Purpose: To study the dosimetric characteristics of amorphous silicon Electronic Portal Imaging Device EPID and 2D array detector for dose verification of radiotherapy treatment plans, and the quality assurance QA testing of IMRT was investigated. Materials and methods: All measurements were done with Varian IX linear accelerator, aSi-1000 EPID and 2D array detector. The dose linearity, reproducibility, output factors, dose rate, SDD and response with slap phantom thickness have been measured and compared against those measured by ion chamber. Results: The characteristics of EPID and 2D array: the response of EPID agreed with 2D array and ion chamber 0.6cc. EPID and 2D array showed short-term output reproducibility with SD = 0.1%. The dose rates of 2D array SD = ±0.7%, EPID = ±0.4% compared with a 0.6 cc SD = ±0.5%. Output factor measurements for the central chamber of the EPID and 2D array showed no considerable deviation from ion chamber measurements. Measurement of beam profiles with the EPID and 2D array matched very well with the ion chamber measurements in the water phantom. The EPID is more sensitive to lower energy photons by increasing solid water phantom thickness. The mean and standard deviation passing rates ( <i> γ </i> % <sub> ≤1 </sub> ) for film, 2D array and EPID for 30 IMRT fields of five patients were 95.93 ± 0.96%, 99.05 ± 0.24%, and 99.37 ± 0.12%, respectively. Conclusion: The study shows that EPID and 2D array are a reliable and accurate dosimeter and a useful tool for quality assurance. We found that the EPID was more accurate compared with both 2D array and ion chamber. The gamma criterion of 3%/3 mm is the most suitable criteria for IMRT plans of QA.
The quality assurance (QA) procedure in radiotherapy generally demands dose measurement as well as patient positioning check. In conventional techniques, the dosimetric verification is based on well-tried methods carried out mostly during treatment sessions [
Van Esch et al. (2004) [
The gamma index (GI) evaluation used to evaluate measured distributions in detector systems against the dose distribution predicted by treatment planning system. The Gamma index (GI) results of each plan were recorded for the passing criteria, and evaluated 3% DD, 3 mm DTA criteria for passing result by using EPID and 2D array detector, calculated the mean and Standard Deviations (SD) for each plan [
This study was carried out to evaluate the dosimetric characteristics performance of EPID and 2D array for IMRT dose verification. To quantify the performance of the device, some of the basic dosimetry tests were carried out and also some of the tests were compared with the ionization chamber measurements. The basic tests included linearity, reproducibility, output factors dependency, dose rate dependency, and sensitivity for photon beams. The measurements carried out by the 2D array and EPID devices for verification of IMRT plans are also presented, and the same was compared with the film dosimetry measurements.
A high-energy linear accelerator (Varian Clinac IX; Varian Medical Systems, Palo Alto, CA) with nominal 6 and 18 MV photon beams has been installed, in radiation oncology department, Ain Shams University Hospital. In the present study, we applied a 300 MU min−1 fixed pulse rate that is used in clinical practice. Portal Vision aSi-1000 imager panel of Varian Clinac was used, with a pixel dimension and spatial resolution of 1024 × 768 and 0.392 mm per pixel, respectively. EPID is a useful tool in the QA process with good evaluation abilities, the Portal Vision Exact-Arm (Medical Systems of Varian). The linear accelerator includes an aS-1000 Portal Vision imager and comprises of an 8 mm thickness main plate, a thin copper slice (1 mm), a 0.5 mm phosphor film. 2D array detector 1500 is 1405 ion chamber matrix in a plane of field size 27 × 27 cm2, and plane-parallel detectors are 4.4 × 4.4 × 3 mm3 in size, with spacing (centre-to-centre) of 7.1 mm for IMRT verification using the VeriSoft software enables physicists to compare radiation dose distributions in IMRT verification plan with those calculated by TPS. PTW Farmer chamber (0.6 cm3) is the chamber to measure the absolute photon and electron dosimetry used in solid-state material. The pinpoint chamber is ideal for dose measurements in small fields. It’s a small-sized sensitive volume of 0.015 cm3 and 2 mm in diameter, vented to air. Very high spatial resolution when used for scans perpendicular to the chamber axis. The slab phantom (RW3) consists of 33 plates machined to 30 × 30 cm2 of various thickness. The mass density of RW3 is 1.045 g・cm−3 and the electron density has a factor relative to water of 1.012. The phantoms are used for monitor calibration and QA measurements, dose is measured by varying the measuring depth.
