|Year : 2017 | Volume
| Issue : 1 | Page : 97-101
Comparison of beam hardening effect of physical and enhanced dynamic wedges at bladder inhomogeneity using EBT3 film dosimeter
Ghazale Geraily1, Nooshin Sharafi1, Alireza Shirazi1, Mahbod Esfehani2, Maryam Masoudifar3, Blookat Eftekhar Rajab4
1 Department of Medical Physics and Medical Engineering, Faculty of Medicine, Tehran University of Medical Sciences, Tehran, Iran
2 Cancer Institute, Imam Hospital, Tehran University of Medical Sciences, Tehran, Iran
3 Department of Radiotherapy, Mahak Hospital, Tehran, Iran
4 Department of Radiotherapy, Shohada Tajrish Hospital, Shahid Beheshti University of Medical Science, Tehran, Iran
|Date of Web Publication||16-May-2017|
Department of Medical Physics and Medical Engineering, Faculty of Medicine, Tehran University of Medical Sciences, Tehran
Source of Support: None, Conflict of Interest: None
Introduction: Using physical wedges (PWs) to modify dose distribution and more homogeneous target coverage is a well-established technique. However, there are many problems with PWs known as beam hardening, which made them problematic. This can be overcome by dynamic wedges which do not filter beam. Comparison of physical properties of physical and enhanced dynamic wedges (EDWs) restricted to homogeneous medium. Hence, the main aim of this study is to compare dosimetric properties of physical and EDWs at bladder inhomogeneous phantom as a most common case implementing wedges.
Materials and Methods: An inhomogeneous pelvic phantom with homogeneities of uterus, femur, soft tissue, rectum, and bladder was designed. Eclipse treatment planning system with the aim of bladder target was used for calculations. All dose distributions were measured with EBT3 films.
Results: Comparison between beam profiles of physical and EDWs at wedged and nonwedged directions shows a greater difference at near inhomogeneous soft tissue interface and also at heel side of wedges.
Conclusion: Little difference observed between dose distribution of physical and EDWs shows neglectable effect of beam hardening produced by PW compared to EDW at inhomogeneous medium. Furthermore, EBT3 films present good feature to measure dose distributions at EDW fields.
Keywords: Bladder inhomogeneity, EBT3 film, enhanced dynamic wedge, physical wedge
|How to cite this article:|
Geraily G, Sharafi N, Shirazi A, Esfehani M, Masoudifar M, Rajab BE. Comparison of beam hardening effect of physical and enhanced dynamic wedges at bladder inhomogeneity using EBT3 film dosimeter. J Can Res Ther 2017;13:97-101
|How to cite this URL:|
Geraily G, Sharafi N, Shirazi A, Esfehani M, Masoudifar M, Rajab BE. Comparison of beam hardening effect of physical and enhanced dynamic wedges at bladder inhomogeneity using EBT3 film dosimeter. J Can Res Ther [serial online] 2017 [cited 2019 Nov 14];13:97-101. Available from: http://www.cancerjournal.net/text.asp?2017/13/1/97/206244
| > Introduction|| |
Delivering high dose to tumor volume while sparing organ at risk is critical to the success of radiotherapy, but curvature of body surface made problem to this goal. To compensate the effect of curvature surface on dose distribution physical wedges (PWs) may be used.
PWs are made from specific materials such as steel, lead, and tungsten were inserted perpendicularly in the trajectory of ray causing progressive attenuation of beam across the field and spinning of isodose curve plate.,
However, the major disadvantage of PWs is the beam hardening effect in the utilization of these filters that introduced the idea of enhanced dynamic wedge (EDW) or non-PW.,
In dynamic wedge, by computer-controlled movement of one collimator jaw across the field, desirably tilted isodose curve is achieved. Hence, beam hardening effect will not be appeared. Therefore, it is expected that dosimetric properties of EDW will be different from that of PWs. Furthermore, several studies have been done to compare dosimetric properties of physical and EDW, but few study focused on this difference at inhomogeneous medium., Furthermore, these dosimetric differences have been strongly dependent on the clinical utilization.
Totally, wedges have a wide application for pelvic malignancies which is high incidence rate in the urinary system malignity. Bladder is the second leading cause of cancer in men and two or three times in women that is rising rapidly due to the alternation of their lifestyles., Hence, in this study, the main purpose is to compare the dosimetric difference between PWs and EDW at most clinical cases implementing wedges as bladder cancer.
| > Materials and Methods|| |
The process of this study is classified to three steps of film calibration, inhomogeneous phantom design, and phantom measurement.
According to other studies, EBT3 films with good spatial resolutions are good two-dimensional dosimeters for EDW field which necessitate measuring accumulated dose during exposure. Hence, they are used for this study.
To calibrate EBT3 films, they were cut into 2 cm × 2 cm pieces and then inserted in a polymethylmethacrylate phantom at 1.5 cm depth. To establish full backscatter condition, 10 cm slab phantoms were placed beneath of films.
Exposures were done under dose levels of 0.5, 2, 4, 6, 8, and 10 Gy at 10 cm × 10 cm field size and source to surface of 100 cm. These measurements were repeated three times.
