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Year : 2014  |  Volume : 10  |  Issue : 4  |  Page : 932-936

A comparison between four immobilization systems for pelvic radiation therapy using CBCT and paired kilovoltage portals based image-guided radiotherapy

1 Department of Radiation Oncology, Fortis International Oncology Center, Noida, India
2 Department of Radiation Oncology, International Oncology Center, Fortis Hospital, Noida, National Capital Region, India
3 Department of Radiation Oncology, Max Cancer Center, Max Super Specialty Hospital, New Delhi, India
4 Max Hospital, Apothecaries Pvt. Limited, Patparhganj and Saket, New Delhi, India

Date of Web Publication9-Jan-2015

Correspondence Address:
Gagan Saini
Department of Radiation Oncology, Fortis International Oncology Center, Sector - 62, Noida, National Capital Region
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0973-1482.138026

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 > Abstract 

Introduction: We commonly use 6- and 4-clamped thermoplastic molds (TMs) for rigid immobilization during pelvic radiotherapy (RT), sometimes a vacuum cushion (VC) is also used as leg support with TM. Our objective was to report the setup margins (SMs) associated with the different systems, to analyze whether any of these systems is superior, and to analyze whether any of them showed better reproducibility in any particular direction.
Materials and Methods: Retrospective analysis was done by dividing the patients into four groups: 6-clamp with VC (6CVC), 6-clamp without VC (6CNC), 4-clamp with VC (4CVC), and 4-clamp without VC (4CNC). A repeat offline review was done for all patients and errors were tabulated. Statistical methods were then applied.
Results: Total 24 patients had 413 image-guided RT (IGRT) sessions, 312 were cone beam computed tomography scan (CBCT) scans and 101 were paired kilovoltage portals (kVp). There was no statistically significant difference between 6CVC and 6CNC. However, while comparing 4CVC and 4CNC, a statistically significant difference was seen in all directions. VC improved precision in vertical and lateral direction mainly, while the 6-clamped TM improved reproducibility in longitudinal direction.
Conclusions: SM was low for all the four immobilization systems studied. There is no added benefit of using a VC with 6-clamped TM for pelvic RT. Use of a VC is recommended with 4-clamped TM to improve overall reproducibility. 6-clamped TM helps keep the errors low.

 > Abstract in Chinese 



结果:共24例病人,413次图像引导放疗(IGRT),312次是锥形束计算机断层扫描(CBCT),101次是配对千伏门户(KVP)。6cnc和 6cvc之间没有统计上的显着差异。然而,当比较4cvc和4cnc时,在所有方向上看到的都有统计学差异。真空垫提高在垂直和横向方向为主的精度,而6卡模具模改进轴方向可重复性。

结论:所有研究的4制动系统中设置范围都是低的。使用真空垫加 6卡模具对盆腔放疗没有额外的好处。推荐使用真空垫加 4卡模具以提高全方位的重复性。6卡模具有助于保持低误差。


Keywords: Error, image-guided RT, knee rest, margin, planning target volume

How to cite this article:
Saini G, Aggarwal A, Jafri SA, Goel V, Ranjitsingh T, Munjal R, Anand AK. A comparison between four immobilization systems for pelvic radiation therapy using CBCT and paired kilovoltage portals based image-guided radiotherapy. J Can Res Ther 2014;10:932-6

How to cite this URL:
Saini G, Aggarwal A, Jafri SA, Goel V, Ranjitsingh T, Munjal R, Anand AK. A comparison between four immobilization systems for pelvic radiation therapy using CBCT and paired kilovoltage portals based image-guided radiotherapy. J Can Res Ther [serial online] 2014 [cited 2019 Nov 17];10:932-6. Available from: http://www.cancerjournal.net/text.asp?2014/10/4/932/138026

 > Iintroduction Top

The basic aim of radiation therapy (RT) planning is to ensure adequate coverage of target with prescribed dose and to save as much normal tissues as possible.

Considerable evolution has taken place to meet these ends in the form of conformal techniques like three-dimensional conformal RT (3DCRT), intensity-modulated RT (IMRT), and volumetric-modulated RT (like RapidArc; ). With these highly conformal techniques, knowledge of margins for setup errors is essential. [1],[2],[3] While adequate margins for setup errors are necessary, the margins applied must not be huge as that would adversely affect normal tissue sparing obtained with IMRT. [1],[2],[4] Rabinowitz et al., [5] have reported large setup variations in pelvis of up to 1.19 cm during pelvic RT. Many studies in the past have discussed immobilization of pelvis to reduce these margins. [6],[7],[8],[9],[10],[11] These reports have discussed rigid immobilization systems like alpha-cradle, hemibody cast, hipfix system, foam cradle, and thermoplastic molds (TMs). While some studies do not report any advantage of using immobilization, [6] others show that use of rigid immobilization significantly reduces the setup errors. [7],[8],[9],[10],[11] Use of leg separator or an ankle immobilizer has also shown to decrease setup errors significantly. [3],[12] TMs are being commonly used to reduce setup errors in modern radiation oncology departments. TMs used in our department for abdominopelvic immobilization are available with four or six clamps. For some patients a vacuum cushion (VC) is also made along with the TM to act as a leg separator. Therefore, either a 6-clamped TM was used or a 4-clamped one. They were used either alone or with VC. It was a felt need to understand the errors associated with each of these systems.

