|Year : 2014 | Volume
| Issue : 4 | Page : 937-941
A comparative study of set up variations and bowel volumes in supine versus prone positions of patients treated with external beam radiation for carcinoma rectum
KR Rajeev1, Smrithy S Menon2, K Beena2, Raghavendra Holla3, R Rajaneesh Kumar3, M Dinesh2
1 Department of Radiation Oncology, Regional Cancer Centre, Trivandrum, India
2 Department of Radiation Oncology, AIMS, Kochi, India
3 Department of Radiation Physics, AIMS, Kochi, Kerala, India
|Date of Web Publication||9-Jan-2015|
K R Rajeev
Department of Radiation Oncology, Regional Cancer Centre, Medical College Campus, Trivandrum, Kerala
Source of Support: None, Conflict of Interest: None
Purpose: A prospective study was undertaken to evaluate the influence of patient positioning on the set up variations to determine the planning target volume (PTV) margins and to evaluate the clinical relevance volume assessment of the small bowel (SB) within the irradiated volume.
Materials and Methods: During the period of months from December 2011 to April 2012, a computed tomography (CT) scan was done either in supine position or in prone position using a belly board (BB) for 20 consecutive patients. All the patients had histologically proven rectal cancer and received either post- or pre-operative pelvic irradiation. Using a three-dimensional planning system, the dose-volume histogram for SB was defined in each axial CT slice. Total dose was 46-50 Gy (2 Gy/fraction), delivered using the 4-field box technique. The set up variation of the study group was assessed from the data received from the electronic portal imaging device in the linear accelerator. The shift along X, Y, and Z directions were noted. Both systematic and random errors were calculated and using both these values the PTV margin was calculated.
Results: The systematic errors of patients treated in the supine position were 0.87 (X-mm), 0.66 (Y-mm), 1.6 (Z-mm) and in the prone position were 1.3 (X-mm), 0.59 (Y-mm), 1.17 (Z-mm). The random errors of patients treated in the supine positions were 1.81 (X-mm), 1.73 (Y-mm), 1.83 (Z-mm) and in prone position were 2.02 (X-mm), 1.21 (Y-mm), 3.05 (Z-mm). The calculated PTV margins in the supine position were 3.45 (X-mm), 2.87 (Y-mm), 5.31 (Z-mm) and in the prone position were 4.91 (X-mm), 2.32 (Y-mm), 5.08 (Z-mm). The mean volume of the peritoneal cavity was 648.65 cm 3 in the prone position and 1197.37 cm 3 in the supine position.
Conclusion: The prone position using BB device was more effective in reducing irradiated SB volume in rectal cancer patients. There were no significant variations in the daily set up for patients treated in both supine and prone positions.
材料与方法：从2011年12月到2012年4月期间， 20例连续的病人用腹板（BB）进行了CT扫描，体位为仰卧或俯卧。所有患者均病理证实为直肠癌，接受术前或术后的盆腔放疗。使用三维计划系统，精确地画出每层轴向CT片上的小肠剂量体积直方图。总剂量为46-50戈瑞（2 Gy /次），使用4野技术实施。研究组的计划变动是由直线加速器电子门户成像装置接收到的数据进行评估的。沿X、Y和Z方向的移动也被记录下。系统和随机误差都被计算，用于确定PTV边界。
Keywords: Belly board, prone, rectum
|How to cite this article:|
Rajeev K R, Menon SS, Beena K, Holla R, Kumar R R, Dinesh M. A comparative study of set up variations and bowel volumes in supine versus prone positions of patients treated with external beam radiation for carcinoma rectum. J Can Res Ther 2014;10:937-41
|How to cite this URL:|
Rajeev K R, Menon SS, Beena K, Holla R, Kumar R R, Dinesh M. A comparative study of set up variations and bowel volumes in supine versus prone positions of patients treated with external beam radiation for carcinoma rectum. J Can Res Ther [serial online] 2014 [cited 2019 Nov 14];10:937-41. Available from: http://www.cancerjournal.net/text.asp?2014/10/4/937/138021
| > Introduction|| |
Pre-or post-operative radiotherapy alone or in combination with chemotherapy plays a significant role in the treatment of rectal cancer. For pelvic irradiation, the small bowel (SB) is the most important dose-limiting structure. The overall incidence of acute and chronic SB complications after pelvic irradiation up to a dose of 50 Gy is in the order of 2-9%. ,
Reducing gastrointestinal toxicity is not the only aim of SB sparing. Improved SB sparing also permits dose escalation for pelvic irradiation. The irradiated small bowel volume (SBV) can be minimized by different surgical techniques such as clip placement in high risk areas, pelvic reconstruction, reperitonealization of the pelvic floor, placement of an omental sling, retroversion of the uterus or by placing a synthetic prosthesis under the SB (a removable pelvic spacer). , The irradiated SBV can also be minimized by nonsurgical radiotherapeutical means, including three-dimensional (3D) conformal radiotherapy, intensity modulated radiotherapy, adaptive radiotherapy, customized shielding, a shrinking field technique, bladder distension, and optimal irradiation positions such as supine, prone or by using a belly board (BB).  The theory behind the BB device is that, by creating a space cephalic to the radiation fields due to a negative pressure effect, the SB loops in the pelvis are expected to be pulled cephalad into the space.
