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Objective:To evaluate the accuracy of measurement of the output factor of high-energy photon small field (Scp) using commercial semi-diodes and ionization chambers in small fields in accordance with the IAEA-483 report, which has been proposed that all kinds of detectors should be revised for small field Scp measurement in clinical practice.Methods:EGSnrc of Monte Carlo (MC) software was utilized to simulate the treatment head of Varian Novalis Tx linear accelerator, and the profile curve and relative dose value were generated by simulation in DOSXYZnrc based on derived phase space file. Measurement of PDD and Profiles was used to adjust and validate the simulation mode. Detectors including ionization chambers A16, A14 sL, CC01, CC13, PFD and EFD and semi-diodes PFD, EFD and Razor under different radiation field settings (0.5 cm to 10.0 cm) were employed to measure the profile curves and Scp of FWHM equivalent rectangular fields, which were compared with data of Monte-Carlo simulation. The measurement of Scp was revised by data given in the IAEA-483 report. The data with or without correction were compared with the data of MC simulation.Results:A curve deviation o F<2.0% between MC simulation and PFD measurement was accepted. MC simulated Profiles were consistent with PFD, EFD and Razor measurements, when the field was<3.0 cm. Razor response in the out-field region was 2.3% higher than those of MC and PFD, and it increased with the increment of field and was 3.0% at 10.0 cm. The maximum 20.0%-80.0% penumbra width was detected as 3.0 mm for CC13 at 10.0 cm rectangular field. With the decrease of the radiation field, the deviation relative to MC simulation was increased as for Scp mean values of 7 detectors before correction. The standard deviation (SD) of the measured value was increased rapidly when it was close to 1.0 cm, ranging from 0.009-0.014 for the field of 5.0 cm-1.5 cm to 0.030-0.089 for the field of 1.0 cm-0.5 cm. The mean value of SD for the whole measurement before correction was 0.030. The mean SD of Scp measured by the six probes was 0.008, 0.013 at 0.8 cm and 0.021 at 0.6 cm after correction. When the equivalent field was ≥1.0 cm, the corrected Scp and MC simulation deviation was ranged from -3.6% to -0.5%. The error was between -6.9% and -1.3% when the radiation field was<1.0 cm. Conclusion:The SD of Scp measured by different detectors after correction in accordance to the IAEA-483 report is small, which is in good agreement with the data of MC simulation, suggesting that it could be applied in clinical dosimetry.
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Objective:To propose a method of image similarity measurement based on structure information and intuitionistic fuzzy set and measure the similarity between CT image and CBCT image of radiotherapy plan positioning, aiming to objectively measure the setup errors.Methods:A total of four pre-registration images of a nasopharyngeal carcinoma patient on the cross-sectional and sagittal planes and a pelvic tumor patient on the cross-sectional and coronal planes were randomly selected. Five methods were used to quantify the setup errors, including correlation coefficient, mean square error, image joint entropy, mutual information and similarity measure method.Results:All five methods could describe the deviation to a certain extent. Compared with other methods, the similarity measure method showed a stronger upward trend with the increase of errors. After normalization, the results of five types of error increase on the cross-sectional plane of the nasopharyngeal carcinoma patient were 0.553, 0.683, 1.055, 1.995, 5.151, and 1.171, 1.618, 1.962, 1.790, 3.572 on the sagittal plane, respectively. The results of other methods were between 0 and 2 after normalization, and the results of different errors of the same method slightly changed. In addition, the method was more sensitive to the soft tissue errors.Conclusions:The image similarity measurement method based on structure information and intuitionistic fuzzy set is more consistent with human eye perception than the existing evaluation methods. The errors between bone markers and soft tissues can be objectively quantified to certain extent. The soft tissue deviation reflected by the setup errors is of significance for individualized precision radiotherapy.
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Objective@#To quantitatively evaluate dose accuracy of radiotherapy for cervical cancer.@*Methods@#A CT image correction algorithm based on image transformation was proposed. Referring to CBCT images, CT images of radiotherapy plan for cervical cancer were corrected to obtain the corrected images which could reflect the actual body position of treatment. The clinical plan was transplanted to the corrected images for dose recalculation as a test plan, and the dosimetry parameters were statistically compared to evaluate the dose accuracy.@*Results@#Both of the target coverage of contrast plans could meet the clinical requirements (>98%), and there was no significant difference in the homogeneity index (P=0.150). The conformability of the test plan was significantly worse than that of the clinical plan (P<0.05). The maximum dose of each organ at risk in the test plan was approximately 30 cGy higher than that of the clinical plan (P<0.05), V50 was slightly higher than that of the clinical plan, whereas the average dose (Dmean) did not significantly differ.@*Conclusion@#The CT image correction algorithm based on image transformation can quantitatively evaluate the dose accuracy of radiotherapy for cervical cancer, which provides reference for resolving similar problems in clinical practice.