Most studies on setup errors are based on analyses performed on EPID (electronic portal imaging devices) images using bony landmarks or implanted markers. [3],[6],[7],[8],[9],[10],[11],[12] With the advent of cone beam computed tomography (CBCT) a three-dimensional image of the patient can be obtained in the treatment position. This image coregistered with planning CT scan gives considerable insight into the overall setup inaccuracy in three dimensions. This is important since rotational errors are common during pelvic RT. [5],[13],[14] The purpose of this study was to report the errors of immobilization and setup associated with each of these four different systems in all the three directions, to analyze whether any of these systems is superior to the others by comparing the differences in error values obtained amongst the four groups. We also aimed to analyze whether any of the systems showed better reproducibility than the other by minimizing errors in any particular direction.

 > Materials and methods Top

Our department is equipped with Varian linear accelerator with On-Board Imager (Varian Medical Systems Inc, Palo Alto, CA). As a part of treatment protocol at our department, all patients receiving pelvic RT are setup on an All-in-one; (AIO; Orifit Industries, Belgium) black board with a suitable head rest and arms above one's head resting on a blue arm rest. The abdomen and pelvis are then immobilized with a TM using either a 6- or 4-clamped type. In some patients a VC is also made, which acts as a leg support. The above-mentioned immobilization system was indexed to the couch, the indexing was fixed for each patient and reproduced in each sitting. For the purpose of this retrospective analysis we categorized the patients into four categories: 6-clamped TM with VC (6CVC), 6-clamped TM without VC (6CNC), 4-clamp with VC (4CVC), and 4-clamp without VC (4CNC) [Figure 1]. Planning was done using Eclipse; planning system (Varian Medical Systems Inc, Palo Alto, CA). As a part of departmental protocol, at our department all patients receiving pelvic RT underwent CBCT scan and kilovoltage portal (kVp) imaging using On-Board Imager on regular basis for image-guided RT (IGRT). For the purpose of this study, all the chosen patients were subjected to a repeat Offline Review; (Varian Medical Systems Inc, Palo Alto, CA). This offline review was done as per agreed method as discussed below.
Figure 1: A patient immobilized with a thermoplastic mold and a vacuum cushion. The vacuum cushion can be seen to be used as a leg separator

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Offline review

Before starting the study, a methodology of offline review was agreed upon within the team of authors. Since this was a study to analyze immobilization and setup errors associated with the use of different immobilization systems, the target to be matched was primarily bones of the pelvis.

For matching or coregistering CBCT scans with planning scans during offline review, the following methodology was used: The first correction to be done was for mismatch in longitudinal direction in the sagittal view, in the section passing approximately through the midline of sacrum. This served as a starting point. Then, the axial cuts were viewed and corrections were done in lateral and vertical directions starting from the superior most cut and then scrolling towards the inferior direction. The reviewer had to scroll back to the superior cuts to verify the corrections. If any alteration in matching was done again, while scrolling in the superior direction, it was to be verified by scrolling down and by visualizing the inferior sections again. The abovementioned methodology was adopted because when mismatch is present in the form of rotations, the corrections done in any one direction like vertical or lateral can lead to complete mismatch in others. Since the purpose of the study was to obtain planning target volume (PTV) margins in the three dimensions for day-to-day use we decided to match the images in the three dimensions only, no rotational match was attempted.

For matching paired kVp the following methodology was used, the first correction was to be done in the longitudinal direction followed corrections in lateral and vertical corrections.

Computation of data

The values were obtained from Offline Review; were considered as errors and the data was tabulated separately for vertical, longitudinal, and lateral directions and subjected to analysis. The "error" values obtained describe any deviation between planned and executed position for an immobilized patient.

Statistical analysis

All the errors were tabulated in each direction for all sessions, and the mean errors and their standard deviation (SD) were calculated. The mean error in each direction for each patient was calculated separately. From these means that were calculated for each patient, a mean value (M) was calculated along with its SD, which was taken as the systematic error in each direction (∑). The root means square (RMS) of previously derived SD values was calculated for each direction to arrive at the group mean of SDs and was reported as random errors (σ). One-way analysis of variance (ANOVA) and unpaired t-test were used to compare within and amongst the groups.

