Abstract
The purpose of this study is to evaluate the accuracy and reproducibility of the IsoCal geometric calibration system for kilovoltage (kV) and megavoltage (MV) imagers on Varian C‐series linear accelerators (linacs). IsoCal calibration starts by imaging a phantom and collimator plate using MV images with different collimator angles, as well as MV and kV images at different gantry angles. The software then identifies objects on the collimator plate and in the phantom to determine the location of the treatment isocenter and its relation to the MV and kV imager centers. It calculates offsets between the positions of the imaging panels and the treatment isocenter as a function of gantry angle and writes a correction file that can be applied to MV and kV systems to correct for those offsets in the position of the panels. We performed IsoCal calibration three times on each of five Varian C‐series linacs, each time with an independent setup. We then compared the IsoCal calibrations with a simplified Winston‐Lutz (WL)‐based system and with a Varian cubic phantom (VC)‐based system. The maximum IsoCal corrections ranged from 0.7 mm to 1.5 mm for MV and 0.9 mm to 1.8 mm for kV imagers across the five linacs. The variations in the three calibrations for each linac were less than 0.2 mm. Without IsoCal correction, the WL results showed discrepancies between the treatment isocenter and the imager center of 0.9 mm to 1.6 mm (for the MV imager) and 0.5 mm to 1.1 mm (for the kV imager); with IsoCal corrections applied, the differences were reduced to 0.2 mm to 0.6 mm (MV) and 0.3 mm to 0.6 mm (kV) across the five linacs. The VC system was not as precise as the WL system, but showed similar results, with discrepancies of less than 1.0 mm when the IsoCal corrections were applied. We conclude that IsoCal is an accurate and consistent method for calibration and periodic quality assurance of MV and kV imaging systems.PACS numbers: 87.55.Qr, 87.56.Fc
Highlights
165 Gao et al.: IsoCal calibration for Varian linacs used for stereotactic radiosurgery (SRS) and/or stereotactic body radiation therapy (SBRT), the coincidence of the MV and kV imaging coordinate systems and the treatment coordinate system within ± 1 mm is highly desired, while for other radiation therapies this coincidence should be within ± 2 mm.[1]. Recently, Task Group 179 of the American Association of Physicists in Medicine (AAPM) recommended that the coincidence of the MV and kV imagers and room lasers should be within ± 1 mm.[2]. Linacs equipped with kV on-board imager (OBI) and MV electronic portal imaging device (EPID) imaging systems have four isocenters to characterize: the mechanical isocenter, the radiation treatment isocenter, the kV imaging system isocenter, and the MV imaging system isocenter
These methods were reviewed by Bissonnette et al[7] and they found that the accuracy of geometric calibration of the OBI system was stable within 2 mm over 28 months
We evaluated the IsoCal system with multiple Varian C-series linacs equipped with OBI-conebeam computed tomography (CBCT) and EPID imaging systems
Summary
165 Gao et al.: IsoCal calibration for Varian linacs used for stereotactic radiosurgery (SRS) and/or stereotactic body radiation therapy (SBRT), the coincidence of the MV and kV imaging coordinate systems and the treatment coordinate system (for four cardinal angles) within ± 1 mm is highly desired, while for other radiation therapies this coincidence should be within ± 2 mm.[1]. Other phantom-based methods have been proposed.[4,5,6] In these methods, a phantom is aligned to the mechanical isocenter using room lasers or some other surrogate for the radiation treatment isocenter; the calibration accuracy relates to the accuracy of the chosen surrogate rather than directly to the treatment isocenter. These methods were reviewed by Bissonnette et al[7] and they found that the accuracy of geometric calibration of the OBI system was stable within 2 mm over 28 months
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