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ORIGINAL ARTICLE
Year : 2021  |  Volume : 17  |  Issue : 1  |  Page : 157-163

Assessment and implication of rectal filling on vaginal motion in postoperative carcinoma endometrium patients during image guided radiotherapy


Department of Radiation Oncology, Max Institute of Cancer Care, Ghaziabad, Uttar Pradesh, India

Date of Submission13-Dec-2019
Date of Decision30-Jan-2020
Date of Acceptance22-Apr-2020
Date of Web Publication15-Mar-2021

Correspondence Address:
Rashi Agrawal
737, Zinnia, Gaur Saundaryam, Sector Techzone 4, Greater Noida West - 201 306, Uttar Pradesh
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jcrt.JCRT_1113_19

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 > Abstract 


Aim: While delivering radiotherapy it is utmost important to minimize target motion to decrease margins in postoperative gynaecological patients. Hence certain bladder and rectum filling protocols are followed by each institute. During cone beam computed tomography (CBCT) verification, we observed that this motion was more affected by rectal filling. To verify, we retrospectively analysed the vaginal movement and its relation with bladder and rectum filling.
Materials and Methods: We evaluated CBCTs of 15 patients of carcinoma endometrium. Bladder and rectum both were contoured offline on each scan. To assess the motion of vagina, two reference points were selected. Posterior movement of bladder and anterior movement of rectum were noted on these points on each scan.
Results: Total 150 scans (135 KV-CBCT scans and 15 planning computed tomography) of 15 patients were studied. Stepwise regression analysis reported that bladder wall changes has a nonsignificant relationship with bladder volume among all the individuals. The significant rectal wall changes both at Point X and Y were observed in six patients and only at Point X in three patients. Rest of the patients showed no significant relationship in their CBCT scans. Thus 60% patients showed significant relation between rectal volume and rectal wall changes.
Conclusion: Hence we suggest to advice our patients more regarding volume of rectum to decrease vaginal motion. Bladder volume is needed to decrease the dose to small intestine. However prospective data with large number of patients in the study is required to confirm these findings.

Keywords: Bladder filling, carcinoma endometrium, motion of vagina, rectal filling


How to cite this article:
Agrawal R, Jafar S, Choudhary P, Anbalagan D, Singh D, Agarwal S. Assessment and implication of rectal filling on vaginal motion in postoperative carcinoma endometrium patients during image guided radiotherapy. J Can Res Ther 2021;17:157-63

How to cite this URL:
Agrawal R, Jafar S, Choudhary P, Anbalagan D, Singh D, Agarwal S. Assessment and implication of rectal filling on vaginal motion in postoperative carcinoma endometrium patients during image guided radiotherapy. J Can Res Ther [serial online] 2021 [cited 2021 Nov 28];17:157-63. Available from: https://www.cancerjournal.net/text.asp?2021/17/1/157/311067




 > Introduction Top


Pelvic radiotherapy is an integral part of management of gynaecological malignancies. Earlier conventional radiotherapy (four field technique) was used to treat these patients. Delivery of conventional radiotherapy in post-operative gynaecological patients is a challenge due to exposure of high radiation dose to organs at risk, most important being small intestine. Several studies have recommended the use of intensity modulated radiotherapy (IMRT) to limit the dose to normal structures (bladder, rectum, small intestine and bone marrow)[1] and now many institutes have adopted IMRT as standard of care.[2],[3],[4],[5],[6] To achieve best results, it is utmost important to minimize clinical target volume (CTV) movement. Vagina has been incorporated into CTV but many studies have reported that the position of vagina is highly uncertain.[7],[8],[9],[10],[11] Internal target volume (ITV) accounts for organ motion. Understanding of the influence of adjacent organ filling is required to determine an appropriate ITV. To minimize these movements certain bladder and rectum filling protocols are followed by each institute. Although, bladder filling protocol is followed more religiously than rectal protocol.

In our hospital cone beam computed tomography (CBCT) verification is routinely done by radiation oncologist. During this verification process, we observed that vaginal movement is more affected by rectal filling in comparison to bladder filling. To verify our observation, we retrospectively analysed the vaginal movement and its relation with bladder and rectal filling.


 > Materials and Methods Top


We selected carcinoma endometrium as a prototype gynaecological malignancy for our study. We evaluated post-operative carcinoma endometrium patients who completed their external beam radiotherapy treatment by image guided IMRT (IG-IMRT) technique between January 2012 and October 2014. Total 15 patients were included in our study. Patients who couldn't follow bladder protocol were excluded from our study.