The position of the opening of the BB in relation to patient anatomy has not been defined in most studies. The position of the BB opening influences the volume of SB within the pelvis. If the lower border of the opening is near the lumbosacral junction, the volume of SB irradiated is the lowest.  Uncertainties in the daily set up of the patients during treatment can give rise to complications or influence the results of the treatment. Those uncertainties can be reduced by the repositioning of the patient according to the set up verification. 
A prospective study was undertaken to evaluate the influence of patient positioning (supine vs. prone using a BB) on the set up variations to determine the planning target volume (PTV) margins and to evaluate the clinical relevance of contouring and volume assessment of the SB within the irradiated volume.
| > Materials and methods|| |
Twenty consecutive patients (7 female and 13 male), with histologically confirmed rectal cancer, scheduled to receive pelvic irradiation were evaluated prospectively from December 2011 to April 2012. Twelve patients had undergone pelvis surgery previously (low anterior resection [LAR]: 10 and abdominoperineal resection [APR]: 2) and in eight patients surgical procedures had been carried out after preoperative chemo radiation. The median age of the patients was 55 years (range 20-80 years). The median body weight of the patients was 65 kg (range 40-120 kg) [Table 1]a and b.
Patients who were unlikely to comply with the treatment, patients who were having a colostomy bag, short-stature patients who could not position themselves in prone BB were excluded from the study. All the patients underwent complete physical examination prior to the treatment. The patients were randomized into two groups:
ARM 1: Patients treated in supine position, receiving either pre-or post-operative radiation treatment
ARM 2: Patients treated in prone position, receiving either pre-or post-operative radiation treatment.
Types of surgery carried out for the primary tumor were either LAR or APR. The type of surgery was decided by the operating surgeon, depending on the location and extent of primary lesion and involvement of the regional lymph nodes.
After informed consent was obtained from all patients, they were made to undergo computed tomography (CT) simulation and were positioned exactly in the same condition in which the treatment will be delivered in the linear accelerator. Patients in the supine position were immobilized by using either wing board or head rest, with knee and ankle fixed in the supine position, while patients in the prone position were immobilized by using BB [Figure 1]a-c. No instructions for bladder or rectum filling were given. Intravenous or oral contrast media was not used obligatorily. The position and alignment of the fiducials were verified by taking a CT slice through the fiducials. The CT scan was performed and slices were obtained from L2 to mid-thigh with a slice thickness of 5 mm.
|Figure 1: (a) Prone belly board device (b) Foot support-belly board (c) Arm support-belly board|
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Planning was done by a 3D planning system. The volumes of interest were defined in each axial CT slice. In the treatment planning system, the beam was placed according to patient coordinates. A suitable beam angle (gantry angle) was chosen in such a way to cover the target volume. Based on the prescription depth, the energy of the beam was chosen. The fields were shaped according to the anatomy of the target volume and normal structures by means of asymmetric jaws and multi-leaf collimators. The dose prescription was done according to ICRU 50. Whole pelvis was treated using the 3-field or the 4-field technique. The tumor bed was irradiated with a dose of 45-50 Gy in 23-25 fractions, 1.8-2 Gy per fraction. Postoperative radiotherapy started as soon as the wound was healed, preferably in 8 weeks. The volume of bowel in relation to the PTV is shown in [Figure 2]a and b.
