CHAPTER 5. STUDY OF ANGULAR RESPONSE OF asi 1000 EPID AND IMATRIXX 2-D ARRAY SYSTEM FOR IMRT PATIENT SPECIFIC QA

Transcription

1 CHAPTER 5 STUDY OF ANGULAR RESPONSE OF asi 1000 EPID AND IMATRIXX 2-D ARRAY SYSTEM FOR IMRT PATIENT SPECIFIC QA 5.1 Introduction With the advent of new techniques like intensity modulated radiotherapy (IMRT), radiotherapy treatment precision requires evolving quality assurance. 2-D arrays with ionization chambers and portal dosimetry system have been widely used and characterized for dose verification of IMRT. Amerio et al. (2004) described the design and construction of ImatriXX 2-D array, whereas Herzen et al. (2007) extensively evaluated it s dosimetric properties. Dosimetric properties of asi1000 electronic portal imaging device (EPID) and it s validation and clinical use were mentioned in a literature survey by Elmpt et al. (2008). In that survey, they reported about the sag of portal imager while using for QA other than zero degree gantry angle. 2-D array detectors have also shown angular dependence while positioned on the treatment couch. Li et al. (2010) and Shimohigashi et al. (2012) reported the angular dependence of central and off-axis detectors in a 2-D ionization chamber array, MatriXX, and suggested the correction methods to improve the accuracy in patient specific QA results. Studies are limited about the dosimetric evaluation of 2-D arrays while positioned in gantry holder as well as the angular dependence of portal dosimetry system for IMRT patient specific QA in true gantry angles. With this background 68

2 this study was performed, to assess the angular response of asi1000 EPID of Varian medical system and the ImatriXX 2-D array system of IBA dosimetry and to validate the detectors for the IMRT patient specific QA measurements in true gantry angles. 5.2 Materials and Methods In this study all the measurements were performed on a 6 MV beam Varian Clinac ix linear accelerator (LINAC). The asi 1000 EPID of Varian medical system and ImatriXX 2-D array system of IBA dosimetry were subjected to study the angular response and to validate both the systems for IMRT patient specific QA with true gantry angles. To study the angular response of the detectors, profiles and output were measured at gantry increments of 10 degrees for a 10x10 cm 2 field. Flatness, symmetry and output values were compared with those for the reference zero degree gantry angle measurements. For ten dynamic IMRT plans (total of 65 fields), patient specific QA tests were performed using both detectors. Two sets of measurements were done (i) with all gantry angles reset to Zero and (ii) with true gantry angles as in the treatment plan. The results of the QA tests were compared using gamma criterion of 3%-3mm. Student s t-test was used to calculate the variation in gamma value due to angular changes in both the detectors. 5.3 Results Flatness, symmetry and output values were compared with those for the reference 0 degree gantry angle measurements. The output stability of the detectors were <0.5% (Figure 5.1 and Figure 5.2). Flatness and symmetry were well within acceptable limits of 2% for all gantry angles in both detectors (Figure 5.3 to 5.6). 69

3 Absolute dose(cgy) Absolute dose(cgy) Figure 5.1 Absolute dose (output) vs. gantry angle- asi1000 EPID Figure 5.2 Absolute dose (output) vs. gantry angle- ImatriXX 70

4 Symmetry(%) Flatness(%) Figure 5.3 Flatness vs. gantry angle- asi1000 EPID Figure 5.4 Symmetry vs. gantry angle- asi1000 EPID 71

5 Symmetry(%) Flatness(%) Figure 5.5 Flatness vs. gantry angle- ImatriXX Figure 5.6 Symmetry vs. gantry angle- ImatriXX 72

6 In case of portal dosimetry with all gantry angle reset to zero, three parameters were studied for each IMRT cases-namely (i) area gamma >1% (ii) average gamma and (iii) maximum gamma. The average values of these parameters for the 10 cases were as follows: 0.89, 0.3 and 1.89 respectively with the standard deviation of 0.46, 0.05 and 0.28 for 3%- 3mm criteria. With true gantry angles, the average value of the studied parameters for the 10 cases were 1.17, 0.34, and 1.66 respectively with the standard deviation of 0.48, 0.06 and 0.23 for the 3%-3 mm criteria (Table 5.1). In the patient specific QA with ImatriXX 2-D array system, on an average 98.9 % pixels passed the criterion of 3%-3 mm with a standard deviation of 0.5 for true gantry angle setup. With all gantry angles reset to zero, 99.2% of the pixels passed the said criterion with standard deviation of 0.3 (Table 5.1). P-value in student s t-test for true gantry angle vs. zero gantry angle was more than 0.05, indicating no significant variation in gamma value due to angular changes in both the detectors (Table 5.2). 73

7 Table 5.1 Patient specific QA results using portal dosimetry and ImatriXX 2-D array system: zero gantry angle vs. true gantry angle IMRT case Portal dosimetry with γ 3% & 3 mm Zero Gantry angle Area γ 1% Avera ge γ Max γ Area γ 1% True Gantry angle Average γ Max γ ImatriXX 2D array system γ = 3% & 3 mm Zero True Gantry Gantry angle angle %of pixels passed %of pixels passed Avg SD Table 5.2 Test of significance (t-test) results: Gamma results of zero gantry angle vs. true gantry angle Parameters measured p-value Area Gamma 1% (in portal dosimetry) 0.16 Average Gamma (in portal dosimetry) 0.10 Maximum gamma (in portal dosimetry) 0.09 % pixels passed Gamma 3%-3 mm (in ImatriXX 2-D array)

8 5.4 Discussion The sag of asi1000 detector and the angular dependence of the 2-D array detectors limited the IMRT patient specific QA measurements in zero gantry angles. The VMAT patient specific QA with these detectors are also a subject of current research. While using the 2-D arrays on treatment couch, the angular dependence that must be compensated by applying correction factor [Anup et al. (2009) and Shimohigashi et al. (2012)]. Currently, available correction factor tables have several underlying assumptions: first, these correction factors assume that the response of all ion chambers is identical for each angle; second, that the ion chamber array response from gantry angles zero degree to 180 degree are equivalent to the response from 180 degree to zero degree; and the third, that the response is independent of the direction of rotation. In this scenario, the use of 2-D arrays in gantry holder or use of portal dosimetry system are the alternate options for the IMRT patient specific QA in true gantry angle. While using the 2-D array detector system in gantry holder as well as the EPID for the measurements in true gantry angles, the QA results can be influenced by the gravity effect, MLC positional errors as well as sag in gantry and detectors. So before performing IMRT QA with true gantry angles, one must make sure that there is no significant variation in profile characteristics and output due to angular changes. In this study, the test results indicate no angular dependence on gamma value while using for measurements in true gantry angle with both detectors. Flatness and symmetry values for profiles did not exhibit any gantry angle dependence and so was the output. Both the detector system can be use for the patient specific QA of IMRT with fields in true gantry angle. Compared to 75

9 ImatriXX 2-D array system, the portal dosimetry system is easy to use for the measurements in true gantry angle and the set up and measurements are also much easier. 76