ObjectiveTo investigate the impact of different dose algorithms and calculation angle intervals (DCAI) on the in vivo dose (IVD) verification of small-field arc therapy in stereotactic body radiation therapy (SBRT).MethodsWe made an exit-dose-measuring and positioning device (EDPD) for the SRS MapCHECK (SMC) using polymethyl methacrylate (PMMA). Computed tomography data for the anthropomorphic head phantom, SMC, and EDPD combination were acquired with 1 mm slice thickness and spacing. SBRT partial arc plans were created using an SBRT cone, block, and a small square open field, with a gantry rotation angle of 60°. The dose distribution was calculated using three different dose algorithms [Pencil Beam (PB), CC Convolution (CCC), and Monte Carlo (MC)], with 1 mm isotropic resolution. We also used three different DCAIs (1°, 3°, 5°) with the PB and CCC algorithms to calculate the dose distribution of each plan three times. The uncertainty of each control point for the MC algorithm was set to 1%. The SMC was used to measure the exit dose outside the phantom for IVD verification, the detector plane was located 182.5 mm outside the scan center.ResultsWithin the phantom, the minimum passing rate of 3D gamma analysis (1%/1 mm) for the dose distributions calculated at different DCAIs was 99.1%, and the maximum relative deviation (RD) of the central point dose (CPD) was <0.2%. The average RD of the CPD for IVD verification was about 30% (range 16.71%–50.0%) for PB; -0.36% ± 1.82% (1° DCAI), -3.18% ± 7.83% (3° DCAI), and 3.69% ± 11.56% (1° DCAI) for CCC; and -0.38% ± 0.76 for the MC algorithm. The passing rates of 2D gamma analysis (3%/3 mm) between the predicted exit dose and the IVD were 100% for MC and >90% for the CCC algorithm at 1° DCAI.ConclusionThe DCAI for exit-dose calculations should be ≤1° using the CCC algorithm. Furthermore, among the three algorithms verified in the current study, the MC algorithm showed the highest accuracy, followed by CCC, with the PB algorithm having the worst performance. The PB algorithm is thus not suitable for exit-dose calculation or IVD verification of SBRT.
The aim of this study was to evaluate the dosimetric variations of static intensity-modulated radiotherapy (sIMRT), dynamic intensity-modulated radiotherapy (dIMRT) and volumetric modulated arc therapy (VMAT) for thoracic esophageal cancer (EC). Ten patients with pathologically-confirmed EC were randomly selected. The Monaco 5.11 treatment planning system was used to design six radiotherapy plans, namely sIMRT with 5 fixedfields (S5) and 7 fixed-fields (S7), dIMRT with 5 fixed-fields (D5) and 7 fixed-fields (D7) and VMAT plan with single arc (VMAT1) and double arc (VMAT2), respectively. The dosimetric parameters were compared among six different plans. The dose to target volumes met the clinical prescription requirements in all six plans. The D5 plan had the lowest dose exposed to the whole lungs, except for V5, which was only 1.7% higher than that in the D7 plan. The low dose to lungs (V5 and V10) in VMAT plans (VMAT1 and VMAT2) were higher than in IMRT plans, but the statistical differences were significant (P < 0.05) only in comparison with dynamic IMRT plans (D5 and D7). The V20 and V30 of lungs in VMAT2 plan were also higher than that in other plans, but the statistical differences were significant (P < 0.05) only in comparison with D5 plan. The doses delivered to the spinal cord and heart showed no statistical significance (P > 0.05). The monitor units (MUs) and treatment time (TT) significantly increased with the increasing number of fields in the same kind of fixed-fields IMRT plans (S5 vs. S7; D5 vs. D7). The MUs in VMAT plans, especially in VMAT2, increased significantly (P < 0.05) when compared with sIMRT and 5-fields dIMRT plans, respectively. VMAT1 plan had the shortest TT and the highest delivery efficiency. For thoracic esophageal cancer, the D5 plan can shorten the TT and improve the delivery efficiency while meeting the dosimetric requirements and sparing the lungs. VMAT can significantly reduce the TT, but at the expense of increasing the dose to lower-dose regions of lungs, which means VMAT plan may not shows obvious dose advantage for thoracic esophageal cancer.
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