All measurements were performed on amorphous silicon aSi-1000 EPID, 2D array 1500, waterproof chamber and IX accelerator (Varian) with nominal 6 and 18 MV energy photon beams. In this process a fixed pulse rate of 300 MU min−1 is used which the pulse rate used in clinical practice.
The properties of 2D array and electronic portal imaging device are verified in this study. The 2D Array 1500 measurement setup used throughout this process is shown in
1) Dose Linearity: To verify the linear response with dose, detectors are irradiated with dose setting of 2, 5, 10, 20, 50, 100, 150, 200, 250, 300 and 500 MUs (monitor Units). The responses were compared with the measurements of 0.6 cc, EPID and 2D array. 2D array in the standard setup as shown in
2) Dose Reproducibility: The reproducibility is the % difference between successive measurements for the same radiation dose. The performance of EPID and 2D array in standard setup
3) Dose rate dependency: The dose rate response of EPID and 2D array in standard setup
4) Field size dependence: Field size response of the EPID and 2D array in standard setup
5) SDD dependency: The SDD dependency was studied for 6 MV and 18 MV photon beams to evaluate the effect of SDD on EPID and 2D array in standard setup
6) Verification of response with slap phantom thickness: Intensity verification of a photon beam is reduced by increasing the thickness of RW3 slab phantom with EPID or 2D array, each phantom is positioned on the beam central axis of the treatment couch and the distance from the source to the centre of the phantom SAD is 100 cm.
measured as the mean pixel value in a 1 cm region of interest in the centre of the image. For the alternative method, the 2D array was inserted in a 30 × 30 cm2 solid water phantom slab, with appropriate build-up (1.5 cm for 6 MV) and 1 cm solid water phantom for back scatter to simulate the EPID.
In this study, IMRT QA with film, 2D array and EPID were used in the gamma index method to compare calculated TPS dose with measured dose, using 3% 3 mm gamma criteria. The traditional method of QA for IMRT was 2Dimensional testing using film (Kodak X-OMAT). The dose was measured at source to axis distance (SAD) of 100 cm, with the film located at 10 cm depth of slab phantom and a gantry angle of 0˚. Similar to film QA, the dose is measured at the standard measurement of setup for the 2D array at a gantry angle of 0˚. For EPID, SAD was set at 100 cm and at a gantry angle of 0˚. Measurement was done by EPID without phantom and EPID dedicated software (Eclipse, Ver. 8.9, Varian Medical System, and USA) was used to verify dose delivery after the beam measurement. IMRT was used for three (two males and one female) head and neck (H&N) plans, one male cervical spine and one male pelvises were selected to evaluate the mean and standard deviations (SD) of gamma index. The Gamma evaluations (DD and DTA) of measured dose against TPS calculated doses were performed for 25 IMRT cases (177 Fields). All the cases were planned in Eclipse treatment planning system and the QA plans for absolute point dose measurements, portal dosimetry, and 2D array were created for the TPS calculated planar dose distributions. The calculated and measured dose for each plan was compared on the basis of 3% 3 mm gamma criteria (DD and DTA). For the portal dosimetry, area gamma > 1%, average gamma, and maximum gamma were measured and tabulated. For the 2D array, the percentage of the pixels passed the acceptance criteria 3% 3 mm were calculated and tabulated. The mean and standard deviation for all the gamma parameters were calculated and compared. The criteria validity accepted as section with gamma value γ%≤1 = 95%. Gamma parameters, γmax, γavg and γ%≤1 were estimated for each field and calculated the mean and standard deviations (SD).
The characteristics of EPID and 2D array detectors have been investigated for periodic QA applications. Study verification of characteristics linearity, Reproducibility, dose rate, Field size, SSD, and response with slap phantom thickness response for both 2D array detector and EPID.
1) Dose Linearity: The comparison of measured values for different monitor units ranging 2 to 500 MU were analyzed for 0.6 cc ion chamber, 2D array and EPID at field size 10 × 10 cm2. The linearity result of 2D array and EPID were compared with ion chamber 0.6 cc as shown in
2) Dose Reproducibility: The reproducibility of the measurements within each set was excellent short-term stability as shown in the
variation from chamber to chamber was ±1%. The EPID and 2D array demonstrates excellent short-term output reproducibility with a maximum standard deviation of 0.1%.
