The capability of error detection of patient-specific QA tools plays an important role in verify ing MLC motion accuracy. The goal of this study was to investigate the capability in error detection of portal dosimetry, MapCHECK2 and MatriXX QA tools in IMRT plans. The 9 fields IMRT for 4 head and neck plans and 7 fields IMRT for 4 prostate plans were selected for the error detection of QA devices. The measurements were undertaken for the original plan and the modified plans, wh ere the known errors were introduced for increasing and decreasing of prescribed dose (±2%, ±4% and ±6%) and position shifted in X-axis and Y-axis (±1, ±2, ±3 and ±5 mm). After measurement, the results were compared between calculated and measured values using gamma analysis at 3%/3 mm criteria. The average gamma pass for no error s introduced in head and neck plans was 96.9%, 98.6%, and 98.8%, while prostate plans presented 99.4%, 99.0%, and 99.7%, for portal dosimetry, MapCHECK2 and MatriXX system, respectively. In head and neck plan, the shifted error detections were 1 mm for portal dosimetry, 2 mm for MapCHECK2, and 3 mm for MatriXX system. In prostate plan, the shifted error detections were 2 mm for portal dosimetry, 3 mm for MapCHECK2, and 5 mm for MatriXX system. For the dose error detection, the portal dosimetry system could detect at 2% dose deviation in head and neck and 4% in prostate plans, while other two devices could detect at 4% dose deviation in both head and neck and prostate plans. Portal dosimetry shows slightly more capability to detect the error compared with MapCHECK2 and MatriXX system, especially in the complicated plan. It may be due to higher resolution of the detector ; however, all three-detector types can detect various errors and can be used for patient-specific IMRT QA.
Intensity modulated radiation therapy (IMRT) technique is one of the advanced radiation treatment techniques, which allows conformal shaping of dose distribution to tumors and significantly gains in the sparing of normal surrounding tissues. IMRT achieves desired dose distribution in a complex shaped volume by modulating the intensity map of each treatment field using the moving of multileaf collimator (MLC). So, the position of MLC pattern needs to be verified as the patient-specific quality assurance (QA) for all IMRT plans before deliver dose to patients [
Li et al. [
The purpose of this study was to evaluate the error detection sensitivity and capability of patient specific IMRT QA tools of portal dosimetry, MapCHECK2, and MatriXX systems.
There were 8 IMRT plans in this study that consisted of four head and neck and four more prostate plans optimized and calculated according to Radiation Therapy Oncology Group dose constraints protocol using Eclipse treatment planning system Version 11.0.31 (Varian Medical Systems, Palo Alto, CA, USA). The 6 Megavoltage (MV) beams were employed with 9 fields arrangement for head and neck IMRT cases as the complicated plans, while 10 MV beams with 7 fields were optimized and calculated for prostate IMRT plans to represent the simple plans. After radiation oncologist approved the plans, the clinical IMRT plans were converted to IMRT QA verification plans with converting the gantry angle to zero degree orientation for all beams and calculating the dose in water phantom for EPID (Varian Medical Systems, Palo Alto, CA, USA), MapCHECK phantom for MapCHECK2 system (Sun Nuclear Corporation, Melbourne, FL, USA) and MultiCube phantom for MatriXX system (IBA dosimetry, Bartlett, TN, USA). The aS1000 flat panel EPID has the area of 40 × 30 cm2 with the matrix size of 1024 × 768 pixels. MapCHECK consists of 1527 N-type diodes arranged in 2D array on the size of 32 × 26 cm2 with 7.07 detector spacing uniform through array. The MatriXX device consists of a 1020 vented parallel plate ionization chamber detectors with a 7.6 mm center-to-center spacing between chambers that arranged in 32 × 32 cm2 grid area.
After creating original IMRT QA plans, the intentional error plans were created. All errors introduced were based on the most realistic clinical situations. The intentional errors were composed of prescribed dose errors and position shifts. Prescribed dose of increasing and decreasing from 2.0%, 4.0% to 6.0% and position shift of 1.0, 2.0, 3.0, and 5.0 mm in positive and negative ways in both X- and Y-directions were applied. The errors were created in treatment plan for measurement of error detection. Then, all of the QA plans were exported to linear accelerator machine for delivery in each QA tool.
