|Year : 2017 | Volume
| Issue : 2 | Page : 313-317
Dosimetric characteristic of physical wedge versus enhanced dynamic wedge based on Monte Carlo simulations
Seied Rabie Mahdavi1, Ghazale Geraily2, Ahmad Mostaar3, Arman Zia2, Golbarg Esmaili1, Somayeh Farahani2
1 Department of Medical Physics and Medical Engineering, Faculty of Medicine, Iran University of Medical Sciences, Tehran, Iran
2 Department of Medical Physics and Medical Engineering, Faculty of Medicine, Tehran University of Medical Sciences, Tehran, Iran
3 Department of Medical Physics and Medical Engineering, Faculty of Medicine, Shahid Beheshti University of Medical Sciences, Tehran, Iran
|Date of Web Publication||23-Jun-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
Aim of Study: Physical wedges (PWs) are widely used in radiotherapy to obtain tilted isodose curves, but they alter beam quality. Dynamic wedges (DWs) using moving collimator overcome this problem, but measuring their beam data is not simple. The main aim of this study is to obtain all dosimetric parameters of DWs produced by Varian 2100CD with Monte Carlo simulation and compare them to those from PWs.
Subjects and Methods: To simulate 6 MV photon beams equipped with PW and DW, BEAMnrc code was used. All dosimetric data were obtained with EDR2 films and two-dimensional diode array detector.
Results: Good agreement between simulated and measured dosimetric data for PW and DW fields was obtained. Our results showed that percentage depth dose and beam profiles at nonwedged direction for DWs are the same as open fields and can be used to each other.
Conclusion: From Monte Carlo simulations, it can be concluded that DWs in spite of PW do not have effect on beam quality and are good options for treatment planning system which cannot consider hardening effect produced by PWs. Furthermore, BEAMnrc is a powerful code to acquire all date required by DWs.
Keywords: Enhanced dynamic wedge, Monte Carlo simulations, physical wedge
|How to cite this article:|
Mahdavi SR, Geraily G, Mostaar A, Zia A, Esmaili G, Farahani S. Dosimetric characteristic of physical wedge versus enhanced dynamic wedge based on Monte Carlo simulations. J Can Res Ther 2017;13:313-7
|How to cite this URL:|
Mahdavi SR, Geraily G, Mostaar A, Zia A, Esmaili G, Farahani S. Dosimetric characteristic of physical wedge versus enhanced dynamic wedge based on Monte Carlo simulations. J Can Res Ther [serial online] 2017 [cited 2020 Apr 10];13:313-7. Available from: http://www.cancerjournal.net/text.asp?2017/13/2/313/183562
| > Introduction|| |
Physical wedge (PW), as a beam modifier, is used in external beam radiotherapy to improve dose homogeneity. They are made of lead, steel, and brass, and when placed in the beam, trajectory makes a gradual decrease in the beam intensity. In this way, they modify beams and produce beam hardening which is not accounted for calculation of the most treatment planning systems (TPSs)., Therefore, it seems to be discrepancy between the actual and calculated dose to target volume. This inaccuracy in dose delivery may be a threat for successful treatment.
Using filter less wedges as enhanced dynamic wedges (EDWs) can overcome this problem somewhat because DW is not as external device mounted in front of the beam trace. On the other hand, in this technology, desirably tilted isodose curve is achieved by computer-controlled movements of one collimator jaw across the field during exposure., At the mean time, dose rate and the velocity of moving jaw are modified according to the precalculated table known as segmented treatment table. However, scattered radiation from the movable jaw can effect on beam quality and so absorbed dose. Therefore, photon beam quality generated by DWs needs to be investigated as well as those from PWs.
