Abstract
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.PACS number: 87.55.Qr, 87.56.Fc
Highlights
199 Shimohigashi et al.: MatriXX angular dependence correction and application in the gantry speed, dynamic multileaf collimator (DMLC) movement, and dose rate
The dose delivery for an advanced radiotherapy technique such as intensity-modulated radiotherapy (IMRT) or volumetric-modulated arc therapy (VMAT) must be verified before clinical implementation in order to ensure that the treatment plan can be executed accurately.[6]. The dose verification method involves the comparison of the dose distribution calculated by the treatment planning system (TPS) in a phantom with dose distribution measured with a film,(7-9) or by two-dimensional (2D) arrays or in an ionization chamber.[10] film dosimetry has very good spatial resolution, it requires careful calibration and real-time measurements are unavailable
The dose distributions for 10 × 10 cm2, 15 × 15 cm2, and 20 × 20 cm2 fields, various simple plans, and IMRT plans were calculated using the Monte Carlo (MC) dose algorithm installed in the iPlan RT (Version 4.1.2, BrainLAB).(31-33) MC calculations were performed via a full MLC geometry simulation with a spatial resolution of 2 mm and variance of 1%
Summary
199 Shimohigashi et al.: MatriXX angular dependence correction and application in the gantry speed, DMLC movement, and dose rate. Ling et al[5] showed that the DMLC movement, variable dose rate, and gantry speed can be precisely controlled using RapidArc. The dose delivery for an advanced radiotherapy technique such as IMRT or VMAT must be verified before clinical implementation in order to ensure that the treatment plan can be executed accurately.[6] The dose verification method involves the comparison of the dose distribution calculated by the treatment planning system (TPS) in a phantom with dose distribution measured with a film,(7-9) or by two-dimensional (2D) arrays or in an ionization chamber.[10] film dosimetry has very good spatial resolution, it requires careful calibration and real-time measurements are unavailable. Remain the gold standard for 2D dose verification
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