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 » Introduction
 »  Materials and Me...
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  Table of Contents  
Year : 2020  |  Volume : 57  |  Issue : 2  |  Page : 182-186

Comparison of thermoplastic masks and knee wedge as immobilization devices for image-guided pelvic radiation therapy using Cone Beam Computed Tomography

Department of Radiation Oncology, Gujarat Cancer Research Institute, Ahmedabad, Gujarat, India

Date of Submission09-Sep-2018
Date of Decision19-Apr-2019
Date of Acceptance29-Apr-2019
Date of Web Publication17-May-2020

Correspondence Address:
Mridul Anand
Department of Radiation Oncology, Gujarat Cancer Research Institute, Ahmedabad, Gujarat
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/ijc.IJC_602_18

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 » Abstract 

Introduction: Pelvic radiotherapy is generally performed with the use of an immobilization and positioning device.
Aim and objective: The objective of the study was to ascertain and compare setup errors between the two positioning devices.
Materials and methods: A total of 35 patients of stage II and III cervical cancers were enrolled in the study and divided into two groups, one using knee wedge and the other using thermoplastic pelvic mask as an immobilization device. Radiation was planned by four field box conformal technique. The random and systematic setup errors were then calculated for each patient in both the groups in the mediolateral (ML), superoinferior (SI), and anteroposterior (AP) directions.
Results: The translational mean setup variation in the lateral, longitudinal, and vertical direction is 0.17 ± 0.24, −0.12 ± 0.48, and −0.18 ± 0.27 cm for thermoplastic pelvic mask and −0.03 ± 0.26, −0.04 ± 0.48, and −0.09 ± 0.37 cm for knee wedge, respectively. The systematic setup error and random errors were 0.24, 0.48, 0.27 cm and 0.31, 0.60, and 0.40 cm for thermoplastic mask and 0.26, 0.48, and 0.37 cm and 0.38, 0.37, and 0.45 cm for knee wedge in ML, SI, and AP axis, respectively. The one way analysis of variance test was applied to compare the setup errors in between the three axes for both the immobilization devices. To compare the positioning accuracy of thermoplastic mask and knee wedge, Student's t-test was applied. Both the tests were found to be insignificant (P value > 0.05).
Conclusion: Thermoplastic mask and knee wedge are equally effective as immobilization devices for treating cervical cancers with conformal techniques.

Keywords: Image-guided radiotherapy, knee wedge, pelvic immobilization

How to cite this article:
Anand M, Parikh A, Shah SP. Comparison of thermoplastic masks and knee wedge as immobilization devices for image-guided pelvic radiation therapy using Cone Beam Computed Tomography. Indian J Cancer 2020;57:182-6

How to cite this URL:
Anand M, Parikh A, Shah SP. Comparison of thermoplastic masks and knee wedge as immobilization devices for image-guided pelvic radiation therapy using Cone Beam Computed Tomography. Indian J Cancer [serial online] 2020 [cited 2021 Jan 15];57:182-6. Available from:

 » Introduction Top

The objective of radiation treatment is to deliver the radiation to the target area as accurately as planned. This reproducibility is generally difficult to maintain, more so in fractionated radiotherapy schedules. With the advent of conformal mode of radiation delivery, accuracy has become even more important and setup errors are known to occur.[1],[2],[3] The margins given for setup error have to be in optimal balance so that the uncertainties of daily reproducibility are met and the surrounding organs at risk are not irradiated to a higher extent.[1],[2],[4]

Pelvic malignancies have been reported to have large setup variations and various immobilization devices have been used to reduce the same.[5],[6],[7] Some studies have shown efficacy with different immobilization techniques, others have shown no difference with the use of such devices. Use of a leg separator or ankle immobilizer has shown to reduce the setup error significantly.[3],[8] For analysis of setup errors, online portal imaging or cone beam computed tomography (CT) can be used. The advantage of the latter is that it allows a three-dimensional image of the patient in treatment position, which leads to assessment of both rotational as well as translational setup error in three directions.

The main objective of this study was to calculate and compare the setup errors between two different immobilization devices.