Currently, Monte Carlo simulation is used as an accurate method to obtain information about the clinical beam, especially in complex situation such as the presence of DW which can be measured hardly due to jaw movement. The main purpose of this study is to compare dosimetric characteristic of PW and EDW in aspect of percentage depth dose (PDD), isodose curves, profiles, and also mean energy, photon and electron spectra both in wedged and nonwedged directions. Although some researchers investigated physical characteristic of physical and EDW, no such complete comparison has been performed by Monte Carlo simulations. In addition, the characteristic of photon beams may be varied between machine to machine and even for the same model., Our study would be the first comprehensive report for comparison of dosimetric differences of PW and EDW with Monte Carlo point of view.
| > Subjects and Methods|| |
The NRCC user code BEAMnrc was used to simulate 6 MV photon beam of Varian 2100CD with open, PW, and EDW fields. To model each component, this code uses a series of component modules. In this simulation, SLABS, CONS3R, FLATFILT, CHAMBER, MIRROR, JAWS, PYRAMIDS, and DJAWS were used to model target, primary collimator, flattening filter, ion chamber, mirror collimator, secondary collimator, PWs, and EDWs, respectively. To simulate EDW, AUTOJAWS, an MATLAB-based program written by Kakakhel, was used.
In all calculations, the EGSnrc transport parameters were set as AE = 700 keV and PCUT = AP = 10 keV. Furthermore, directional bremsstrahlung splitting was used as variance reduction method to reduce simulation time and improve uncertainty. Number of photon histories was set to 500,000,000 to acquire statistical uncertainties better than 1%. Phase space file, which is the output of the BEAMnrc code, was used as an input to DOSXYznrc code to simulate PDD, beam profiles, and isodose curves in water phantom. BEAMDP, another user code of NRCC, was used to obtain mean energy as well as photon and electron spectrums.
To verify the simulations, PDD and beam profiles at different field sizes and depths (i.e., dmax and 10 cm) and also for different wedge angles (15, 30, 45, and 60) were measured with calibrated Kodak EDR2 films and Sun nuclear profiler2 in perspex slab phantoms. For measuring beam profiles, sun nuclear profilers 2 (1174 model) including 139 diode detectors were used. For measuring PDD of wedges, EDR2 films were sandwiched between slab phantom parallel to central axis of beam and the entrance point of beam which is clear with laser light marked on it. The film edges were adjusted to be aligned with phantom surface, and the slab phantoms were tight carefully to avoid air gap between films and slabs. After irradiation, to minimize the errors related to day-to-day variations of processor, all the films were developed in 1 day. Then, they were scanned with the Microtek scanner (9800XL model). Then, the resulting PDD and profiles were compared to those from simulations.
| > Results|| |
Comparison of measured and simulated PDD and beam profiles for open, PW, and EDW fields show differences up to 2% and 2 mm which is within the accepted criteria.
[Figure 1] shows the PDD for 45 PWs and EDWs along with the open field. For clarity, magnification is done at buildup and depth regions. These results show that there is no difference between PDD of open and EDW fields (below 1%). However, comparison of PDD of open and PW fields shows differences up to 3.5%. For clarity, just data for 45 wedges are illustrated here. Further, the comparison of PDD from PW and EDW fields shows discrepancy up to 5.5%.
|Figure 1: Comparison of percentage depth dose for (a) open field and physical wedge 45. (b) Open field and enhanced dynamic wedge 45|
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[Figure 2] shows the comparison of beam profiles for PW and EDW along wedged direction at depth of maximum dose and 10 cm. These results show discrepancy around 1.5% and 1 mm at low- and high-dose gradient regions.
|Figure 2: Comparison of beam profiles at wedged direction for physical wedge and dynamic wedge 45 at (a) 10 cm depth. (b) Depth of maximum dose|
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[Figure 3] shows that the differences between PW and open fields, EDWs and open fields, PWs and EDWs along nonwedged direction are within 1.5 mm, 1%; 0.5 mm, 0.5%; and 1 mm, 1%, respectively. Isodose curves for 45 PW and EDW angles are also illustrated at [Figure 4].
|Figure 3: Comparison of beam profiles at nonwedged direction for physical wedge and dynamic wedge 45 at (a) depth of maximum dose. (b) 10 cm depth|
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|Figure 4: Isodose curves for (a) physical wedge and (b) dynamic wedge for 45° angle|
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Comparison of mean energy at wedged directions for PWs and DWs along with open fields in [Figure 5] shows that there is an increase in mean energy from toe to heel for PWs. However, mean energy for DWs is the same as open field. In addition, mean energy at central axis for PWs is higher than DW and open field.