The process of radiotherapy planning and treatment was same for all the patients and is described in earlier publication.[12]

For treatment of patients with gynaecological malignancies by IG-IMRT technique following online imaging verification protocol was followed. During first 3 days, KV CBCT scans were taken to quantify the status of target volume and normal structures. After that twice weekly CBCTs were taken. During pre-treatment imaging, if there was discrepancy in bladder or rectal volume and probability of target being missed then patient was again instructed regarding bladder and rectum filing and imaging (CBCT) was repeated. This online patient verification was performed by radiation oncologist only.

For this study, bladder and rectum both were contoured offline on each KV-CBCT and this was done for all 15 patients. Contours were verified by two radiation oncologists. Our CBCT included whole bladder and rectum was included from rectosigmoid junction to lower border of pubic symphysis. Bladder and rectum volumes of CBCT and planning computed tomography (CT), both were recorded for each patient. Each CBCT was fused with planning CT scan of the same patient. Treatment area of 15 cm length was scanned through CBCT.

Tissue between posterior wall of bladder and anterior wall of rectum was considered as vagina. To assess the motion of vagina, two reference points were selected. An imaginary line was drawn at the level of uppermost border of femoral head at acetabulam on the axial CT scan image. This was the first reference line. Second reference line was drawn 2 cm below this line. Points at mid plane of these lines were first (Point X) and second (Point Y) reference points respectively [Figure 1] and [Figure 2]. Posterior movement of bladder and anterior movement of rectum were noted on these lines on each CBCT scan.
Figure 1: First reference point

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Figure 2: Second reference point

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Statistical analysis

All the collected data was entered in the Microsoft Word Excel Sheet 2007 version and the data obtained was analysed using the IBM SPSS v21 for the descriptive analysis and statistical tests of significance. The mean and standard deviations were calculated for each clinical parameter and the various statistical test of significance were used.

Regression analysis was used to estimate the relationships among variables. Regression analysis helps one to understand how the typical values of the movements of rectal and bladder wall change when their volumes are varied. Spearman's correlation was used to correlate rectal wall changes with rectal volume and bladder wall changes with bladder volume among the patients. Significance for all statistical tests was predetermined at a probability (P) value of 0.05 or less.


 > Results Top


This is a retrospective descriptive study. Total 150 scans (135 KV-CBCT scans, and 15 planning CT) of 15 patients were studied. 66% patients had at least ten CBCT scans. 26% had more than 7 scans. On an average, 9 CBCT scans per patient were available for analysis.

The bladder volume on planning CT (reference bladder volume) ranged from 51.4 to 424.4 cc. Mean reference bladder volume was 214 cc. In spite of bladder filling protocol, bladder volume during treatment ranged from 31.3 to 799.1 cc. In three patients bladder volumes during CBCT scans were less than the minimum planning volume and in one patient it was more than the maximum volume. The rectal volume on planning CT scan (reference rectal volume) ranged from 32.6 to 123.5cc. Mean volume was 64.32 cc. In two patients, rectal volumes during their entire CBCT scans were even less than the minimum rectal planning volume. [Table 1] shows detailed values of bladder and rectal volumes in each patient.
Table 1: Mean, standard deviation, minimum and maximum values of bladder and rectum volume among the patients

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Mean bladder wall motion at point X ranged from −0.42 to 1.49 and rectal wall from −1.22 to 1.38 cm. Motion anterior to reference point was considered as positive motion while posterior to that point as negative motion.

Mean and standard deviation of bladder volume, bladder wall change and regression of bladder volume on bladder wall changes at points X and Y are presented in [Table 2] and it was found that bladder wall changes have a non significant relationship with bladder volume among all the individuals. In two patients (patient 3 and 4), average bladder wall motion at point X in anteroposterior direction was zero in spite of changes in bladder volume.
Table 2: Mean and standard deviation of bladder volume, bladder posterior wall change and regression of bladder volume on bladder posterior wall changes at points X and Y

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[Table 3] shows mean and standard deviation of rectal volume, rectal wall change and regression of rectal volume on rectal wall changes at points X and Y. The significant rectal wall changes both at Point X and Y were observed among patient no 3, 6, 8, 10, 14, 15 and only at Point X were observed among patient no. 4, 7, and 11. Rest of the patients showed no significant relationship in their CBCT scans. Thus 60% patients showed significant relation between rectal volume and rectal wall changes.
Table 3: Mean and standard deviation of rectal volume, rectal anterior wall change and regression of rectal volume on rectal anterior wall changes at points X and Y

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Bladder and rectal volume did not follow any specific pattern during treatment. The bladder and rectal volume didn't decrease towards the end of treatment as shown in [Graph 1]. In fact, bladder volume was more than the planning bladder volume on last day in five patients.