Forty-eight hours after radiations, films were scanned by Microtek 9800XL scanner at a resolution of 150 dpi, and the average results were obtained for each dose level. According to the study of Devic et al., net optical density from the following formula was determined:
where Iunexp, Iexp, and Ibckg are intensities measured for unexposed films, exposed films, and zero-light transmitted intensities, respectively.,,
To fit the best model of calibration curve, proposed following function by Devic et al. was used:
D = a × net OD + b × net OD n.
where a and b are the fitting parameters.,,
In this study, an average scan of 25 women structure values with bladder cancer has been used to make the dimension of phantom. Pelvic phantom with uterus, femur, rectum, and bladder inhomogeneities and dimension of 35 cm × 28 cm × 35 cm were designed.
This phantom is symmetric and was designed such that films can be placed between them. This phantom has five cavities and two holes of kickstand established by the correspond materials to facilitate accurate setup positioning.
The material used for soft tissue is Perspex according to the International Commission on Radiological Units and Measurements 44. All the material used in this design are listed in [Table 1]. Schematic of designed phantom is depicted in [Figure 1].
|Table 1: Comparison of measured dose profiles by difference dose and distance to agreement parameters in physical wedge and enhanced dynamic wedge at two directions|
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|Figure 1: Anterior view of inhomogeneous pelvic phantom including bladder, rectum, and head of femur heterogeneity|
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After design of phantom, computed tomography scans of it were obtained and transferred to Eclipse treatment planning system for planning purpose. By continuing rectum, bladder, and femur, a three field plans with target of bladder cancer were designed. Characteristic of plan is listed in [Table 2].
Then, EBT3 films were inserted in the phantom at bladder level, and irradiation was repeated for both physical and EDWs according to designed plan listed in [Table 3] [Figure 2]a and [Figure 2]b.
|Table 3: Characteristics of designed plan for inhomogeneous pelvic phantom|
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|Figure 2: (a) Schematic of measuring beam profiles at nonwedged. (b) Schematic of measuring beam profiles at wedged directions|
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After scanning of EBT3 films, implementing the calibration formula, received isodose, and beam profiles by EBT3 films were obtained.
Discrepancy of dose distribution values was defined by the following formula, based on already theoretical considerations.
Δ (d) = ([T − M]/M)×100.
where T and M are calculated and measured dose, respectively.
| > Results|| |
Calibration curve of EBT3 films was shown at [Graph 1]. Because the delivered dose is below 10 Gy, red channel was used in this study. It can be seen that R2 and fit power of curve are equal to 0.996 and 3.4, respectively.
Comparison of dose profile
[Graph 2] and [Graph 3] show the comparison of beam profile of physical and EDWs at inhomogeneous phantom at both wedged and nonwedged direction.
The result of this comparison in terms of distance to agreement and difference dose is listed in [Table 1].
Comparison of isodose curves
[Figure 3]a and [Figure 3]b shows the measured 100%, 90%, and 80% isodose curves by EBT3 films for both physical and EDW treatment. The result of isodose curve comparison in terms of delta index is listed in [Table 4] [Graph 4].
|Figure 3: (a) Dose distribution of enhanced dynamic wedge. (b) Dose distribution of physical wedge|
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|Table 4: Comparison of measured isodose curves by delta index in physical wedge and enhanced dynamic wedge|
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| > Discussion|| |
As it can be seen from [Graph 1], fit power of calibration curve is equal to 3.4 which is in good agreement with others., Slight difference observed with those may be related to different film model and also scanner. R2 obtained by this study shows that EBT3 films are good dosimeter to measure dose distribution at radiotherapy.
From [Table 1], it can be seen the difference between beam profiles of physical and EDW at nonwedged direction is lower than wedged direction.
[Graph 2] shows that difference between beam profiles of physical and EDW at center is smaller than peripherals.
It can be explained by transmission of beam from soft tissue at center comparison to bone inhomogeneity at peripheral. In other words, when beam transfers through soft tissue at center, contribution of lateral scattering is reduced at absorbed dose. However, when radiation is closed to bone inhomogeneity, this effect is significant. In the presence of PW which introduces beam hardening, angle of scattering is reduced in comparison with EDW. This is responsible for horns observed between two profiles.
[Graph 3] which is drawn in wedged direction at the soft tissue shows that difference between two curves at heel side of the wedge is bigger than two sides. Because this curve is drawn at soft tissue and further away from bone/tissue interface, contribution of scattering at differences in absorbed dose is neglected. The difference between two curves is smaller at center than peripheral. This can be contributed to beam hardening effect produced at PW field which results in higher percentage depth dose curve and also lower difference between peripheral dose and prescribed dose (which is located at the center of curve).
From [Graph 4], it can be seen that difference between two isodose curves at bone inhomogeneity is bigger than soft tissue. However, in overall from [Table 4], it can be seen that this difference is not significant.
| > Conclusion|| |
Complete comparison of PW and EDW with EBT3 films shows that there is no significant difference between these wedges at inhomogeneous media. Hence, beam hardening effect produced by PW does not change isodose distribution at inhomogeneous situation compared with EDW. Furthermore, EBT3 film is a good dosimeter and is recommended for measuring at EDW fields.
This research has been supported by grant no. 27128 of Tehran University of Medical Sciences, and the authors would like to thank the Tehran University of Medical Sciences, for financial support of this work.
Financial support and sponsorship
This research has been supported by grant no. 27128 of Tehran University of Medical Sciences, Tehran, Iran.
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2], [Table 3], [Table 4]