Monte Carlo dose verification for a single-isocenter VMAT plan in multiple brain metastases.

The purpose of this study was to verify the accuracy of dose calculation algorithms of a treatment planning system for a single-isocenter volumetric modulated arc therapy (VMAT) plan in multiple brain metastases, by comparing the dose distributions of treatment planning system with those of Monte Carlo (MC) simulations. We used a multitarget phantom containing 9 acrylic balls with a diameter of 15.9 mm inside a Lucy phantom measuring 17 × 17 × 17 cm3. Seven VMAT plans were created using the multitarget phantom: 1 multitarget plan (MTP) and 6 single target plans (STP). Three of the STP plans had a large jaw field setting, almost equivalent to that of the MTP, while the other plans had a jaw field setting fitted to each planning target volume. The isocenter for all VMAT plans was set to the center of the phantom. The VMAT dose distributions were calculated using the analytical anisotropic algorithm (AAA) and were also recalculated through Acuros XB (AXB) and MC simulations under the same irradiation conditions. The AAA and AXB methods tended to overestimate dosage compared with the MC method in the MTP and in STPs with large jaw field settings. The dose distribution in single-isocenter VMAT plans for multiple brain metastases was influenced by jaw field settings. Finally, we concluded that MC-VMAT dose calculations are useful for 3D dose verification of single-isocenter VMAT plans for multiple brain metastases.

Dependence of volume dose indices on dose calculation algorithms for VMAT-SBRT plans for peripheral lung tumor.

The purpose of this study was to investigate the dependence of volume dose indices on dose calculation algorithms for volumetric modulated arc therapy (VMAT) for stereotactic body radiotherapy (SBRT) plans to treat peripheral lung tumors by comparing them with those of Monte Carlo (MC) calculations. VMAT-SBRT plans for peripheral lung tumors were created using the Eclipse treatment planning system (TPS) for 24 patients with nonsmall cell lung cancer. VMAT dose distributions for gross tumor volume (GTV), internal target volume (ITV), and planning target volume (PTV) were calculated using the analytical anisotropic algorithm (AAA), the Acuros XB (AXB) algorithm, and a MC algorithm. VMAT dose distributions of the 3 algorithms were compared using their volume dose indices from dose volume histograms (DVHs), a dose difference map, and 3-dimensional gamma analysis. The DVHs for GTV and ITV from AAA, AXB, and MC were in good agreement. The difference between the ITV and PTV volume dose indices from AAA and MC increased as D98, D95, D80, D50, and D2. In particular, the difference between D98 for PTV from AAA and MC was up to 48%. A >5% difference between D95 for PTV from AAA and MC was 11 patients, but only 2 patients for ITV. The volume dose indices for AXB were near those of MC. AAA tended to overestimate the PTV volume dose indices compared to AXB and MC. Thus, we propose that the volume dose indices for the ITV be used because they are independent of dose calculation algorithms.

Accuracy of positioning and irradiation targeting for multiple targets in intracranial image-guided radiation therapy: a phantom study.

This study investigated the accuracy of positioning and irradiation targeting for multiple off-isocenter targets in intracranial image-guided radiation therapy (IGRT). A phantom with nine circular targets was created to evaluate both accuracies. First, the central point of the isocenter target was positioned with a combination of an ExacTrac x-ray (ETX) and a 6D couch. The positioning accuracy was determined from the deviations of coordinates of the central point in each target obtained from the kV-cone beam computed tomography (kV-CBCT) for IGRT and the planning CT. Similarly, the irradiation targeting accuracy was evaluated from the deviations of the coordinates between the central point of each target and the central point of each multi-leaf collimator (MLC) field for multiple targets. Secondly, the 6D couch was intentionally rotated together with both roll and pitch angles of 0.5° and 1° at the isocenter and similarly the deviations were evaluated. The positioning accuracy for all targets was less than 1 mm after 6D positioning corrections. The irradiation targeting accuracy was up to 1.3 mm in the anteroposterior (AP) direction for a target 87 mm away from isocenter. For the 6D couch rotations with both roll and pitch angles of 0.5° and 1°, the positioning accuracy was up to 1.0 mm and 2.3 mm in the AP direction for the target 87 mm away from the isocenter, respectively. The irradiation targeting accuracy was up to 2.1 mm and 2.6 mm in the AP direction for the target 87 mm away from the isocenter, respectively. The off-isocenter irradiation targeting accuracy became worse than the positioning accuracy. Both off-isocenter accuracies worsened in proportion to rotation angles and the distance from the isocenter to the targets. It is necessary to examine the set-up margin for off-isocenter multiple targets at each institution because irradiation targeting accuracy is peculiar to the linac machine.