The correlation between bladder posterior wall changes with bladder volume (Pearson's correlation test) and rectal anterior wall changes with rectal volume are shown in [Table 4]. It was found that there was no significant correlation between bladder wall changes and bladder volume. However, there were 8 patients who have shown significant correlation between rectal wall changes and rectal volume.
Table 4: Correlation analysis showing bladder posterior wall changes with bladder volume and rectum anterior wall changes with rectal volume

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 > Discussion Top


Potential benefits of IMRT in post-operative carcinoma endometrium are well established. Toxicities related to normal tissues irradiation have been reduced with this technique.[13],[14],[15]

IMRT delivers highly conformal dose with steep dose gradient. Hence understanding of target volume motion in relation to adjacent organs is very essential. The information of this movement helps us in deciding about margins for ITV. The normal tissue volume spared by IMRT is very sensitive to margin dimension. If there is 5mm increase in margin size around CTV then the volume of small bowel receiving 30 Gy or more has been found to increase by as much as 40%.[16]

Various studies have suggested different margins to incorporate ITV. Movement of vaginal apex in antero-posterior (AP), mediolateral (ML) and supero-inferior (SI) direction has been assessed. Movement in AP direction is most significant and least in ML direction. Harris et al. reported that margins of 3.1, 9.5 and 12.1 mm along right-left (RL), SI and AP axes will cover vaginal fiducials in most of the treatment. He suggested that PTV margin of 16 mm will encompass all the organ motion in postoperative gynaecological malignancy patients.[17] Another study reported maximum shift in AP direction of 7 mm.[8] In a recent study, portable bladder scan was used to keep bladder full during treatment. Twenty nine postoperative cervical cancer patients were included in this study. The authors suggested 5, 9 and 14 mm margin in the RL, SI and AP directions respectively for planning tumor volume.[18] For definitive radiotherapy also random errors of the fiducials placed in fornices were 8, 14 and 26 mm in the RL, SI and AP direction in cervical cancer.[19] Almost all studies reported that maximum movement was in AP direction. Hence, in our study we assessed the movement of posterior bladder wall and anterior rectal wall in AP direction only. To assess the movement of vagina, we considered two reference points. Point X and Y as described earlier. Wang et al. assessed the movement at line A and line B.[20] Line A was defined as the midsagittal line across the superior border of the pubic symphysis and line B was the parallel line 1.5 cm above line A.In another study markers were placed at the apex of vagina to assess the movement.[21] We do not place fiducial markers in vaginal vault in our institute. It was difficult to recognise apex of vaginal vault in CBCT hence we studied vaginal motion at point X and point Y.

In our study mean bladder posterior wall movement in AP direction was 0.3 cm (range −0.42–1.49) at Point X and 0.166 cm (range −0.34–0.8) at Point Y. Mean value of motion of posterior boundary of bladder at line A was 0.08 cm (range −1.7–2.9) and at line B −0.4 (range −2.1–1.9).[20] In the study with fiducial markers, the mean shifts ranged from 0.3 to 1.71 cm (median, 0.64 cm) in AP direction.[21]

On the basis of above mentioned studies we have incorporated 1 cm PTV margin in anteroposterior and SI directions and 5 mm at ML direction. But we do not follow this practice universally, if first three consecutive CBCT scans suggested that rectal volume of patient is increasing in spite of instructions then we increased our margins accordingly. Systematic review on organ motion concluded that pelvic organ motion is patient specific and application of population based PTV margins will lead to larger normal tissue irradiation.[22]

In post hysterectomy patients, movement of CTV depends mainly upon bladder and rectal filling. If there is variation in the volume of bladder and rectum from that in the planning CT, then the probability of normal tissue complications will be affected.[23]

No approach regarding bladder and rectal filling has been standardized. Bladder filling protocol is followed to move small bowel away from the treatment field in pelvis and also to minimize vaginal cuff movement. If full bladder protocol is not followed then bowel volume in the pelvis may increase but it was possible with IMRT to decrease the dose to small bowel.[24]

As far as our knowledge, all studies are following bladder filling protocol or bladder and rectum filling protocol both.

