Electron Virtual Wedge: Validation and Dosimetric Evaluation
Mostafa A. Hashem1*; Ahmed L. Elattar2; Mahmoud A. Hefni2; Mohamed I. Elsaid1; and Moamen M. Aly1
1 Radiotherapy and Nuclear Medicine Department, South Egypt Cancer Institute, Assiut University, Assiut, Egypt.
2 Physics Department, Faculty of Science, Assiut University, Assiut, Egypt.
*Corresponding Author:
Dr Moostafa Ahmed Hashem, PhD
Medical Physicist,
Radiotherapy and Nuclear Medicine Department,
South Egypt Cancer Institute, Assiut University,
Assiut, EGYPT
Tel.: +2(0)882348612
Fax: +2(0)882348609
Email:
Abstract
Virtual wedge (VW) was developed in mid-nineties and has been used in photon radiotherapy since then. The idea was that a dose profile similar to that obtained by physical wedge can be generated by moving one of the collimator jaws at a constant speed while changing the dose rate. In this paper, validation of electron VW was made and the dosimetric properties of 7, 10, 12 and 14 MeV electron beams from a Siemens Mevatron linear accelerator, including percentage depth doses and dose profiles for 10 × 10 cm2 and 15 × 15 cm2 applicators, were determined. In addition, comparison between depth doses for open and wedged fields for all studied applicators and energies was made. This study showed that the validation of VW on electron radiotherapy is visible and the behavior of VW in electron beams is similar to that of photon beams.
Keywords: Virtual Wedge; Electron Beam; Radiotherapy
Introduction
Wedge-shaped iso-doses required in many clinical situations. Sloping distributions obtained by inserting a physical wedge in the beam. However, physical wedges have several unfavorable functional and dosimetric characteristics such as beam hardening, fixed wedge angles, and limited field size[1].
Varian produced Enhanced Dynamic Wedge (EDW) to provide seven wedge angles (10, 15, 20, 25, 30, 45, and 60 degrees), generating sloping dose distributions by moving one of the jaws with variable speed while the opposite remains steady. The relationship between jaw position and the percentage of total monitor units delivered described in a segmented treatment table. The segmented treatment table used to generate the primary intensity function matrix used by the pencil-beam convolution (PBC)[1].
The Siemens virtual wedge (VW) produced a beam profile similar to that with a physical wedge by moving one of the upper collimator jaws across the field during irradiation. This jaw motion produces a pattern of energy fluence across the field that Siemens expresses by the formula [2-5]:
(1)
where x is the distance away from the central axis, μ is a generic effective linear attenuation coefficient, α is the desired wedge angle, and c is a factory-set attenuation calibration factor near unity, used to more accurately specify μ for the specific beam.
The VW angle can be set to any desired value when treating a patient. For Siemens linear accelerator, the VW is only implemented in the direction of Y-jaws. For each Y-jaw, the maximum open position is 20 cm, and the maximum to pass the central axis is 10 cm. Since only one jaw is employed in the wedge operation, the maximum range of the moving jaw is +20 to -10: 30 cm.
In this study, we developed the application of VW in electron beams based on the principles of photon VW.
Materials and Methods
A. VW implementation
The option of irradiation using VW in the electron mode is not available in the Siemens linear accelerator. Therefore, a way to implement this option was developed. For each inserted electron applicator, the secondary collimator opens for a larger field size to cover the upper aperture of the applicator. This large field size was divided to smaller fields (segments) by closing one jaw that is in the direction of the desired wedge angle. The number of MU per each segment was calculated using the equation (1) and then delivered for each segment. This process was repeated until the jaw reaching the original location.
B. Measurements
The measurements of the beam central axis depth doses and off-axis beam profiles for 7, 10, 12 and 14 MeV electron beams were carried out using Kodak X-Omat V radiographic films which is a well established method for electron beam dosimetry [6, 7]. These measurements were carried out by removing the film from its envelope in a dark room, sandwiching it between two slabs of solid water with the edges sealed using black tape. The film placed parallel to the electron beam , as shown in Figure 1, and additional solid water is add on both sides until there is at least 5 cm of excess phantom on each side of the field to ensure reaching a good electron equilibrium around the film. Finally, the film assembly is align to the surface of the phantom and entice assembly is tighten with a clamp to prevent artifacts which might arise due to air pockets between the film and the phantom.
Figure 1: X-Omat V radiographic film setup in solid water for PDD and Beam profiles measurements.
The X-Omat films were irradiated with VW angles of 15°, 30°, 45°, and 60° for applicator sizes 10 × 10 cm2 and 15 ×15 cm2. The surface of the film was placed at the source to surface distance (SSD) of 100 cm. The central axis percentage depth dose and the off-axis beam profiles at depths of dmax, d90%, d80%, d70%, d60% and d50% cm were carried out using mechanical laser film densitometer scanner (DynaScan Dosimetry System, CMS Associates Inc.).