 » Materials and Methods Top

A total of 35 patients of carcinoma of the cervix to be treated with radical radiotherapy and chemotherapy were included in the study. They were randomly divided to use two different immobilization devices. One arm of patients used thermoplastic device and the other arm used knee wedge.

All patients underwent treatment planning scan in supine position on carbon base plate using respective immobilization device. Skin marking to assure reproducibility was done. Bowel preparation and bladder protocols were adhered to in all the patients. A contrast-enhanced CT scan was taken after verifying the topogram. Three (3) mm contiguous cuts were taken on CT simulator and transferred to treatment planning system, Monaco. Target volumes were contoured according to the Radiation Therapy Oncology Group (RTOG) guidelines by radiation oncologist and given for planning. Four field boxes planning by 3DCRT was done for all patients.

At the time of verification, a kilo-voltage cone beam computed tomography (kV-CBCT) was done to match the treatment planning scan with the scan taken at the time of delivery of radiation. Thereafter, KV-CBCT was taken alternate or once a week as and when required. The setup error was calculated by registration of planning CT with the current CBCT using the bony anatomy after confining the volume for automatic bone match with the help of an alignment box. All translational and rotational errors were recorded in centimeters and shifts were applied. For the purpose of uniformity and analysis, anterior, superior, and right-sided shifts were coded as positive shifts and posterior, inferior, and left-sided shifts as negative shifts. Daily quality assurance was done to eliminate other potential sources of errors. The mean and standard deviations (SDs) were calculated for all patients in all three axes, i.e. x, y, and z for individually recorded errors.

Systematic and random errors were calculated as per conventionally defined norms.[9],[10] The systematic component of the displacement represents displacement that was present during the entire course of treatment and the random errors represent day to day variation in the daily setup.

Statistical analysis

Shifts on three axes: x-axis (mediolateral; ML), y-axis (superoinferior; SI), and z-axis (anteroposterior; AP) of individual patient were noted and entered in excel sheet to prepare master chart. The systematic of individual patient on particular axis was calculated by arithmetic mean of shifts on particular axes for total days of radiation received by particular patient (Σ-individual). The random error of individual patient on particular axis was calculated by standard deviation (SD) of shifts on particular axes for total days of radiation (σ-individual). Systematic error of population was found out by calculation of arithmetic mean of SE of individual patient (Σ-population), while random error of population was calculated as SD of random error of individual patient (σ-population). The random and systematic errors were then used to calculate the Clinical Tumor Volume-Planning Tumor Volume (CTV-PTV) margins by Van Herk's margin formula (2.5Σ-population + 0.7 σ-population).[11],[12]

 » Results Top

The translational errors were calculated for a total of 35 patients and 190 KV-CBCT were taken. The mean errors in ML, SI, and AP directions were −0.17, −0.12 and −0.18; −0.03, −0.04 and −0.09 cm for Orfit (thermoplastic mask) and knee wedge immobilization devices, respectively. This data is shown in [Table 1]. The systematic setup error and random errors were 0.24, 0.48, 0.27 cm; 0.31, 0.60 and 0.40 cm for Orfit and 0.26, 0.48, and 0.37 cm; 0.38, 0.37, and 0.45 cm for knee wedge in ML, SI, and AP axis, respectively. The analysis of variance (ANOVA) test was applied to compare the errors between the three different axes within each group and was found to be insignificant, i.e., the systematic and random setup errors did not differ with respect to the three directions. Student's unpaired t-test was applied to compare the errors between the two immobilization device and this was also found to be insignificant (P > 0.05).
Table 1: Mean setup error for thermoplastic mask and knee wedge in all three axes (cm)

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 » Discussion Top

The successful delivery of radiation therapy depends on accuracy and reproducibility of patient's position during fractionated radiotherapy. Many studies have proven that positioning of patients for pelvic radiotherapy is relatively inaccurate and subject to setup variations that are probably greater than other sites in the body.[6],[7] Pelvic area is relatively difficult to setup accurately due to many factors including motion of external skin marks relative to internal structures, the nonrigid nature of the area, patient rotation, and day to day variations in rectal and bladder filling.[6]