The original plans and plans with intentional errors were measured on ClinaciX linear accelerator (Varian Medical Systems, Palo Alto, CA, USA) by all QA tools, EPID, MapCHECK2 and MatriXX systems. The EPID, which was fixed with Varian ClinaciX machine, was set at 100.0 centimeter (cm) source to detector distance (SDD) as shown in
The gamma evaluation index was recommended to analyze the dose distribution plane between treatment planning system (TPS) calculation and each QA tool measurement. In order to observe the error detection of QA tools, the measurements with intentional errors were compared with the original plans that no any error applied. The Varian portal dosimetry software, SNC patient software, and OmniPro-I’mRT software were used to evaluate using gamma index concept for portal dosimetry, MapCHECK2, and MatriXX, respectively. The gamma index of 3.0% dose difference and 3.0 mm distance to agreement with 10.0% threshold were selected for the evaluation criteria. The gamma-passing rate was set at 95.0% in our routine passing criteria and an error was considered detected when the gamma failure rate was higher than 5.0%. We then analyzed the gamma result of modified plans and evaluated the error detection capability of each QA tool. According to the ethic consideration, this study respect for person authority, principle of beneficence/non-maleficence and justice rule. Although this study does not contact directly to the patients for data collection, the research was approved by Ethics Committee of Faculty of Medicine, Chulalongkorn University.
The measurements were undertaken by measuring the original plan as the first step using Portal dosimetry, MapCHECK2 and MatriXX systems.
After measuring original plans that pass 95.0% gamma result, the plans introduced with 1.0 mm to 5.0 mm shifted in X- and Y-axis errors were measured to evaluate the error detection sensitivity upon position shift of portal dosimetry system using gamma pass criteria of 3.0%/3.0 mm. The smallest error
Case No. | Gamma passing rate (%) | |||||
---|---|---|---|---|---|---|
Portal dosimetry | MapCHECK2 | MatriXX | ||||
H & N | Prostate | H & N | Prostate | H & N | Prostate | |
1 | 95.2 | 99.6 | 97.9 | 100.0 | 97.5 | 99.8 |
2 | 96.6 | 99.9 | 98.6 | 100.0 | 99.3 | 99.9 |
3 | 98.9 | 99.8 | 99.3 | 96.5 | 99.7 | 99.5 |
4 | 96.8 | 98.3 | 98.3 | 99.4 | 98.2 | 99.8 |
Average | 96.9 ± 1.3 | 99.4 ± 0.7 | 98.6 ± 0.6 | 99.0 ± 1.7 | 98.8 ± 1.0 | 99.8 ± 0.2 |
detections were 1.0 mm shifted in head and neck and 2.0 mm shifted in the prostate plans. Considering to different directions, the error detection in some axis showed error started from 3.0 mm shifted in head and neck plan and 5 mm shifted in the prostate. The detected results of portal dosimetry system in head & neck and prostate plans are shown in
Position shift error (mm) | Gamma passing rate (%) | ||
---|---|---|---|
H & N | Prostate | ||
X-axis | 1.0 | 96.5 ± 1.1 | 99.4 ± 0.7 |
2.0 | 96.3 ± 1.7 | 99.1 ± 1.0 | |
3.0 | 92.7 ± 1.9 | 97.9 ± 1.5 | |
4.0 | 70.1 ± 5.2 | 71.3 ± 13.4 | |
X-axis | −1.0 | 94.5 ± 2.0 | 99.1 ± 0.9 |
−2.0 | 86.1 ± 2.3 | 94.2 ± 3.4 | |
−3.0 | 73.0 ± 4.7 | 76.5 ± 12.4 | |
−4.0 | 55.9 ± 9.0 | 60.4 ± 17.6 | |
Y-axis | 1.0 | 94.9 ± 1.9 | 98.9 ± 1.0 |
2.0 | 87.8 ± 3.4 | 94.2 ± 3.5 | |
3.0 | 82.3 ± 3.2 | 83.8 ± 4.6 | |
4.0 | 74.3 ± 5.9 | 70.6 ± 4.6 | |
Y-axis | −1.0 | 96.9 ± 1.4 | 99.4 ± 0.7 |
−2.0 | 96.6 ± 1.5 | 99.3 ± 0.9 | |
−3.0 | 95.2 ± 1.9 | 98.4 ± 0.6 | |
−4.0 | 83.8 ± 4.3 | 80.5 ± 2.5 |
Prescribed dose error (%) | Gamma passing rate (%) | ||
---|---|---|---|
H & N | Prostate | ||
Increasing | 2.0 | 89.5 ± 2.8 | 98.6 ± 1.4 |