In a prospective study on vaginal movement by Jhingran et al., patients were given detailed written instructions to empty bladder before simulation and then to drink 24 ounces of liquid in 10 min. After 50 min, simulation was done. No instructions were given regarding rectal filling.[21] The ITV was decided on the basis of only full and empty bladder volume.

In some institutes a portable ultrasound bladder was used to help patients to prepare the bladder, with approximately 200–350 ml of urine.[6] Many centres recommended comfortably full bladder approach. Wang et al. instructed their patients to have 500 ml of water 1 h before scanning so that their bladder can be comfortably full. They were also instructed to have empty rectum.[20]

Although most of the centres preferred empty rectum treatment, but no strict written instructions were followed. In our institute, patients were asked to drink 500 ml of water within 10 min. The patients were scanned and treated after 30–40 min of taking last glass of water. Laxatives were used to keep rectum empty as per need basis. We have given proctoclysis enema in our patients on the very 1st day of planning if they had loaded rectum.

In a study on patients of carcinoma prostate for bowel preparation protocol, all patients were given mild laxative (macrogol 4000) 1 day before (after dinner) treatment planning and during treatment sessions. Without bowel preparation protocol, the rectal volumes were more variable, the maximum variation in rectal volume was as high as +96%. With bowel preparation protocol, the variations were <25%.[25]

Mean bladder volume in planning CT scan was 214 cc (51.4–424.4) and 189.8 cc (31.3–799.1) during treatment. Mean rectal volume during planning CT scan was 64.32 cc (32.6–123.5) and 68.2 cc (18–172.3) during treatment. Jhingran et al. reported mean full bladder volumes measured from initial planning CT scans were 480 ml (range, 122–1052).[21] This was higher in comparison to our study because of more drinking water volume and waiting time. The median rectal volume during treatment was 104 cc (range, 46–193). Differences between the pre treatment full bladder and each subsequent CT scan ranged from 53.2 to 698.0 cc. In another study differences ranged from −471.0 to 264.4 cc.[20] The mean rectum volume was 48.2 (11.3–139.7 cc) with an SD of 18.9 cc.

Bladder volume tends to decrease 4 cm3/day through time and rectum displayed no pattern.[26] Some studies has also shown this pattern due to reduced bladder capacity and radiation cystitis.[27],[28] But in our study, bladder and rectum volume did not follow any specific pattern during treatment.

In all institutes, bladder protocol is being followed more strictly in comparison to rectal filling. But during our online CBCT verification we observed that vaginal movement is more affected by rectal filling.

The impact of bladder filling was more on tip of uterus. Several studies reported the alteration in position of uterus with changing bladder volume. In a review article by Jadon et al., authors reported five studies that included 103 patients and studied the impact of rectal filling on cervix – uterine motion.[22] Rectal filling had more impact on cervix motion and bladder filling on uterine motion.[29],[30] Correlation between rectal volume and anterior – posterior shift in the CTV was found to be significant with the correlation coefficient of 0.79.

But in a study by Wang et al., it was demonstrated that the position of vagina was significantly affected by the filling status of bladder, not by the rectum.[20] The correlation of the motion of the vagina in the AP direction in the midline with bladder volume was found to be weak but significant on lines A and B (P < 0.05 for both; the Pearson's correlation coefficient was 0.37 and 0.44, respectively). However, no correlation was found between the motion of the vagina and rectal volume on lines A and B (P > 0.05 for both; the Pearson's correlation coefficient was 0.026 and 0.011, respectively). While in a prospective study by Jhingran et al., in six cases, variations in bladder volume were significantly correlated with posterior–anterior or inferior–superior shifts in vaginal seed positions.[21] Variations in rectal volume were also found to correlate significantly with seed position in six cases; the direction of shifts produced by variations in rectal filling seemed to be predominantly in the anterior–posterior direction. In 11 of 16 cases, significant shifts in the location of the vaginal apex were correlated with variations in the volume of the bladder or rectum.