|Figure 2: (a) Three-dimensional conformal radiation therapy plan-prone: Image showing small bowel pushed away from the radiation field (b) Three-dimensional conformal radiation therapy plan-supine: More amount of small bowel receiving 100% and 98% of the prescription dose|
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Treatment set up
The set up variation of the study group was assessed from the data received from electronic portal imaging device (EPID). In the eye view using template matching, the port film was matched with digitally reconstructed radiographs. The shift along X (lateral shift), Y (in-out shift) and Z (up-down shift) was noted. For each patient, approximately six port films were taken. The variations along X, Y, and Z directions were noted.
The set up variation was calculated as follows:
For each patient, x1, x2, x3,…, x6 were the variations along X direction, y1, y2, y3,…, y6 the variations along Y direction and z1, z2, z3,…, z6 the variations along Z direction. The average value along each direction was calculated, i.e., X = x1 + x2 + x3+… + x, Y = y1 + y2 + y3+…+ y, Z = z1 + z2 + z3+… + z. For all 20 patients, the average values X1, X2, X3,…, X20 were taken. The standard deviation of x1, x2, x3,…, x6, the variation along X direction, y1, y2, y3,…, y6, the variation along Y direction and z1, z2, z3,…, z6, the variation along Z direction were calculated using "nonbiased" or "n - 1" method. Twenty such average and standard deviation values were determined separately along X, Y, and Z directions. With the help of these mean and standard deviation values the systematic and random errors were calculated.
Systematic errors were calculated by taking the standard deviation of the average values along X, Y, and Z directions for prone and supine positions while random errors were calculated by taking the root mean square values of the standard radiation values along X, Y, and Z directions. Using both these values the PTV margin was calculated using the formula,
Planning target volume margin = 2.5 ∑ +0.7 σ,
Where ∑ is the quadratic sum of standard deviation of all preparation (systematic) errors and σ is the quadratic sum of standard deviation of all execution (random) errors.
| > Results|| |
When contoured the bowel as loops, the mean bowel volume within the PTV was 204.66 cm 3 in prone position and 480.918 cm 3 in supine position [Figure 3]a and b. When contoured the peritoneal cavity, the mean volume of the peritoneal cavity was 648.65 cm 3 in prone position and 1197.37 cm 3 in supine position [Figure 4]a and b.
|Figure 3: (a) Bowel contour as loops-prone (b) Bowel contour as loops- supine|
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A total of 120 ports (six ports/patient) were taken, including 10 patients in prone and 10 patients in supine position. The mean variations in X, Y, and Z directions for each patient treated in both supine and prone positions are given in [Table 2] and [Table 3].
The systematic errors of patients [Table 4] treated in supine positions were 0.87 (X-mm), 0.66 (Y-mm), 1.6 (Z-mm) and of those treated in prone positions were 1.3 (X-mm), 0.59 (Y-mm), 1.17 (Z-mm). The random errors of patients [Table 5] treated in supine positions were 1.81 (X-mm), 1.73 (Y-mm), 1.83 (Z-mm) and of those treated in prone positions were 2.02 (X-mm), 1.21 (Y-mm), 3.05 (Z-mm).
The calculated PTV margin values [Table 6] in supine positions were 3.45 (X-mm), 2.87 (Y-mm), 5.31 (Z-mm) and those in prone positions were 4.91 (X-mm), 2.32 (Y-mm), 5.08 (Z-mm).
