The purpose of the present work was to measure the performance characteristics in the penumbra region and on the leaf‐end of an innovative dual‐layer micro multileaf collimator (DmMLC). The DmMLC consists of two orthogonal (upper and lower) layers of leaves; a standard MLC consists of one layer. The DmMLC provides unique performance characteristics in smoothing dose undulation, reducing leaf‐end transmission, and reducing MLC field dependence of the leaf stepping angle. Two standard MLCs (80‐leaf and 120‐leaf versions: Varian Medical Systems, Palo Alto, CA), a DmMLC (AccuKnife: Initia Medical Technology, Canton, MA), and a Cerrobend (Cerro Metal Products, Bellefonte, PA) block were used in performance studies involving a triangular field, a cross leaf‐end field, and a circular field. Measurements were made with 6‐MV X‐rays and extended dose range film at a depth of 5 cm in Solid Water (Gammex rmi, Middleton, WI) at a source–axis distance of 100 cm. The field penumbra width measured between the 20% and 80% isodose lines through the MLC‐80, MLC‐120, DmMLC, and Cerrobend block were 9.0, 5.0, 3.0, and 2.0 mm respectively. The dose undulation amplitude of the 50% isodose line was measured as 5.5, 2.0, and 0.5 mm for the MLC‐80, MLC‐120, and DmMLC respectively. The planar dose difference between the MLC‐80, MLC‐120, and DmMLC against Cerrobend block was measured as ranging at ±52.5normal%,±35.0normal%, and ±20.0normal% respectively. The leaf‐end transmission was measured at 22.4% in maximum and 15.4% in average when closing a single layer of the DmMLC, and at 2.4% in maximum and 2.1% in average when closing both layers. The MLC dependence of the leaf stepping angle with the DmMLC ranged from 45 degrees to 90 degrees. The standard MLC leaf stepping angle ranged from 0 degrees to 90 degrees. In conclusion, the dose undulation, leaf‐end transmission, and MLC field dependence of the leaf stepping angle with the DmMLC were remarkably reduced as compared with those of the standard MLCs. And as compared with Cerrobend block, the DmMLC provided very comparable performance in field‐edge smoothing and in the shaping of complex fields.PACS numbers: 87.56.Jk, 87.56.Nk, 87.56.Nj, 87.57.Nt
Purpose: The aim of the study is to evaluate and compare the four different commercial devices for Rapid Arc and Sliding Window IMRT QA. Materials and methods: Fifteen consecutive RapidArc plans and ten IMRT plans were used for this study. All plans were optimized using Eclipse TPS. For all plans, the corresponding verification dose distributions were calculated using four (n=4) geometries. This include an acrylic phantom with film, the IBA with the MatrixX, the PTW Octavious phantom with the seven29 array, and the Scandidos Delta4 device. All plans and deliveries were calculated and delivered using the actual planned gantry and collimator angles respectively. All measurements were compared against the calculated ones, first by taking into account all points in the measurement planes and second by applying a threshold value where the points that received less than 20% of the normalized dose were excluded from the gamma index calculation. Results:Among these 25 plans, using all available points for the gamma calculation, all of them passed the criteria (3.0% and 3mm DTA) of having gamma values ≤ 1 (more than 90% of points). A few verification plans failed to pass the set gamma criteria when the evaluated points were limited to those only receiving more than 20% of the maximum dose. On average all the QA devices produced very similar results. This was further supported by the Bland‐Altman analysis that was performed and showed that all the calculated gamma values of all detectors were within 5% from those of film. Conclusions: The results showed that there insignificant differences between the detectors. All patient QAs passed the routine clinically criteria of gamma index values of 3% dose difference and 3mm DTA. We conclude that the dosimetric systems under investigation can be used interchangeably for routine patient specific QA. “Research sponsored by PTW‐Freiburg Company.”
Purpose: To determine the effective source distance (ESD) for electron fields used in modulated electron radiotherapy (MERT) using an existing photon multileaf collimator and to examine their behavior with changes in field size and beam energy. Methods and Materials: In order to accurately calculate the MU needed for MERT treatment the electron effective source distance should be determined. ITwo methods were used to determine the virtual source positions of electron beams at energies of 6, 9, 12, 16, and 20 MeV. For field sizes varying from 2×2 cm2 to 10×10 cm2, parallel plate ion chamber measurements in a plastic water phantom were taken, and the inverse slope method was used to determine the ESD for each field at each energy. In addition, beam profiles measured with film were used to determine the virtual source location (VSL). Results: The ESDs calculated using the inverse slope method increased with increasing field size and energy. It was observed that the ESD increase with field size was less steep at high energies than at lower energies. Comparing the ESD and VSL measurements for a 10×10 cm2 field, it was observed that the VSL was slightly larger than the ESD at both 9 and 16 MeV electron beams. Conclusions: The ESDs measured were strongly dependent on both field size and electron beam energy. Differences between ESD values may be due to differences in the electron beam scatter that is energy dependent. Discrepancies between ESD and VSL values may be due to the method used to measure ESD which more closely matches clinical conditions than the film VSL method. Determination of the effective source distance can be used for correct MU calculations for modulated electron radiotherapy.
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