3) Dose rate dependency: The response of dose rate for EPID and 2D array compared to 0.6cc ion chamber measurements for 6 MV photon energy, at 10 × 10 cm2 field size at various dose rates (100, 200, 300, 400, 500 & 600 MU/min) at a fixed distance of 100 cm as shown in
4) Field size dependence: The output factor measurement results are shown in
5) SDD dependency: The responses of detectors of SDD for 6 MV and 18 MV photon beams are displayed in
6) Verification of response with slap phantom thickness: The intensity of a photon beam was reduced as the absorbing material thickness was increased.
Both the detector system showed good response for IMRT patient specific QA. The EPID field verification could be done very effectively with an excellent spatial resolution. The disadvantages of the 2D array system are: the low resolution of the detectors, the time taken to set up the detectors, phantom and connect to the external computer system with analysis software. Syamkumar S. A. et al. (2012) [
The Dose was calculated using TPS compared with dose measured by the dosimetric tools based on gamma evaluation (3%/3 mm).
Dosimetry tools | Patient 1 | Patient 2 | Patient 3 | Patient 4 | Patient 5 | Mean ± SD |
---|---|---|---|---|---|---|
Film | 97.41 | 95.45 | 96.03 | 94.82 | 95.95 | 95.93 ± 0.96 |
2D array | 99.12 | 99.28 | 98.74 | 98.87 | 99.26 | 99.05 ± 0.24 |
EPID | 99.43 | 99.22 | 99.32 | 99.54 | 99.34 | 99.37 ± 0.12 |
of five patients were 95.93% ± 0.96%, 99.05% ± 0.24%, and 99.37% ± 0.12%, respectively. All acceptable passing rate of 95%, these tools showed some differences in measuring the same beam, with the gamma index being much lower for film than for the other tools. This comparison of gamma indices for film, 2D array, and EPID showed differences in dose distribution when using various dosimetric tools to carry out the QA for the same patients IMRT, results of QA dependency on dosimetric tools. We found that the EPID was more accurate compared with both 2D array and film.
A dosimetric IMRT field verification of 177 IMRT fields was carried out with 2D array detector by comparing the measured dose distribution to portal dosimetry measurement and TPS calculations. The results of gamma evaluation for 25 cases were tabulated as shown in the
Studies of dosimetric characteristics are essential before using at all dosimetric tools for the clinical purpose. At the present time portal dosimetry and 2D array
Patient No. | Site | Portal Dosimetry | 2D array | ||||
---|---|---|---|---|---|---|---|
γ%≤1 (%) | γmax | γavg | γ%≤1 (%) | γmax | γavg | ||
1 | Head & Neck | 98.1 | 3.35 | 0.24 | 97.62 | 2.95 | 0.26 |
2 | Head & Neck | 97.8 | 2.53 | 0.26 | 96.82 | 2.42 | 0.28 |
3 | Head & Neck | 99.4 | 1.66 | 0.21 | 97.84 | 1.31 | 0.25 |
4 | Head & Neck | 97.9 | 1.42 | 0.22 | 98.29 | 1.44 | 0.31 |
5 | Pelvis | 99.5 | 1.47 | 0.25 | 97.37 | 1.39 | 0.29 |
6 | Pelvis | 99.4 | 1.71 | 0.24 | 98.55 | 1.58 | 0.34 |
7 | Pelvis | 99.1 | 1.49 | 0.35 | 97.1 | 1.62 | 0.27 |
8 | Pelvis | 98.7 | 1.21 | 0.23 | 97.4 | 1.16 | 0.23 |
9 | Head & Neck | 98 | 3.35 | 0.29 | 98.83 | 2.76 | 0.32 |
10 | Head & Neck | 99.2 | 2.71 | 0.21 | 97.61 | 1.83 | 0.27 |
11 | Pelvis | 100 | 1.81 | 0.18 | 98.74 | 1.24 | 0.23 |
12 | cervical spine | 98.5 | 2.15 | 0.28 | 97.83 | 1.31 | 0.28 |
13 | Head & Neck | 98.7 | 2.52 | 0.27 | 97.52 | 1.37 | 0.32 |
14 | Head & Neck | 99.4 | 1.91 | 0.25 | 97.43 | 1.69 | 0.29 |
15 | Head & Neck | 99.8 | 1.94 | 0.27 | 98.4 | 1.39 | 0.24 |
16 | Pelvis | 99.2 | 2.6 | 0.19 | 98.35 | 1.85 | 0.23 |
17 | Pelvis | 99.9 | 1.8 | 0.23 | 97.53 | 1.58 | 0.21 |