There were no significant variations in the set up for patients treated in both supine and prone positions. The calculated PTV margins were around 0.5 cm in both the treatment groups (P < 0.001).
| > Discussion|| |
A combined modality approach with surgery and chemo radiation or preoperative chemo radiation followed by surgery is the standard of care for rectal cancer treatment. , In radiotherapy of rectal carcinoma the dose-limiting normal tissue is first and foremost the SB, but organs or structures such as the urinary bladder, ureters, rectum, femoral head, skin, and nerves are at risk of adverse effects.  The reduction of toxicity is a major challenge for clinicians even in the era of sophisticated surgical and radiation treatment modalities. There have been various improvements in the surgical and radiation treatment modalities with the availability of newer techniques. These help to reduce the side-effects of radiation treatment. The use of prone BB device helps to reduce the SBV in the irradiation field. ,, However, the reproducibility of such newer techniques remains unverified, especially in the Indian scenario. The major challenges in treating the patient in the prone position are probable daily set up variations, patient compliance and suspected technical issues in female patients due to difficulties in achieving pubic locking and breast positioning on the BB device .  The authors report that this procedure takes about 1 h, meaning high personal costs and therefore low practicability for routine simulation. 
There have been many trials evaluating the benefits of prone BB reducing the SBV in the radiation treatment field . , In the present study, volume of bowel was less in patients treated in the prone position using prone BB. Even though, the bowel contouring was not standardized, the data showed that the volume of bowel was minimal either we contour the entire peritoneal cavity or single bowel loops. Although, this study demonstrated that the use of a BB was beneficial in reducing the irradiated SBV, it was a comparative study between two treatment positions and plans, but not real treatments. Thus, clinical outcomes such as bowel toxicity could not be studied. However, the reduced volume of the SB irradiated may be beneficial to patients with rectal cancer who receive treatment in the prone BB device.
The concern remains regarding the set up variation and reproducibility with prone BB either with 3D conformal radiation therapy (CRT) or intensity modulated radiation therapy (IMRT) techniques. Adli et al. have reported that even though there may be more set up uncertainties in prone positioning than in supine positioning, they have generally not observed a greater rate of field corrections or a greater rate of tumor/nodal failure in patients treated in the prone position compared with those treated in the supine position.  In the present study, all our patients received radiation treatment with 3D CRT except one patient who received adjuvant IMRT. Keys et al. have reported more systematic and random errors in treating patients with the prone BB device.  However, the methods used for the verification were conventional X-ray simulation film systems instead of EPIDs. The trial has reported problems in accommodating patients in the prone BB position. Thus, the extent of benefits from the use of prone BB cannot be assessed from the available data. There are not many studies available in this scenario, addressing the Indian population. The present study was carried out to evaluate the feasibility of using prone BB in patients receiving external radiation therapy for rectal cancer.
Of the 20 patients, seven were females of which three were treated with prone BB device. There was no significant mean variation in the daily set up for any of these three female patients. Both the systematic and random errors were not higher in the one patient who received IMRT. The body weight found to be a factor that shows a trend toward more daily patient set up variations in the prone position. The variations in each direction (X, Y and Z) were more in the two patients with bodyweight >100 kg.
In the present study, we compared both the systematic and random errors in both supine and prone positions. None of the values was significantly higher in either arm. Thus, the PTV margin calculated was the same, i.e., 0.5 cm in both the treatment groups. There was no major variation in daily patient set up. Patient compliance was good with adequate reproducibility
| > Limitations of the study|| |
This study was performed on 20 patients divided into two groups of 10 each in prone and supine positions. Determination of systematic and random errors on a group of 10 patients gives a reasonable, but not definite estimate of the errors. Larger studies are required to improve the statistical power of the analyzed variations. The study does, however, give a good estimate of the order of magnitude and especially the heterogeneity of systematic and random errors for shape variation, especially in Indian patients. There were technical problems in strictly not following bladder filling protocol as in some patients this found difficult, especially for patients in prone position. There were other issues in getting the patient data from the supine and prone position for the same patient. The impact of bowel volume variations between different surgical modalities may have an influence which cannot be analyzed in the present study.
| > Conclusion|| |
The present study found that the use of BB was more effective in reducing irradiated SBV in rectal cancer patients. These dosimetric findings may result in a reduction in SB complications in patients receiving external beam radiation for rectal cancer. Both prone and supine positions are acceptable for pelvic radiotherapy in patients with rectal cancer as no significant variation was observed in daily patient set up either in the prone or supine position. Factors like weight of the patient, patient comfort, and technical expertise are important when deciding the treatment position for pelvic radiation.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]