4.0 | 82.6 ± 3.0 | 90.9 ± 6.6 | |
6.0 | 75.1 ± 4.2 | 82.8 ± 10.4 | |
Decreasing | −2.0 | 95.1 ± 4.6 | 95.7 ± 3.4 |
−4.0 | 93.3 ± 5.7 | 83.4 ± 12.1 | |
−6.0 | 87.3 ± 7.4 | 73.5 ± 16.1 |
The MapCHECK2 system could detect error starting from −2.0 mm and +3.0 mm in X-axis and +2.0 mm and −3.0 mm in Y-axis for the head neck plan, while the error starting from 3.0 mm in both directions for prostate were detected by MapCHECK2 as shown in
Position shift error (mm) | Gamma passing rate (%) | ||
---|---|---|---|
H & N | Prostate | ||
X-axis | 1.0 | 98.6 ± 0.7 | 98.8 ± 1.7 |
2.0 | 98.0 ± 1.0 | 98.2 ± 2.0 | |
3.0 | 93.4 ± 2.9 | 92.9 ± 3.7 | |
4.0 | 64.3 ± 6.7 | 60.8 ± 13 | |
X-axis | −1.0 | 96.8 ± 1.8 | 98.8 ± 2.0 |
−2.0 | 91.1 ± 3.1 | 97.1 ± 3.4 | |
−3.0 | 75.7 ± 3.3 | 84.1 ± 10 | |
−4.0 | 49.6 ± 2.4 | 53.9 ± 19 | |
Y-axis | 1.0 | 97.0 ± 1.7 | 99.0 ± 1.7 |
2.0 | 92.6 ± 2.6 | 95.5 ± 4.4 | |
3.0 | 87.3 ± 3.7 | 87.4 ± 5.7 | |
4.0 | 75.0 ± 4.3 | 75.1 ± 11 | |
Y-axis | −1.0 | 98.5 ± 0.4 | 98.7 ± 1.5 |
−2.0 | 96.9 ± 0.6 | 96.9 ± 2.5 | |
−3.0 | 93.6 ± 2.2 | 90.1 ± 5.1 | |
−4.0 | 81.1 ± 2.3 | 68.7 ± 5.7 |
Prescribed dose error (%) | Gamma passing rate (%) | ||
---|---|---|---|
H & N | Prostate | ||
Increasing | 2.0 | 98.1 ± 0.3 | 98.8 ± 1.0 |
4.0 | 96.0 ± 1.2 | 91.1 ± 6.0 | |
6.0 | 89.3 ± 2.9 | 82.8 ± 8.1 | |
Decreasing | −2.0 | 95.4 ± 4.2 | 95.4 ± 4.9 |
−4.0 | 88.9 ± 8.7 | 85.3 ± 8.0 | |
−6.0 | 80.3 ± 12.2 | 75.9 ± 5.3 |
The results of position shifted error measured by MatriXX system is shown in
Position shift error (mm) | Gamma passing rate (%) | ||
---|---|---|---|
H & N | Prostate | ||
X-axis | 1.0 | 97.4 ± 1.5 | 99.4 ± 0.4 |
2.0 | 95.1 ± 2.1 | 98.6 ± 0.5 | |
3.0 | 83.3 ± 4.6 | 96.7 ± 0.9 | |
4.0 | 66.8 ± 7.8 | 92.0 ± 1.2 | |
X-axis | −1.0 | 98.2 ± 1.4 | 99.5 ± 0.5 |
−2.0 | 95.5 ± 2.7 | 99.2 ± 0.7 | |
−3.0 | 89.3 ± 5.2 | 98.1 ± 1.1 | |
−4.0 | 72.2 ± 7.1 | 93.5 ± 1.3 | |
Y-axis | 1.0 | 98.6 ± 1.2 | 99.7 ± 0.2 |
2.0 | 97.9 ± 1.5 | 99.0 ± 0.8 | |
3.0 | 95.8 ± 1.7 | 97.6 ± 0.9 | |
4.0 | 88.0 ± 2.5 | 92.8 ± 2.1 | |
Y-axis | −1.0 | 98.2 ± 1.4 | 99.7 ± 0.2 |
−2.0 | 96.9 ± 2.1 | 99.4 ± 0.4 | |
−3.0 | 93.9 ± 3.1 | 98.1 ± 0.9 | |
−4.0 | 85.7 ± 4.2 | 94.1 ± 2.4 |
Prescribed dose error (%) | Gamma passing rate (%) | ||
---|---|---|---|
H & N | Prostate | ||
Increasing | 2.0 | 97.9 ± 1.1 | 98.9 ± 1.1 |
4.0 | 94.8 ± 1.2 | 97.4 ± 1.5 | |
6.0 | 84.3 ± 6.3 | 96.0 ± 1.8 | |
Decreasing | −2.0 | 95.1 ± 3.1 | 98.8 ± 0.9 |
−4.0 | 85.5 ± 8.1 | 97.3 ± 1.2 | |
−6.0 | 75.6 ± 11.3 | 95.5 ± 2.1 |
The percent gamma passing rate of the original head and neck plans using by 3.0%/3.0 mm criterion with 10.0% threshold were 96.9 ± 1.3, 98.6 ± 0.6 and 98.8 ± 1.0 for portal dosimetry system, MapCHECK2 system and MatriXX system, respectively. The results of prostate were 99.4 ± 0.7, 99.0 ± 1.7 and 99.8 ± 0.2 for portal dosimetry system, MapCHECK2 system and MatriXX system, respectively. All of the plans were pass the tolerance limit of 95.0% gamma pass. These results were agreed with the gamma passing rate reported by Son et al. [
The 95.0% gamma pass was set as the criteria to determine the error detection ability. The position-shifted errors were applied by shifting in X-axis (lateral direction) and Y-axis (longitudinal direction) with the magnitude of 1.0 mm, 2.0 mm, 3.0 mm and 5.0 mm. For the head and neck IMRT plans, the error detection by the portal dosimetry showed the smallest error detection of 1.0 mm shifted in X-axis (negative direction) and Y-axis (positive direction). At the same condition the error of 2.0 mm and 3.0 mm shifted were detected by MapCHECK2 system and MatriXX system, respectively.