|Figure 5: Mean energy at wedged directions for physical and dynamic wedges along with open fields|
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Mean energy at nonwedged direction for 45 PWs and EDWs as well as open fields is shown in [Figure 6]. It can be seen that mean energy at the central axis of the field increased for PW. However, no such differences were found for dynamic wedged field.
|Figure 6: Mean energy at nonwedged directions for physical and dynamic wedges along with open fields|
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Photon and electron spectra calculated by BEAMDP for open, PW, and EDW fields are illustrated in [Figure 7] and [Figure 8]. These spectra were scored at air and SSD of 100 cm.
|Figure 7: Photon spectrum calculated by BEAMDP for open, physical, and enhanced dynamic wedges|
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|Figure 8: Electron spectrum calculated by BEAMDP for open, physical, and enhanced dynamic wedges|
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| > Discussion|| |
Comparison of PDD from open field and PWs in [Figure 1] shows that absorbed dose of PWs at depth beyond dmax is higher than open fields. However, at buildup region, PDD for PWs are less than open fields. The first phenomenon is related to beam-hardening effect produced by PWs which made them more penetrating than open fields. The later can be attributed to electron contamination produced by PWs, which may increase dose at buildup regions.
Significant difference between PDD of PWS and EDWs (about 5.5%) did not recommend using PDD of PWs instead of DWs. However, by considering the similarity of PDD for EDWs and open fields, it can be recommended using PDD from open field instead of DWs without any error. This results in simplicity of monitor unit calculation and also a good advantage in TPS which cannot consider beam-hardening effect produced by wedges.
Dose differences obtained between beam profiles of PW and EDW at wedge direction can be attributed to the hardening effect produced by PWs, which is not presented in EDWs. This finding is in agreement with Ahmad et al., who measured beam profiles of physical and EDWs.
General shape of beam profile at nonwedged direction for PWs is similar to DWs and open field but it can be seen that off-axis dose for PWs is less than that for open and EDWs. This effect is due to beam softening at off-axis distance which made them be absorbed more while transition through PWs.
Similarity of beam profiles at nonwedged direction of DWs and open fields shows that beam profiles of open fields can be used for dose calculation at off-axis distance for enhanced dynamic wedged at nonwedged direction.
From comparison of isodose curves illustrated in [Figure 4], it can be shown that due to beam hardening of PWs, isodose curves are appeared at upper level than that for DWs. However, another important clinical result, which is illustrated in this figure, is that penumbra region for DWs is smaller than PWs. This effect is seen in other studies too.
Differences between mean energy at wedged direction for PWs and EDWs can be related to different beam hardening introduced by PWs. In other words, different thickness of PWs from toe to heel made different beam hardening which in turn results in different absorbed dose at wedged directions.
From comparison of mean energy at nonwedged direction illustrated in [Figure 6], it can be concluded that in spite of DWs, PWs may result in beam hardening not only in wedged direction but also in nonwedged direction. Matching general shape of mean energy for DWs and open fields at wedged and nonwedged direction shows that DWs cannot change beam quality at all.
From [Figure 7], it can be seen that both PWs and DWs reduce photon spectrum differently. On the other hand, general shape of photon spectrum in DWs follows as open fields, which indicates that DW only modulates the beam. However, differences obtained between overall shape of PWs and open field shows that PWs change the photon spectrum. In fact, they attenuate low-energy photons while introduce scattered photon. This effect is responsible for the changes introduced in general shape of the spectrum.
From [Figure 8], it can be seen that PWs dramatically reduce secondary electrons in spite of DWs. This effect is responsible for the decrease of buildup dose as shown in [Figure 1]. This funding is in accordance with Shih et al., who simulated electron spectrum generated by PWs and DWs.,
| > Conclusion|| |
Complete comparison of DWs with PWs with Monte Carlo shows that DWs do not introduce beam hardening in spite of PWs. Hence, it is a good advantage for using in TPS who cannot consider beam hardening. Further, some dosimetric parameters of DW are the same as open fields, so it is recommended that using those dosimetric data of open field for DW fields.
In addition, BEAMnrc is a powerful code at predicting dosimetric data for DWs and it can be replaced for cumbersome dosimetric data for EDW.
The authors would like to thank staff of Pars Hospital for support of this work.
Financial support and sponsorship
Tehran University of Medical Science has financially supported the work.
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8]