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Objective To construct and investigate the multi-leaf collimator (MLC) fault prediction model of Varian NovalisTx medical linear accelerator based on BP neural network.Methods The MLC fault data applied in clinical trial for 18 months were collected and analyzed.The total use time of accelerator,the quantity of patients per month,average daily working hours of accelerator,volume of RapidArc plans and time interval between accelerator maintenance were used as the input factors and the prediction of MLC fault frequency was considered as the output result.The BP neural network model of MLC fault prediction was realized by AMORE package of R language and the simulation results were validated.Results The model contained 3 layers of network to realize the input-output switch.There were 5 nodes in the input layer,13 nodes in the hide layer and 1 node in the output layer,respectively.The transfer function from the input layer to the hide layer selected the tansig function and purelin function was used from the hide layer to the output layer.The maximum time of training was pre-set as 150 in the designed model.Actually,111 times of training were performed.The pre-set error was 3％ and the actual error was 2.7％,which indicated good convergence.The simulation results of MLC fault applied in clinical trial for 18 months were similar to the actual data.Conclusions The BP neural network model realized by R language of MLC fault prediction can describe the mapping relationship between fault factors and fault frequency,which provides references for the understanding of accelerator fault and management of spare parts inventory.
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Objective To examine the application of On-Board Imaging (OBI) system-based image-guided radiotherapy (IGRT) in the improvement of the precision of intensity-modulated radiotherapy (IMRT) for nasopharyngeal carcinoma.Methods Ten patients with nasopharyngeal carcinoma were treated with IMRT using the OBI system. The IGRT images after positioning, position adjustment, and treatment were observed and recorded to investigate the image difference between CT simulation and IGRT. Results The deviations in the x (lateral), y (cranial-caudal), and z (ventral-dorsal) directions between CT simulation and IGRT images were 0.22±1.00 mm,-0.37±1.28 mm, and 0.04±1.36 mm, respectively, after positioning, 0.29±0.76 mm,-0.04±0.78 mm, and -0.01±0.92 mm, respectively, after position adjustment, and 0.20±0.78 mm, 0.16±0.80 mm, and 0.05±0.92 mm, respectively, after treatment. The probabilities of a ≤1 mm deviation in the x, y, and z directions were 81.0%, 77.6%, and 88.2%, respectively, after positioning, 92.5%, 96.4%, and 96.4%, respectively, after position adjustment, and 91.7%, 94.9%, and 96.8%, respectively, after treatment. Conclusions The application of OBI system-based IGRT is very important in the improvement of the precision of fractionated IMRT for patients with nasopharyngeal carcinoma. The position of the patient should be adjusted based on the IGRT image after positioning in order to correct set-up error and effectively increase the precision of fractionated IMRT.
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Objective To test intraoperative radiotherapy with mobile photon beam using the INTRABEAM system ( Germany) , and to analyze the dosimetric characteristics of low?energy photon beam using X?ray source and spherical applicators and explore its potential limitations in clinical application. Methods A special water phantom, a parallel?plate ionization chamber, and an electrometer were used to measure the depth dose rates and isotropy of dose distribution in x/y plane of X?ray source and different spherical applicators in the INTRABEAM system. Those data were then compared with the system data. Results For the X?ray source, the deviation of observed depth dose rate and isotropy in the x/y plane from the system data were-2.16%± 1. 36% and-1.9%~ 2. 1%, respectively. For applicators with different diameters, the deviation of observed depth dose rate, transfer coefficient, and isotropy in x/y plane from the system data were-10.0%~2. 3%,-8.9%~4. 2%, and-1.6%~2. 6%, respectively. Surface dose rate and dose gradient became larger with the decrease in the diameter of the spherical applicator. The measurement of depth dose rate and isotropy of X?ray source and spherical applicators showed good repeatability. The influencing factors for measurement accuracy included the positioning error of ionization chamber, energy response, noise current, and correction factor f ’ ( R ) . Conclusions This study reveals the dosimetric characteristics of the INTRABEAM system, verifies the accuracy of the system data, and obtains the data for clinical application and routine quality assurance. However, large dose gradient and small therapeutic range may limit its wide clinical application.