ABSTRACT
PURPOSE: We aimed to prove that the locally absorbed doses in tissues and organs are affected by inaccurate table height in computed tomography. MATERIALS AND METHODS: We compared the volume CT dose index (CTDIvol) and the absorbed doses using an anthropomorphic phantom combined with a breast phantom. The phantom was set at the gantry center, from which the table height was changed every 20 mm between-40 mm and 40 mm. Data acquisition was performed using auto table height correction (AHC) for each table height. The CTDIvol was obtained from the CT console and the tube current value for each image slice (DICOM tag: 0018, 1151). The absorbed dose was measured by a glass dosimeter that was implanted at various positions in the phantom. RESULTS: The tube current values in the lung were lower at a table height of + 40 mm than those at other heights. The CTDIvol was slightly lower at + 40 mm than at the center (12.78 mGy vs. 13.42 mGy, p < 0.05). The CTDIvol values were almost the same at the other table heights (13.30-13.40 mGy). The absorbed doses at the lens and mammary gland were significantly different from those at the gantry center (-27.27%-17.77% and -24.31%-12.83%, respectively). Compared with the center, both the lens and mammary gland had higher absorbed doses at a table height of -40 mm. CONCLUSION: The absorbed dose was affected by the table height, but the CTDIvol was maintained by AHC. The operator should appropriately position patients even when using AHC.
Subject(s)
Breast , Tomography, X-Ray Computed , Breast/diagnostic imaging , Humans , Phantoms, Imaging , Radiation Dosage , ThoraxABSTRACT
Objective:To provide a new morning check method for the output dose stability of the multileaf collimator (MLC) of the CyberKnife M6 (CK-M6) system.Methods:The CT images of a verification phantom with a size of 20 cm × 20 cm × 10 cm were transmitted into the Precision Treatment Plan ning System (ver. 1.1.1.1). The high-precision alignment between the accelerator output front and the fixed position of the phantom surface was achieved using the fiducial tracking method. A 10 cm × 10 cm radiation field was formed by the MLC and a DailyCheck plan with an output of 200 MU was designed. The repeatability, sensitivity, and accuracy of the DailyCheck plan were measured, and the CK-M6 system was continuously tested for one month using the artificial fixed method and the DailyCheck plan designed in this study. Results:The average and the standard deviation of 10 repeated measurements by the DailyCheck plan were 492.28 pC and 0.09, respectively, indicating good stability. There was a linear correlation between the measured values and the output dose, with a correlation coefficient of R2 > 0.999. Moreover, there was a position deviation of 2 mm between the phantom and the accelerator output front, and the result ant effect on the measured values was equivalent to a dose deviation caused by an output of 1.24 MU. The result from the continuous measurement of both the artificial fixed method and the DailyCheck plan fell within permissible limits, showing high consistency. Conclusions:The DailyCheck plan established through the fiducial tracking of a verification phantom can achieve the convenient, quick, and accurate daily detection of the output dose stability of the MLC of CK-M6. Therefore, this method can be widely applied in the clinical quality control of the CK-M6 system.
ABSTRACT
Objective To probe an accurate,rapid and effective method for the calculation and measurement of output dose of the accelerator with IAEA regulation.Methods The parameter value of measurement was determined according to IAEA regulations,and absorbed dose calculation forma was briefed to the form that the reading of instrument multiplies cumulative correction factor.The output dose was measured in water phantom on sandard condition,and then regrafted into plexiglass phantom,which made the measurement more convenience.Results Staistics of measurement had been done to PRIMUS accelerator in 66 weeks output dose.Over 85% of the measurement had the error less then 1%,and others were less then 2%.Conclusion This measurement can be used for the measuremen of output dose in precise radiotherapy of linear accelerator.