Uncertainties in radiation delivery can arise due to setup variation caused by both, interfraction and intrafraction motion, former being the greater of two components.[13] Patient setup error has a component of random and systematic variation. Systematic errors (Σ) are defined as variations that are persistent during the whole course of treatment. For individual patients, it is defined by the mean value of displacements along a specified coordinate. For the whole group, the distribution of systematic deviations is determined by the standard deviation of the values of the mean shifts of individual patients along a specified coordinate. Random errors (σ) are defined as variations that may occur by chance and correspond to day-to-day setup variations during the course of treatment, and are represented by the amount of dispersion of individual points around the mean.

Setup error accuracy is influenced by various factors like laser misalignment, skin mark movement, patient mobility, and the precision with which the radiation technologists are able to position the patient using anatomical reference mark.[7] Patient immobilization is proposed by many authors as the first solution to reduce setup errors. However, immobilization devices do not always eliminate all errors and some institutions have failed to find evidence of significant improvement in setup with the use of immobilization devices but the data are limited.

Numerous setup error studies have measured both systematic and random errors using different techniques. The aim of this study was to evaluate the patient systematic and random setup error during external beam radiotherapy of cervical cancer patients at our institute and compare them between the two immobilization devices, thermoplastic mask and knee wedge, used at our institute. The translational and rotational setup errors were recorded by using KV-CBCT imaging before each treatment fraction and automatic registration using bone matching was done for all patients. The systematic error in ML (x), SI (y), and AP (z) direction was found to be 0.24, 0.48 and 0.27 cm and 0.26, 0.48, and 0.37 cm for Orfit and knee wedge, respectively. The random error in ML (x), SI (y), and AP (z) direction was found to be 0.31, 0.60, and 0.40 cm and 0.38, 0.37, and 0.45 cm for Orfit and knee wedge, respectively. Similar studies were done by Santanam et al., Laursen et al., and Yao et al.[14],[15],[16] All this data has been evaluated for the translatational errors since rotational errors are mostly ignored. This is because it is not feasible to apply every correction due to clinical and treatment couch limitations.[17]

The random and systematic errors were then used to calculate the CTV-PTV margin with the help of Van Herk's margin formula (2.5 Σpop + 0.7 σpop).[11] This margin is needed to counteract the daily setup errors and ensure adequate coverage of the target, while reducing the dose to organs. Factors like different patient immobilization positions (prone or supine), varying measurement methods (electronic portal imaging device, kV-CBCT, Megavoltage Computed Tomography (MVCT), magnetic resonance imaging), measurement frequencies (daily or weekly), and different treatment techniques [3-Dimensional Conformal Radiotherapy (3-DCRT), Intensity Modulated Radiotherapy (IMRT), and Volumetric Arc Therapy (VMAT)] can all influence decisions regarding the optimal PTV. In the treatment of gynecologic malignancies, a wide range of different margins of PTV is observed in the published literature and this should be individualized for an institution given the earlier determining factors.[14],[15]

In our study, these margins were found to be 0.91, 1.45, and 1.23 cm and 0.81, 1.62, and 0.95 cm in ML, SI, and AP axis for Orfit and knee wedge immobilization devices, respectively. In a study by Yao et al., the margins calculated were 0.56, 0.83, and 0.76 cm along AP, SI, and ML direction.[16] Laursen et al. calculated margins of 1.16 AP, 0.82 SI, and 0.96 cm ML, respectively.[15] Li et al. studied daily MVCT images with online correction before treatment and no immobilization device was used for patients with pelvic malignancies. They calculated the CTV-PTV margins at 0.83 cm.[18] Santanam et al. recommend a 7 mm CTV-PTV margins in all directions when using daily imaging and daily setup corrections.[14] Lim et al. could show that a 5 mm margin might be appropriate for most patients treated with IMRT with the use of a small bowel displacement system, if daily setup control is used.[19] A study done by Patni et al. reported CTV to PTV margins as 0.58, 1.03, and 0.56 mm in AP, SI, and ML direction, respectively.[20] The findings of the present study have been summarized in [Table 2] and comparison with similar studies has been shown in [Table 3]. [Figure 1] highlights the percentage of patients with set up errors within +/- 0.5 cm in both the arms.
Table 2: Setup errors and margins required on the CTV (cm)