18 | Pelvis | 99.9 | 1.49 | 0.21 | 98.75 | 1.32 | 0.29 |
19 | Head & Neck | 98.4 | 2.15 | 0.22 | 97.46 | 1.61 | 0.25 |
20 | Pelvis | 97.9 | 2.73 | 0.26 | 98.21 | 1.89 | 0.32 |
21 | Pelvis | 99.7 | 2.57 | 0.18 | 97.52 | 1.79 | 0.26 |
22 | Pelvis | 99.9 | 2.1 | 0.2 | 98.87 | 1.37 | 0.19 |
23 | Pelvis | 99.8 | 1.85 | 0.17 | 99.68 | 1.81 | 0.24 |
24 | Pelvis | 98.5 | 1.92 | 0.21 | 99.74 | 1.38 | 0.26 |
25 | Pelvis | 98.5 | 2.02 | 0.22 | 98.12 | 1.42 | 0.32 |
Mean | 99.01 | 2.10 | 0.23 | 98.06 | 1.66 | 0.27 | |
SD | 0.74 | 0.57 | 0.04 | 0.75 | 0.45 | 0.04 |
detector verification systems are adopted for the patient specific QA due to excellent dosimetric characteristics and easiness to use. Dosimetric properties of aSi1000 EPID and 2D array system proved its worth over film and other dosimetric system. The dosimetric characteristics are required for the development of an effective and efficient algorithm and dosimetric measurement tool for the better accuracy. Both the detector system showed good response for IMRT and VMAT patient specific QA. With the introduction of aSi1000 EPID individual field verification can be done very effectively with an excellent spatial resolution. The disadvantages of the 2D array system are the low resolution of the detectors and the time taken to setup the detectors and phantom connect with the external computer system with analysis software. The values obtained with the portal dosimetry system were found to be relatively more consistent compared to those obtained with 2D array detector system. For pretreatment verification of IMRT plans have been carried out using the 2D array and verification by EPID dose measurement, the passing criteria for IMRT plans was based on the percent of pixels passing gamma > 95% within the passing criteria of dose difference (DD) 3%, distance to agreement (DTA) 3 mm DTA. The result shows an agreement between the measurement by the EPID and 2D array. Every point measured in these plans agreed to within ±5% acceptability criteria of the dose calculated by the planning system and the chamber measured dose.
The results showed that both of 2D array and EPID can be used in patient specific QA measurements for IMRT. It is a useful tool for the quality assurance and the verification of radiotherapy plans. The 2D array provides an overall accuracy when compared with single ionization chamber measurements for IMRT delivery. Moreover, the dose calibration for the 2D array is easy and stable. But EPID is more accurate dosimeter and a useful tool for quality assurance. The EPID of IMRT patient specific QA is great potential for saving time and for the verification of individual IMRT fields. The disadvantages of the 2D array system are: the low resolution of the detectors, the time taken to set up the detectors, phantom and connect to the external computer system with analysis software. The results showed that the gamma criterion of 3%/3 mm is the most suitable criteria for IMRT plans QA. The result shows a very good agreement between measured dose and calculated dose of the TPS, proving that our treatment planning using patient specific IMRT QA is the sufficient practice for IMRT treatment.
The authors declare no conflicts of interest regarding the publication of this paper.
Ibrahim, A.G., Mohamed, I.E. and Zidan, H.M. (2018) Dosimetric Comparison of Amorphous Silicon EPID and 2D Array Detector for Pre-Treatment Verification of Intensity Modulated Radiation Therapy. International Journal of Medical Physics, Clinical Engineering and Radiation Oncology, 7, 438-452. https://doi.org/10.4236/ijmpcero.2018.74037