, multi-breath-hold cardiac cine MR imaging

PURPOSE: To correct for spatial misregistration of multi-breath-hold short-axis (SA), two-chamber (2CH), and four-chamber (4CH) cine cardiac MR (CMR) images caused by respiratory and patient motion. MATERIALS AND METHODS: Twenty CMR studies from consecutive patients with separate breath-hold 2CH, 4CH, and SA 20-phase cine images were considered. We automatically registered the 2CH, 4CH, and SA images in three dimensions by minimizing the cost function derived from plane intersections for all cine phases. The automatic alignment was compared with manual alignment by two observers. RESULTS: The processing time for the proposed method was <20 seconds, compared to 14-24 minutes for the manual correction. The initial plane displacement identified by the observers was 2.8 +/- 1.8 mm (maximum = 14 mm). A displacement of >/=5 mm was identified in 15 of 20 studies. The registration accuracy (defined as the difference between the automatic parameters and those obtained by visual registration) was 1.0 +/- 0.9 mm, 1.1 +/- 1.0 mm, 1.1 +/- 1.2 mm, and 2.0 +/- 1.8 mm for 2CH-4CH alignment and SA alignment in the mid, basal, and apical regions, respectively. The algorithm variability was higher in the apex (2.0 +/- 1.9 mm) than in the mid (1.4 +/- 1.4 mm) or basal (1.2 +/- 1.2 mm) regions (ANOVA, P < 0.05). CONCLUSION: An automated preprocessing algorithm can reduce spatial misregistration between multiple CMR images acquired at different breath-holds and plane orientations.