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Table 3: Comparison of data as obtained by our study and similar studies done in past (all values are in cm)

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Figure 1: Graph showing percentage of patients with average setup error within ±0.5 cm

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All these studies used IMRT planning for patients, using 0.5-1 cm CTV-PTV margin. In our institute, according to the institutional policy, we planned 3-Dimensional Conformal Radiotherapy (3-DCRT) box field for all patients and used a margin of 1 cm. Our study is consistent with other studies of similar nature, in the fact that the largest setup error has been found in longitudinal (y-axis) direction for both the immobilization devices and also, asymmetric CTV-PTV margins are to be applied for the three directions. Large longitudinal differences were also seen by Drabik et al. for other sites using MVCT scans for daily patient positioning.[21],[22] The margins estimated in this study are based only on setup errors and do not account for organ motion.

ANOVA test was applied for comparison between the three axes for both the devices and Student's t-test has been used for comparison between the two immobilization devices. Both the test results had P value >0.05 and hence were found to be statistically insignificant. This meant that the difference in errors between the three axes did not differ significantly in our patient series in both thermoplastic Orfit and knee wedge immobilization device. Also, the difference in the translational setup errors and CTV-PTV margins did not differ significantly between the two immobilization devices.

Thus, regarding the difference between the usage and superiority of one immobilization device over other, there was no significant difference found between the errors and the margins required while treating with the two devices. Given that the thermoplastic device is individualized for a patient and can be used for one patient only, knee wedge immobilization devices offers advantage of a single device being used for a large number of patients and can be considered as an economical option.

 » Conclusion Top

Image guidance in cervical cancers is required given the large variation of setup errors as noted by various studies. Comparison between the different immobilization devices has been attempted in a small number of studies. In our study, the setup margins were found to be consistent with those done in the past and yielded the institutional CTV-PTV margins. Kv-CBCT is a satisfactory method of accurate patient positioning in treating cervical cancers with high-precision techniques resulting in avoiding geographical miss. Also, the two immobilization devices available in the department do not differ in the needed setup margins, are equally effective, and thus can be used interchangeably.

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Conflicts of interest

There are no conflicts of interest.