[Comparison of dose calculation algorithms in stereotactic radiation therapy in lung].

Dose calculation algorithms in radiation treatment planning systems (RTPSs) play a crucial role in stereotactic body radiation therapy (SBRT) in the lung with heterogeneous media. This study investigated the performance and accuracy of dose calculation for three algorithms: analytical anisotropic algorithm (AAA), pencil beam convolution (PBC) and Acuros XB (AXB) in Eclipse (Varian Medical Systems), by comparison against the Voxel Monte Carlo algorithm (VMC) in iPlan (BrainLab). The dose calculations were performed with clinical lung treatments under identical planning conditions, and the dose distributions and the dose volume histogram (DVH) were compared among algorithms. AAA underestimated the dose in the planning target volume (PTV) compared to VMC and AXB in most clinical plans. In contrast, PBC overestimated the PTV dose. AXB tended to slightly overestimate the PTV dose compared to VMC but the discrepancy was within 3%. The discrepancy in the PTV dose between VMC and AXB appears to be due to differences in physical material assignments, material voxelization methods, and an energy cut-off for electron interactions. The dose distributions in lung treatments varied significantly according to the calculation accuracy of the algorithms. VMC and AXB are better algorithms than AAA for SBRT.

[Dose impact of a carbon fiber couch for stereotactic body radiation therapy of lung tumors].

The aim of this study was to measure the dose attenuation caused by a carbon fiber radiation therapy table (Imaging Couch Top; ICT, BrainLab) and to evaluate the dosimetric impact of ICT during stereotactic body radiation therapy (SBRT) in lung tumors. The dose attenuation of ICT was measured using an ionization chamber and modeled by means of a treatment planning system (TPS). SBRT was planned with and without ICT in a lung tumor phantom and ten cases of clinical lung tumors. The results were analyzed from isocenter doses and a dose-volume histogram (DVH): D95, Dmean, V20, V5, homogeneity index (HI), and conformity index (CI). The dose attenuation of the ICT modeled with TPS agreed to within ±1% of the actually measured values. The isocenter doses, D95 and Dmean with and without ICT showed differences of 4.1-5% for posterior single field and three fields in the phantom study, and differences of 0.6-2.4% for five fields and rotation in the phantom study and six fields in ten clinical cases. The dose impact of ICT was not significant for five or more fields in SBRT. It is thus possible to reduce the dose effect of ICT by modifying the beam angle and beam weight in the treatment plan.

Angular dependence correction of MatriXX and its application to composite dose verification.

We measured the angular dependence of central and off-axis detectors in a 2D ionization chamber array, MatriXX, and applied correction factors (CFs) to improve the accuracy of composite dose verification of IMRT and VMAT. The MatriXX doses were measured with a 10° step for gantry angles (θ) of 0°-180°, and a 1° step for lateral angles of 90°-110° in a phantom, with a 30 × 10 cm2 field for 6 MV and 10 MV photons. The MatriXX doses were also calculated under the same conditions by the Monte Carlo (MC) algorithm. The CFs for the angular dependence of MatriXX were obtained as a function of θ from the ratios of MatriXX-measured doses to MC-calculated doses, and normalized at θ = 0°. The corrected MatriXX were validated with different fields, various simple plans, and clinical treatment plans. The dose distributions were compared with those of MC calculations and film. The absolute doses were also compared with ionization chamber and MC-calculated doses. The angular dependence of MatriXX showed over-responses of up to 6% and 4% at θ = 90° and under-responses of up to 15% and 11% at 92°, and 8% and 5% at 180° for 6 MV and 10 MV photons, respectively. At 92°, the CFs for the off-axis detectors were larger by up to 7% and 6% than those for the central detectors for 6 MV and 10 MV photons, respectively, and were within 2.5% at other gantry angles. For simple plans, MatriXX doses with angular correction were within 2% of those measured with the ionization chamber at the central axis and off-axis. For clinical treatment plans, MatriXX with angular correction agreed well with dose distributions calculated by the treatment planning system (TPS) for gamma evaluation at 3% and 3 mm. The angular dependence corrections of MatriXX were useful in improving the measurement accuracy of composite dose verification of IMRT and VMAT.