Results and Discussions
Figure 2 displays the percentage depth doses for open and virtual wedge angles of 15°, 30°, 45°, and 60° with the applicator size of 15 × 15 cm2 at SSD of 100 cm for 7, 10, 12 and 14 MeV electron beams. It is clear that there is no change in the PDDs due to the use of virtual wedge even at different wedge angles, compared to the open PDD. These results are in a good agreement with [8, 9] where they found that down to 15 cm, in case of photon beams, the PDD for open and virtual wedge were identical. This can be justifying due to the absence of any hardening materials in the beam direction that could alter the PDD.
Figure 2: The percentage depth dose of 15 × 15 cm2 applicator at SSD of 100 cm for open and 15°, 30°, 45°, and 60° virtual wedges for (A) 7 MeV, (B) 10 MeV, (C) 12 MeV and (D) 14 MeV electron beams.
Off-axis profiles for the open and wedged (15°, 30°, 45° and 60°) fields in the wedges direction at the depth of maximum dose, dmax and SSD of 100 cm for the applicator sizes of 10 × 10 cm2 and 15 × 15 cm2 in the 7 and 14 MeV electron beams were presented in figures 3 and 4 respectively.
Off-axis profiles in the wedge direction showed a small change with the electron beam energy at the same virtual wedge angle, which found to be similar to those found in photon beams [10]. However, the profiles shape and tilt were overlapped for each virtual wedge angle (15°, 30°, 45°, and 60°) and energy (7, 10, 12, and 14 MeV) while changing the applicator size (10 × 10 cm2 and 15 × 15 cm2). The Penumbra values (Penumbra is the average distance separating the 80% and 20% isodose lines) for VW fields were less than open fields for all energies and applicator sizes.
Figure 3: Off-axis profiles of applicator sizes 10 × 10 cm2 and 15 × 15 cm2 at the depth of dmax while SSD equal to 100 cm for (A) 15°, (B) 30°, (C) 45° and (D) 60° virtual wedge angles in 7 MeV electron beam.
Figure 4: Off-axis profiles of applicator sizes 10 × 10 cm2 and 15 × 15 cm2 at the depth of dmax while SSD equal to 100 cm for (A) 15°, (B) 30°, (C) 45° and (D) 60° virtual wedge angles in 14 MeV electron beam.
Table (1) show the penumbra values for open and (15°, 30°, 45° and 60°) wedges at the depth of dmax and SSD of 100 cm in 7, 10, 12 and 14 MeV beams with applicator sizes of 10 × 10 cm2 and 15 × 15 cm2.
The electron open field penumbra (20%–80% intensity) decreased with higher energies. For the same energy, it has been found that the penumbra decreased with increase wedge angle. At high energies, the beam is more forward scattered, with less lateral scattering, giving rise to a narrow penumbra. This is expected because high-energy electrons are subject to less scattering [11].
7 MeV / 10 MeV / 12 MeV / 14 MeVOpen / 10 × 10 cm2 / 1.44 / 1.35 / 1.33 / 1.32
15 × 15 cm2 / 1.54 / 1.46 / 1.45 / 1.41
VW 150 / 10 × 10 cm2 / 0.97 / 1.02 / 1.1 / 1.29
15 × 15 cm2 / 1.05 / 1.04 / 1.2 / 1.31
VW 300 / 10 × 10 cm2 / 0.95 / 1.01 / 1.05 / 1.04
15 × 15 cm2 / 0.99 / 1.02 / 1.08 / 1.14
VW 450 / 10 × 10 cm2 / 0.9 / 0.85 / 0.84 / 0.96
15 × 15 cm2 / 0.95 / 0.9 / 0.99 / 0.99
VW 600 / 10 × 10 cm2 / 0.87 / 0.84 / 0.79 / 0.84
15 × 15 cm2 / 0.92 / 0.87 / 0.87 / 0.86
Table (1): Penumbra values of applicator sizes 10 × 10 cm2 and 15 × 15 cm2 at the depth of dmax while SSD equal to 100 cm for open and (A) 15°, (B) 30°, (C) 45° and (D) 60° virtual wedge angles in 7 MeV, 10 MeV, 12 MeV, 14 MeV electron beams.
Figures (5) and (6) show the isodose distributions crossing the central axis for 7 MeV and 14 MeV beams respectively, for open and, 15°, 45°, and 60° virtual wedge angles in 15 × 15 cm2 field sizes at 100 cm SSD. It is clear the effect of using the virtual wedge on the iso-dose curves.
Figure 5: The iso-dose curves of 15 × 15 cm2 applicator at SSD of 100 cm for (A) open and, (B) 15°, (C) 45°, and (D) 60° virtual wedges for 7 MeV beam.
Figure 6: The iso-dose curves of 15 × 15 cm2 applicator at SSD of 100 cm for (A) open and, (B) 15°, (C) 45°, and (D) 60° virtual wedges for 14 MeV beam.
Conclusions
This is a simple technique to apply the use of virtual wedges in electron beams. The paper explained that we could use the idea of virtual wedges used in x-ray radiotherapy and implement it in electrons radiotherapy. This study showed that the behavior of the electron beams using virtual wedge is similar to that of x-ray beams.
References
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