 > Results Top

The total number of patients in this study was 24, of which most patients were females [Table 1]. The total number of available IGRT sessions was 413, out of which 312 were CBCT scans and 101 were paired kVps. On an average, there were 17 IGRT sessions per patient. The minimum number of sessions for individual patients was nine with a maximum of 27. There was no patient with only kVp data. All sessions were reviewed again as described before and error in each direction for each session for each of the groups was tabulated. Coefficient of variance (CV) was calculated to understand the extent of variations of all the errors in all the directions [Table 2]. One-way ANOVA test was utilized for comparing error values within each group, considering the source of variation to be the error values obtained for all the directions, the results were significant for all groups.
Table 1: Gender distribution in various groups studied

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Table 2: Statistical Analysis of the error values obtained for the four groups

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The results of one-way ANOVA were calculated within each of the three directions (vertical, longitudinal, and lateral), while considering the error values obtained in all groups for each direction as a source of variation, these were found to be statistically significant for all direction [Table 3].
Table 3: Statistical Analysis of error values obtained for each direction

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For each direction separately, two groups at a time were compared with each other using unpaired t-test and values were reported [Table 3]. The systematic and random errors have also been reported separately.

 > Discussion Top

Daily setup errors are an inherent part of conventionally fractionated RT. Setup errors during RT for pelvic malignancies are perhaps larger than for any other site in body. [5] These large errors not only have the potential to alter the day-to-day dose delivery to the target, but accounting for large errors by applying margins can offset dosimetric advantage of highly conformal treatment modalities like IMRT, RapidArc, etc. [1],[2],[4] Indeed a failure in delivering prescribed radiation doses to the target volume can ultimately lead to a lower cure rate. [16],[17] Therefore, knowledge of setup errors associated with day-to-day clinical practice is important.

Nowadays, TMs are being used regularly for immobilization for pelvic RT. These are available in 6- and 4-clamps types and are used in our department with or without a VC for leg support. In our department, it was assumed that 6-clamped TM would be more effective than its 4-clamped counterpart for immobilization for pelvic RT and a VC whenever added as a leg support would add to the overall precision. With this study, we aim to objectively address the abovementioned assumptions and to report setup margins (SM) associated with each of these devices.

For the purpose of standardization of our results, it was decided that all patients shall be reviewed again. The purpose of this study was to report errors in immobilization and setup associated with each of these systems and to compare their precision with each other, hence it was decided to use bony landmarks during matching. Rotational errors are very common during pelvic RT, [5],[13],[14] a common methodology employed during matching and computing errors is to ignore them. [18],[19] The reason for this practice is the fact that it is not possible to apply all the corrections obtained to the patient due to clinical limitations and limitations of the treatment couch. [18] The CBCT images aid in understanding the three-dimensional geometry of the patient and hence help us appreciate the rotational errors better. Since this was a retrospective study and the aim was to understand SM related to each immobilization system, it was planned to incorporate corrections for rotational errors within the translational errors themselves as described before.

The average shifts noted [Table 2] in our patients ranged from - 0.02 to 0.18 mm, irrespective of the group. A past study investigating the importance of leg immobilization reported mean values ranging from 0.3 to -1.55 mm, [6] another study which reported results of errors with pelvic immobilization reported errors ranging between - 0.1 and 0.4 mm. [7] Our reported averages are very low as compared with the above results. The maximum single value of error in our data is 2.9 cm seen in longitudinal direction.

A PTV margin (for setup error) of 0.5-1 cm is generally recommended during pelvic RT. [21] The maximum percentage of instances in which the error was greater than 0.5 cm was observed in the longitudinal direction (23.85%) with 4CNC. We subjected each individual group to one-way ANOVA considering the sources of variation as the tabulated error values individually obtained for each of the three directions. The result obtained was statistically significant for all groups [Table 2]. This means that differences in errors for all the three directions for each group are indeed statistically significantly different from each other. In each group, the CV indicates that the longitudinal errors are the maximum [Table 3]. The rates of errors >0.5 cm, while using pelvic immobilization as reported previously by other authors are high with values of 53.2% by Kneebone et al., and Fiorino et al., [5],[6] an even higher rate of up to 66% has been reported in an older report. [1] In our study, the instances of errors of >0.5 cm were comparatively lower.

Further, we checked the influence of use of different immobilization systems upon errors in each direction. For comparing all the four groups one-way ANOVA was applied, while considering the error values obtained in all groups for each direction as a source of variation. The P values obtained were statistically significant for all directions [Table 2], which rejects the null hypotheses and means that the use of immobilization device effects the errors obtained in each direction.