 » References Top

Siebers JV, Keall PJ, Wu Q, Williamson JF, Schmidt-Ullrich RK. Effect of patient setup errors on simultaneously integrated boost head and neck IMRT treatment plans. Int J Radiat Oncol Biol Phys 2005;63:422-33.  Back to cited text no. 1
Song S, Yenice KM, Kopec M, Liauw SL. Image-guided radiotherapy using surgical clips as fiducial markers after prostatectomy: A report of total setup error, required PTV expansion, and dosimetric implications. Radiother Oncol 2012;103:270-4.  Back to cited text no. 2
Baumert BG, Zagralioglu O, Davis JB, Reiner B, Luetolf UM, Ciernik IF. The use of a leg holder immobilisation device in 3D-conformal radiation therapy of prostate cancer. Radiother Oncol 2002;65:47-52.  Back to cited text no. 3
Ahamad A, D'Souza W, Salehpour M, Iyer R, Tucker SL, Jhingran A, et al. Intensity-modulated radiation therapy after hysterectomy: Comparison with conventional treatment and sensitivity of the normal tissue sparing effect to margin size. Int J Radiat Oncol Biol Phys 2005;62:1117-24.  Back to cited text no. 4
Rabinowitz I, Broomberg J, Goitein M, McCarthy K, Leong J. Accuracy of radiation field alignment in clinical practice. Int J Radiat Oncol Biol Phys 1985;11:1857-67.  Back to cited text no. 5
Song PY, Washington M, Vaida F, Hamilton R, Spelbring D, Wyman B, et al. A comparison of four patient immobilization devices in the treatment of prostate cancer patients with three dimensional conformal radiotherapy. Int J Radiat Oncol Biol Phys 1996;34:213-9.  Back to cited text no. 6
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
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. 8
Hurkmans CW, Remeijer P, Lebesque JV, Mijnheer BJ. Set-up verification using portal imaging; review of current clinical practice. Radiother Oncol 2001;58:105-20.  Back to cited text no. 9
Stroom JC, Heijmen BJ. Geometrical uncertainties, radiotherapy planning margins, and the ICRU-62 report. Radiother Oncol 2002;64:75-83.  Back to cited text no. 10
Gupta T, Chopra S, Kadam A, Agarwal JP, Devi PR, Ghosh-Laskar S, et al. Assessment of three-dimensional set-up errors in conventional head and neck radiotherapy using electronic portal imaging device. Radiat Oncol 2007;2:44.  Back to cited text no. 11
Van Herk M, Remeijer P, Rasch C, Lebesque JV. The probability of correct target dosage: Dose-population histograms for deriving treatment margins in radiotherapy. Int J Radiat Oncol Biol Phys 2000;47:1121-35.  Back to cited text no. 12
Hanley J, Lumley MA, Mageras GS, Sun J, Zelefsky MJ, Leibel SA, et al. Measurement of patient positioning errors in three-dimensional conformal radiotherapy of the prostate. Int J Radiat Oncol Biol Phys 1997;37:435-44.  Back to cited text no. 13
Santanam L, Esthappan J, Mutic S, Klein EE, Goddu SM, Chaudhari S, et al. Estimation of setup uncertainty using planar and MVCT imaging for gynecologic malignancies. Int J Radiat Oncol Biol Phys 2008;71:1511-7.  Back to cited text no. 14
Laursen LV, Elstrøm UV, Vestergaard A, Muren LP, Petersen JB, Lindegaard JC, et al. Residual rotational set-up errors after daily cone-beam CT image guided radiotherapy of locally advanced cervical cancer. Radiother Oncol 2012;105:220-5.  Back to cited text no. 15
Yao L, Zhu L, Wang J, Liu L, Zhou S, Jiang S, et al. Positioning accuracy during VMAT of gynecologic malignancies and the resulting dosimetric impact by a 6-degree-of-freedom couch in combination with daily kilovoltage cone beam computed tomography. Radiother Oncol 2015;10:104.  Back to cited text no. 16
Van Herk M. Different styles of image-guided radiotherapy. Semin Radiat Oncol 2007;17:258-67.  Back to cited text no. 17
Li XA, Qi XS, Pitterle M, Kalakota K, Mueller K, Erickson BA, et al. Interfractional variations in patient setup and anatomic change assessed by daily computed tomography. Int J Radiat Oncol Biol Phys 2007;68:581-91.  Back to cited text no. 18
Lim K, Kelly V, Stewart J, Xie J, Cho YB, Moseley J, et al. Pelvic radiotherapy for cancer of the cervix: Is what you plan actually what you deliver? Int J Radiat Oncol Biol Phys 2009;74:304-12.  Back to cited text no. 19
Patni N, Burela N, Pasricha R, Goyal J, Soni TP, Kumar TS, et al. Assessment of three-dimensional setup errors in image-guided pelvic radiotherapy for uterine and cervical cancer using kilovoltage cone-beam computed tomography and its effect on planning target volume margins. J Can Res Ther 2017;13:131-6.  Back to cited text no. 20
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Drabik DM, MacKenzie MA, Fallone GB. Quantifying appropriate PTV setup margins: Analysis of patient setup fidelity and intrafraction motion using post-treatment megavoltage computed tomography scans. Int J Radiat Oncol Biol Phys 2007;68:1222-8.  Back to cited text no. 21
Eskadmas YB. Evaluating setup accuracy of a positioning device for supine pelvic radiotherapy. A research report submitted to the Faculty of Science, University of Witwatersrand, Johannesburg, in partial fulfilment of the requirements of the degree of Master of Science. Johannesburg, 2011.  Back to cited text no. 22


  [Figure 1]

  [Table 1], [Table 2], [Table 3]


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