Feasibility study of multi-purpose quality assurance phantom for pretreatment verification of volumetric modulated arc therapy

Background: the purpose of this study was to evaluate the feasibility of a multipurpose quality assurance [QA] phantom for pretreatment verification of volumetric modulated arc therapy [VMAT]

Materials and Methods: the QA phantom was constructed with polymethyl methacrylate [PMMA] to perform relative dosimetry using EBT3 film and Map CHECK, as well as absolute dosimetry using an ionization chamber. The QA phantom was constructed to perform relative dosimetry using EBT3 film and Map CHECK, as well as the absolute dosimetry using ionization chamber. In order to verify the pretreatment plans, 25 patients treated with VMAT were selected. The pretreatment plans were calculated in the Eclipse treatment planning system using the Acuros XB dose calculation algorithm and CT images for the QA phantom, with the same beam setup and monitor units [MUs] as those for patient treatment. All plans were delivered to the Varian True Beam accelerator equipped with a high-definition multi-leaf collimator

Results: the multi-purpose QA phantom is developed for convenient VMAT dose verification. By using the QA phantom, all 25 cases passed +/- 3% acceptability criteria in absolute dosimetry with an ionization chamber for pretreatment verification. The relative dosimetry using EBT3 film and Map CHECK system also showed high agreement of more than 90% for 2%/2-mm and 3%/3-mm criteria

Conclusion: the results of this study demonstrated the good multipurpose capabilities of the phantom for the absolute and relative dosimetry. Therefore, the developed multi-purpose QA phantom was applied in our institution for routine VMRT dose verification

Dosimetric accuracy of the Acuros XB and Anisotropic analytical algorithm near interface of the different density media for the small fields of a 6- MV flattening-filter-free beam

Background: This study was conducted to assess the accuracy of dose calculation near the air-phantom interface of a heterogeneous phantom for Acuros XB [AXB] and Anisotropic Analytical Algorithm [AAA] algorithm of a 6-MV flattening-filter-free beam, compared with film measurements

Materials and Methods: A phantom included air gap was specially manufactured for this study. In order to evaluate the dose near air gap-phantom interface, Eclipse treatment planning system equipped both AXB and AAA was used for the dose calculations. Measurements in this region were performed with radiochromic film. The central-axis dose [CAD] and off-axis dose [OAD] between calculations and measurements were analyzed for various field sizes and air gaps. The root-mean-square-error [RMSE] was used to evaluate the difference between the calculated and measured OAD. In order to quantify agreement between the calculated and measured dose distributions, the gamma analysis was performed with the 2%/2 mm and 3%/3 mm criteria

Results: For all fields traveling through 1 and 3 cm air gap, the maximum difference in the calculated CAD was -5.3% for AXB and 214.8% for AAA, compared to the measured CAD. For the RMSE between the calculated and measured OAD, the calculated OAD using AXB showed interval in the RMSE [from 4.4 to 12.7] while using AAA indicated broad [from 7.7 to 101.0]. In addition, the gamma passing rates showed that AXB was higher agreement than AAA

Conclusion: This study demonstrated that AXB was more accurate in heterogeneous media near air-phantom interface than AAA when comparing the measured data