To compare the errors obtained for each direction amongst different groups we applied the unpaired Student's t-test and tested for the influence of immobilization system on errors in each of the directions [Table 3]. It is evident from the table that the P values obtained are not statistically significant in any direction when comparing between 6CVC and 6CNC, however, while comparing between 4CVC and 4CNC there is a statistically significant difference in all directions.

Analyzing the values obtained with unpaired Student's t-test in light of a statistically significant one-way ANOVA values obtained for each direction, we can make a few conclusions. With regards to the vertical direction, we observed that 6CVC contributed least errors in vertical direction. With regards to longitudinal direction, we found that the 4CNC group fared the worst. Both the groups with 6-clamped TM, that is, 6CVC and 6CNC were better than their 4-clamped counterparts. Thus, 6-clamped TM can be seen to be superior with regards to errors obtained in longitudinal direction. For the lateral direction, we can again see that 4CNC group did worst. It is, however, interesting to see that 6CVC, 6CNC, and 4CVC fare similarly. We can infer, therefore, that presence of 6-clamp system or a presence of VC helps keep errors in lateral direction in check. The aforementioned results clearly point out that when a 6-clamped TM is being used, there is no need for leg immobilization with VC, however, if use of a 4-clamp TM is being planned then using a leg immobilization like a VC adds to the precision in a statistically significant manner for all directions. It has been reported in past [8],[9] that setup accuracy improved simply by the use of leg support. These authors did not use any thermoplastic immobilization or any other rigid immobilization and reported a superior setup only by an additional use of leg support. The above results also indicate that 4CNC fares the worst with regards to errors in each direction and that a leg support indeed adds to the overall precision when a 4-clamp mold is used. Perhaps an added leg support is not useful with six clamps since the AIO board itself acts as a leg separator [Figure 1]. Presence of 6-clamp mechanism helps minimize errors in all three directions in general. In a past report by Song et al., where they compared four immobilization systems, they did not find any differences in errors recorded in different systems. [6] As proposed by van Herk, [15] systematic and random errors were calculated for all the groups [Table 4]. It is discussed that the systematic errors should be close to zero and that the component of random error should be more than that of systematic error. We notice that in our results, the systematic errors are very low and random errors are higher.
Table 4: Values of group systematic and random errors calculated (cm)

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In this era of modern radiation techniques of planning and treatment, we understand that setup errors are not the only source of errors and consideration of various other sources of errors are important for computation of PTVs. As per a report by van Herk et al., apart from setup errors, organ motion, respiratory motion, and errors in delineation contribute to calculation of PTV. [20] The tables provided by our report can be useful to understand the SM and can aid in calculation of PTV by suitably accommodating other parameters. [21]

Our study of comparison of four common immobilization systems for pelvic RT is indeed a need of the day. To our understanding, such an analysis has not been reported in the recent era of IGRT using On-Board Imager. One of the important drawbacks of our data is that it is retrospective and the patient numbers are not well-matched. However, TMs are being commonly used in radiation oncology departments all over and an insight into their accuracy is likely to influence our day-to-day practices. Our study not only helps us appreciate the errors associated with these systems, but also helps us appreciate the overall accuracy of the immobilization systems.

 > Conclusions Top

The SMs for all the four immobilization systems studied were low. When using a 6-clamp TM for pelvic RT there is no added benefit of using a feet separator like VC. However, when using a 4-clamped TM use of a VC is recommended to improve overall reproducibility. Use of 6-clamped TM helps keep the errors low.

 > Acknowledgment Top

We would like to thank Dr. Kanika Sharma Sood, Dr. Rachna Jain and Mr. Bharat Arora (patient care coordinator) at Department of Radiation Oncology, Max Cancer Center; New Delhi for their invaluable contribution to the work.

 > References Top

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Bentel GC, Marks LB, Sherouse GW, Spencer DP, Anscher MS. The effectiveness of immobilization during prostate irradiation. Int J Radiat Oncol Biol Phys 1995;31:143-8.  Back to cited text no. 7
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Kneebone A, Gebski V, Hogendoorn N, Turner S. A randomized trial evaluating rigid immobilization for pelvic irradiation. Int J Radiat Oncol Biol Phys 2003;56:1105-11.  Back to cited text no. 9
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Catton C, Lebar L, Warde P, Hao Y, Catton P, Gospodarowicz M, et al. Improvement in total positioning error for lateral prostatic fields using a soft immobilization device. Radiother Oncol 1997;44:265-70.  Back to cited text no. 12
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  [Figure 1]

  [Table 1], [Table 2], [Table 3], [Table 4]


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