ABSTRACT
Objective:To study the impact of the Varian real-time position management (RPM) respiratory gating system on radiotherapy planning dosimetry.Methods:The radiotherapy plans of 40 cases with thoracic or abdominal tumors were retrospectively selected in this study. The motion phantom for quality control was adopted to generate respiratory gating signals, and the 30%-60% stable phase at the end of expiratory was selected as the respiratory gating window. The dose verification for the abovementioned radiotherapy plans was performed using the Portal Dosimetry (PD) system under RPM respiratory gating mode with the Edge accelerator. Afterwards, dose analysis was performed with different γ passing rate criteria and the distribution characteristics of γ values were analyzed. Finally, the verification results between the non-gating mode and the gating mode were compared.Results:Under the respiratory gating mode, the passing rates of all intensity-modulated radiation therapy/volumetric-modulated arc therapy (IMRT/VMAT) plans with or without flattening filters were over 95.5% by γ criteria of (3%, 3 mm) or (3%, 2 mm) and were over 90% by stricter γ criteria of (2%, 2 mm). All plans met the clinical requirements recommended by the American Association of Physicists in Medicine (AAPM). The passing rates of dose verification under non-gating mode were slightly better than those under respiratory gating mode, and the differences between the two modes were statistically significant (3%/3 mm, Z =-1.45; 3%/2 mm, Z =-2.86; 2%/2 mm, Z =-3.70; 1%/1 mm, Z =-4.52; P<0.05). There was no significant difference in the minimum and maximum values of γ and the share of γ > 1.5 of plan verification result under the two modes. However, the average value and standard deviation of the γ were generally smaller under the non-gating mode. Conclusions:The impact of the introduction of RPM respiratory gating technology on dose is clinically acceptable, and the execution of these plans in this gating mode is safe and reliable.
ABSTRACT
Aim: The aim of this study is to commission and validate the portal dosimetry (PD) system using an indirect method for flattening filter free (FFF) photon beam of the upgraded c-series linear accelerator. Background: Varian Medical System clinacs with amorphous-silicon portal imager panel (aSi-1000) do not have PD for FFF beams. Recently, our c-series linear accelerator was upgraded to deliver 6MV FFF (6MVFFF) photon beam with the highest dose rate of 1400 monitor unit (MU)/min. The study, therefore, focuses on the commissioning and validation of PD for the 6MVFFF beam. Materials and Methods: An indirect method was implemented to predict the portal dose for FFF beam in Eclipse as the treatment planning system does not have direct prediction algorithm for FFF beam (version. 11). Dosimetrical characteristics of aSi-electronic portal imaging device (EPID) were evaluated for 6MVFFF beam and validation of PD for 6MVFFF beam was performed for open fields along with pretreatment quality assurance of intensity-modulated radiation therapy (IMRT), volumetric-modulated arc therapy (VMAT), and stereotactic radiosurgery (SRS) techniques for 30 patients planned with 6MVFFF beam. Results: ASi-EPID saturates between 100 and 130 cm source to detector distance (SDD) for 6MVFFF beam and resolved at more than 140 cm SDD. The squared correlation coefficient (R2) for MU linearity was found to be 1 (R2 = 1), and instantaneous dose response linearity at different SDD's was found to be 0.999 (R2 = 0.999) for the 6MVFFF beam. Maximum gamma area index (GAI) for 3% dose difference and 3 mm distance-to-agreement criteria for IMRT, VMAT, and SRS/stereotactic radiotherapy plans was 97.9% ± 0.3%, 96.3% ± 0.5%, and 98.2% ± 0.2%, respectively. Conclusion: The results reveal that this novel method can be used to commission portal dosimetry for 6MVFFF beam as it is a convenient, faster, and accurate method
ABSTRACT
Objective To compare the dosimetric verification results of Varian Portal Dosimetry and Matrixx,and to assess the reliability of the clinical application of electronic portal imaging device (EPID) verification.Methods Varian TrueBeam linear accelerator,which was equipped with a 120-leaf multileaf collimator and an amorphous silicon EPID,as well as portal dose prediction software.IBA I′mRT Matrixx ion chamber array was used.EPID algorithm configuration,dose calibration,and testing before use were performed.The sliding-window protocol was used.There were 77 patients with tumors involving the head and neck (mainly nasopharyngeal carcinoma),mediastinum,abdomen,and pelvic cavity were selected.The verification plan of the portal dose was created with a source-detector distance of 100 cm,and the gantry angle was kept the same as the treatment plan.The verification plan was carried out in the TrueBeam machine,and the data were collected at the same time by EPID.Comparison between the measured and calculated dose images was performed,and the evaluation standard was gamma index (3%/3 mm).The paired t-test was used for difference analysis.Results For the 77 patients,the Gamma passing rates of both methods were above 97%.Except for head and neck carcinoma were a significant difference between the results of dosimetric results using EPID and Matrixx in intensity-modulated radiotherapy (P=0.018) other remaining all P> 0.05.Conclusions The dosimetric verification results of EPID are consistent with those of Matrixx.EPID can be used for dosimetric verification,and Matrixx ion chamber array can be used only in case of a low Gamma passing rate.
ABSTRACT
Purpose: To validate a locally fabricated phantom of Imatrixx-2D Array by comparing its results with ArcCheck phantom and comparing portal dosimetry measurements with the two phantom studies. Materials and Methods: Electronic Portal Imaging Devices and Epiqa software were used for portal dosimetry. An Imatrixx-2D array with a locally fabricated phantom and ArcCheck cylindrical phantom were used for phantom studies. Eclipse-TPS with RapidArc treatment planning and portal dose prediction software was used for planar dose calculations. Three verification plans were created for each of the 15 patient plans of various sites, making a total of 45 plans to be delivered on 3 QA systems as above. Fifteen plans each with 2 arcs were delivered on the EPIDs of the Linacs, on Imatrixx-2D array phantom and on ArcCheck cylindrical phantom respectively. The planar dose matrices were analysed using global Gamma Index criteria of 3mm DTA and 3% dose difference. Results: The maximum deviations of percentage in dose points, in which γ>1, are 1.94, 1.89 and 1.5 in Imatrixx phantom, ArcCheck phantom and Portal dosimetry, respectively. Similarly, the mean deviations and SD values are less in portal dosimetry than that of phantom studies. The smaller deviations in portal dosimetry are attributed to closely embedded chambers in the EPID compared to the distance between the detectors placed in the phantom measurements. Conclusion: After carrying out the comparison of results, the locally fabricated phantom has been validated and accepted for the dosimetric studies. The conclusion is that all the three dosimetric QA systems are suitable for the patient-specific QA of RapidArc treatments.
ABSTRACT
PURPOSE: The purpose of this study is to investigate fundamental aspects of the dose response of fluorescent screen-based electronic portal imaging devices (EPIDs). MATERIALS AND METHODS: We acquired scanned signal across portal planes as we varied the radiation that entered the EPID by changing the thickness and anatomy of the phantom as well as the air gap between the phantom and the EPID. In addition, we simulated the relative contribution of the scintillation light signal in the EPID system RESULTS: We have shown that the dose profile across portal planes is a function of the air gap and phantom thickness. We have also found that depending on the density change within the phantom geometry, errors associated with dose response based on the EPID scan can be as high as 7%. We also found that scintillation light scattering within the EPID system is an important source of error. CONCLUSION: This study revealed and demonstrated fundamental characteristics of dose response of EPID, as relative to that of ion chambers. This study showed that EPID based on fluorescent screen cannot be an accurate dosimetry system