However in our study, the relation of rectum wall changes with rectum volume using regression analysis showed significant correlation in 60% patients and Pearson's correlation test showed that 53% patients had significant correlation. Bladder wall changes had a non significant relationship with bladder volume in regression analysis and in Pearson's correlation test. But ours is a retrospective study with 135 CBCTs and 15 planning CT scan. We have not analysed the relation between rectum volume and the movement of anterior rectal wall although some have reported rectal volume more than 70 cm3 required greater margins in posterior direction.[11] In their study, Jhingran et al. also reported that variation in rectal filling can also have a major impact on vaginal position, particularly in the anteroposterior direction.[21] They advised their patients to evacuate rectum or put a rectal tube but if these methods didn't work then they increased CTV to PTV margins in posterior direction.


 > Conclusion Top


Hence we suggest to advice our patients more regarding volume of rectum to decrease vaginal motion. Bladder volume is needed to decrease the dose to small intestine. However prospective data with large number of patients in the study is required to confirm these findings.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
 > References Top

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Cilla S, Macchia G, Digesù C, Deodato F, Romanella M, Ferrandina G, et al. 3D-conformal versus intensity-modulated postoperative radiotherapy of vaginal vault: A dosimetric comparison. Med Dosimetry 2010;35:135-42.  Back to cited text no. 1
    
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Portelance L, Chao KS, Grigsby PW, Bennet H, Low D. Intensity-modulated radiation therapy (IMRT) reduces small bowel, rectum and bladder doses in patients with cervical cancer receiving pelvic and para-aortic irradiation. Int J Radiat Oncol Biol Phys 2001;51:261-6.  Back to cited text no. 2
    
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Beriwal S, Jain SK, Heron DE, Kim H, Gerszten K, Edwards RP and Kelley JL. Clinical outcome with adjuvant treatment of endometrial carcinoma using intensity-modulated radiation therapy. Gynecol Oncol 2006;102:195-9.  Back to cited text no. 5
    
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Wang W, Zhang F, Hu K, Hou X. Image-guided, intensity-modulated radiation therapy in definitive radiotherapy for 1433 patients with cervical cancer. Gynecol Oncol 2018;151:444-8.  Back to cited text no. 6
    
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Luo H, Jin F, Yang D, Wang Y, Li C, Guo M, et al. Interfractional variation in bladder volume and its impact on cervical cancer radiotherapy: Clinical significance of portable bladder scanner. Med Phys 2016;43:4412.  Back to cited text no. 7
    
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Rash D, Hagar Y, Cui J, Hunt JP, Valicenti R, Mayadev J, et al. Interfraction motion of the vaginal apex during postoperative intensity modulated radiation therapy: Are we missing the target? Int J Gynecol Cancer 2013;23:385-92.  Back to cited text no. 8
    
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Haripotepornkul NH, Nath SK, Scanderbeg D, Saenz C, Yashar CM. Evaluation of intra-and interfraction movement of the cervix during intensity modulated radiation therapy. Radiother Oncol 2011;98:347-51.  Back to cited text no. 9
    
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Bondar L, Hoogeman M, Mens JW, Dhawtal G, de Pree I, Ahmad R, et al. Toward an individualized target motion management for IMRT of cervical cancer based on model-predicted cervix-uterus shape and position. Radiother Oncol 2011;99:240-5.  Back to cited text no. 10
    
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van de Bunt L, Jürgenliemk-Schulz IM, de Kort GA, Roesink JM, Tersteeg RJ, van der Heide UA. Motion and deformation of the target volumes during IMRT for cervical cancer: What margins do we need? Radiother Oncol 2008;88:233-40.  Back to cited text no. 11
    
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Agrawal R, Prithiviraj S, Singh D, Zamre V, Agrawal S, Goel AK, et al. Long-term results of post-operative pelvic image guided intensity modulated radiotherapy in gynaecological malignancies. Gulf J Oncol 2017;1:30-7.  Back to cited text no. 12
    
13.
Klopp AH, Moughan J, Portelance L, Miller BE, Salehpour MR, Hildebrandt E, et al. Hematologic toxicity in RTOG 0418: A phase 2 study of postoperative IMRT for gynecologic cancer. Int J Radiat Oncol Biol Phys 2013;86:83-90.  Back to cited text no. 13
    
14.
Folkert MR, Shih KK, Abu-Rustum NR, Jewell E, Kollmeier MA, Makker V, et al. Postoperative pelvic intensity modulated radiotherapy and concurrent chemotherapy in intermediate-and high-risk cervical cancer. Gynecol Oncol 2013;128:288-93.  Back to cited text no. 14
    