In the prostate plans, the smallest error detection of 2.0 mm shifted can be seen with the portal dosimetry system in X-axis (negative direction) and Y-axis (positive direction). For other directions, the error of 5.0 mm shifted was detected by portal dosimetry. The other two devices, MapCHECK2 system can detect at 3.0 mm shifted with and 5.0 mm shifted with MatriXX system in all directions. For the prostate, the target organ was quite round shape and the error detection in the different direction showed the same magnitude, while the error detection in different direction showed different results in the head and neck plan because of irregular shape of the target volume.
When the error magnitudes increase, the gamma passing results decrease. The decrease in percent gamma pass for 1.0 mm shifted was around 1.0% for all devices. However, when the higher magnitude of error was introduced, the decrease percent gamma result was more different for each device. At 5.0 mm shifted, the average error from all position shifted and in both regions from MapCHECK2 system showed 32.7% gamma pass decreases from the original plan results, whereas portal dosimetry decreased 27.3% gamma pass and MatriXX system was only 13.7% gamma pass reduction. The MatriXX was less sensitive to the more error introduced.
Because of the fine resolution of portal dosimetry system (0.39 × 0.39 mm) compared to the other two devices, small displacement in fluence map of the IMRT plan results the higher drop of the percent gamma pass [
Increasing or decreasing of dose from 2.0% to 6.0% was performed to detect the prescribed dose error measurements. The results were not evidently different for the error detection of all devices. These attributes to the response of the dose of all detectors were not significantly different. The 2.0% increasing dose error in head and neck plan was detected by portal dosimetry, while other two devices can detect at 4.0% dose error. In the prostate plan, portal dosimetry system and MapCHECK2 system can find out of 4.0% dose error, while the 6.0% dose error was detected by MatriXX system. The gamma passing rate of three devices showed not significantly drop like in the position shifted error plan.
When the dose error was introduced, the percent gamma was exactly decreased. The higher decreasing of results was seen in portal dosimetry system, which the average passing rate in both increase and decrease in dose error and in both regions was 94.7%, 87.6% and 79.7% for the 2.0%, 4.0% and 6.0% dose errors, respectively. The results of MapCHECK2 system were 96.9%, 90.3% and 82.1% and the results of MatriXX were 97.7%, 93.8% and 87.9%. As the error magnitude was increased, the portal dosimetry system reacted evidently with the rapid change of the percent gamma pass. Bojechko and Ford [
The introduced of prescribed dose was distributed over all area of the fluence rather than at a single point. Even the measurement over the entire fluence map was same for all devices but fine resolution point could meet all deviation of dose from original plan and modified plans. Once the error was introduced (2.0%, 4.0% and 6.0% prescribed dose increasing or decreasing) the fine detection point of the portal dosimetry system could show more deviation of the fluence from the original plan rather than MapCHECK2 system and MatriXX system. So, even the same magnitude of dose change, the portal dosimetry could find more point of disagreement for original plan and the modified plans. It was the reasons that portal dosimetry system could detect the small magnitude of error than other two QA devices.
All the devices perform well in terms of error detection, and error detection of each QA tool depending on the error types, sites, tumor shapes, plan complexities and also gamma criteria used to analyze. Each device has advantages and disadvantages upon its usage. The configurations of QA tools also play an important role in the sensitivity of error detection. The higher the resolution of QA tools, the better the sensitivity of error detection. From the results, it can be concluded that portal dosimetry system has marginally higher sensitivity than MapCHECK2 and MatriXX systems in both shifted positions and dose errors because it has higher resolution of detectors.
The authors declare no conflicts of interest regarding the publication of this paper.
Sanghangthum, T., Lat, S.Z. and Suriyapee, S. (2019) Investigation of Error Detection Capabilities of Various Patient-Specific Intensity Modulated Radiotherapy Quality Assurance Devices. International Journal of Medical Physics, Clinical Engineering and Radiation Oncology, 8, 21-31. https://doi.org/10.4236/ijmpcero.2019.81003