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Portelance L, Moughan J, Jhingran A, Miller BE. Salehpour MR, D'Souza D, et al. A phase II multi-institutional study of postoperative pelvic intensity modulated radiation therapy (IMRT) with weekly cisplatin in patients with cervical carcinoma: Two year efficacy results of the RTOG 0418. Int J Radiat Oncol Biol Phys 2011;81:S3.  Back to cited text no. 15
    
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Ahamad A, D'Souza W, Salehpour M, Iyer R, Tucker SL, Jhingran A, Eifel PJ. 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 2005:15:1117-24.  Back to cited text no. 16
    
17.
Harris EE, Latifi K, Rusthoven C, Javedan K, Forster K, et al. Assessment of organ motion in postoperative endometrial and cervical cancer patients treated with intensity-modulated radiation therapy. Int J Radiat Oncol Biol Phys 2011;81:645-50.  Back to cited text no. 17
    
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Okamotoa H, Murakamia N, Carvajalb CC, Miuraa Y, Wakitaa A, Nakamuraa S, et al. Positional uncertainty of vaginal cuff and feasibility of implementing portable bladder scanner in postoperative cervical cancer patients. Physica Medica 2018;45:1-5.  Back to cited text no. 18
    
19.
Ahmad R, Hoogeman MS, Bondar M, Dhawtal V, Quint S, De Pree I, et al. Increasing treatment accuracy for cervical cancer patients using correlations between bladder-filling change and cervix uterus displacements: Proof of principle. Radiother Oncol 2011;98:340-6.  Back to cited text no. 19
    
20.
Wang X, Yu M, Wang J, Zhong R, Shen Y, Zhao Y, et al. An assessment of interfractional bladder, rectum and vagina motion in postoperative cervical cancer based on daily cone-beam computed tomography. Mol Clin Oncol 2016;4:271-7.  Back to cited text no. 20
    
21.
Jhingran A, Salehpour M, Sam M, Levy L, Eifel PJ. Vaginal motion and bladder and rectal volumes during pelvic intensity-modulated radiation therapy after hysterectomy. Int J Radiat Oncol Biol Phys 2012;82:256-62.  Back to cited text no. 21
    
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Jadon R, Pembroke CA, Hanna CL, Palaniappan N, Evans M, Cleves AE, et al. A systematic review of organ motion and image-guided strategies in external beam radiotherapy for cervical cancer. Clin Oncol (R Coll Radiol) 2014;26:185-96.  Back to cited text no. 22
    
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O'Doherty UM, McNair HA, Norman AR, Miles E, Hooper S, Davies M, et al. Variability of bladder filling in patients receiving radical radiotherapy to the prostate. Radiother Oncol 2006;79:335-40.  Back to cited text no. 23
    
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Georg P, Georg D, Hillbrand M, Kirisits C, Potter R. Factors influencing bowel sparing in intensity modulated whole pelvic radiotherapy for gynaecological malignancies. Radiother Oncol 2006;80:19-26.  Back to cited text no. 24
    
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Heng SP, Low SH, Sivamany K. The influence of the bowel and bladder preparation protocol for radiotherapy of prostate cancer using kilo-voltage cone beam CT: Our experience. Indian J Cancer 2015;52:639-44.  Back to cited text no. 25
[PUBMED]  [Full text]  
26.
Eminowicz G, Motlib J, Khan S, Perna C, McCormack M. Pelvic organ motion during radiotherapy for cervical cancer: Understanding patterns and recommended patient preparation. Clin Oncol 2016;28:e85-91.  Back to cited text no. 26
    
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Collen C, Engels B, Duchateau M, Tournel K, De Ridder M, Bral S, et al. Volumetric imaging by megavoltage computed tomography for assessment of internal organ motion during radiotherapy for cervical cancer. Int J Radiat Oncol Biol Phys 2010;77:1590-5.  Back to cited text no. 27
    
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Ahmad R, Hoogeman MS, Quint S, Mens JW, de Pree I, Heijmen BJ. Inter-fraction bladder filling variations and time trends for cervical cancer patients assessed with a portable 3-dimensional ultrasound bladder scanner. Radiother Oncol 2008;89:172-9.  Back to cited text no. 28
    
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Taylor A, Powell ME. An assessment of interfractional uterine and cervical motion: Implications for radiotherapy target volume definition in gynaecological cancer. Radiother Oncol 2008;88:250-7.  Back to cited text no. 29
    
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