Objective To test the accuracy and reliability of Axesse accelerator for volumetric modulated arc therapy (VMAT).Methods The accuracy and reliability of Axesse accelerator for VMAT were tested in a stepwise manner, from the simple to the complex and from the part to the whole.For the parts of the system, the stability of dosimetric output and the position accuracy of multi-leaf collimator (MLC) were tested.For the process of the system, the variable VMAT dose rates and gantry speed modulation, the MLC speed and dose rate modulation, and dosimetric verification in patients were tested.Results Compared with fixed gantry irradiation, the variation in dosimetric output was below 1.0% for rotary irradiation including the slide rotary irradiation of the dynamic MLC.The MLC position error of 0.5 mm was visible using the electronic portal imaging system of Axesse, iViewGT 3.40.The MLC position accuracy was within 1 mm for fixed gantry irradiation and rotary irradiation.In the range of the dose rate applied in clinical practice, the testing results of variable VMAT dose rates and gantry speeds as well as variable dose rates and MLC speeds showed that the variation between different strip-field beam intensities was below 2.0%.Using a gamma criterion of 3 mm/3%, the pass rates in dosimetric verification of patients with cervical cancer, prostate cancer, and breast cancer were 96.52%, 95.72%, and 98.83%, respectively.Conclusions The Axesse system can precisely control MLC motion, variable dose rates, and gantry speeds in VMAT.The Axesse system is accurate and reliable for VMAT.

Objective To analyze the patient-specific dosimetric verification results of 2010 intensity-modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) plans from different treatment sites,and provide a reference for improving the patient-specific dosimetric verification program.Methods A total of 2 010 (965 IMRT and 1 045 VMAT) patient-specific dosimetric verification results were reviewed for isocenter dose difference and percentage of pixels passing planar dose γ analysis.All plans were designed with Eclipse planning system and delivered with Trilogy linear accelerator from February 2012 to February 2016.The dosimetric verification was performed with MatriXX array together with Muhicube phantom.Point dose difference larger than ± 3％ and/or γ pass rate (3％/ 3 mm) less than 90％ was defined as plan failure.Additional analysis was conducted for trends in difference of pass rates with treatment site and delivery technique (IMRT vs.VMAT).Results The mean isocenter difference between measured and calculated doses was-0.3％ ± 2.4％ for 2 010 plans.The mean percentage of pixels passing the γ criteria was 97.9％ ± 3.4％.88.2％ and 96.7％ of plans passed the point and planar dose verification,respectively.The γ pass rate was different among the treatment sites (F =3.09,P ＜ 0.05).The pass rate of point and planar dose difference was different among the treatment sites(x2 =40.93,39.15,P ＜0.05).There was no difference between IMRT and VMAT plans for both point dose and planar dose evaluation (P ＞ 0.05).Conclusions Most of IMRT and VMAT plans passed the tolerance criteria of ±3％ and 90％ for point and planar dose verification with MatriXX together with Multicube phantom,respectively.Both point and planar dose verification results varied among treatment sites,whereas no significant difference was found between IMRT and VMAT.

Objective To investigate the feasibility and efficiency of electronic portal imaging device for dosimetric verification of intensity-modulated radiation therapy.Methods 10 patients treated with intensity-modulated radiation therapy were verified using the electronic portal imaging device of aS1000 from Varian and MatriXX from IBA.The gamma pass rate and time needed was recorded and compared.Results The gamma pass rate at 3％/3 mm was 95.82％ (94.77％-96.86％) and 99.08％ (98.72％-99.45％) with aS1000 and MatriXX,respectively.The average time needed for dosimetric verification with aS1000 and MatriXX was 12.7 and 47.8 min,respectively.Conclusions The electronic portal imaging device of aS1000 has the potential as a tool for IMRT dosimetric verification.It is more convenient and efficient than MatriXX.

Objective To investigate the dosimetric effect of multi-leaf collimator (MLC)transmission on intensity-modulated radiation therapy (IMRT).Methods MLC transmission through theleaves and rounded ends were measured with ion chamber for Varian Trilogy linear accelerator with the X-ray of 6 MV and 10 MV.The intraleaf and interleaf transmission were also measured with the electronic portal imaging device of aS1000.10 tumor patients treated with IMRT were used to evaluate the MLC parameters modeling in the Eclipse treatment planning system.Results The average transmissions of Millennium MLC were 1.6％ and 1.8％ for 6 MV and 10 MV X-rays.The transmission increased with the field size and depth of measurement.The transmissions at off axis position were a little lower than those on the central axis.The intraleaf transmissions measured with aS1000 were 0.8％-1.2％ and 1.1％-1.6％,the interleaf transmission were 1.3％-1.9％ and 1.6％-2.5％ for 6 MV and 10 MV X-rays respectively.Modeling with the measured MLC parameters,the Eclipse treatment planning system could calculate the dose distribution accurately.The γ pass rate at 3％ 3 mm was above 95％,except for two patients with nasopharyngeal cancer with the pass rate of 93.6％ and 94.5％.Conclusions The transmissions through the MLC leaves and the leaf ends contribute to the dose throughout the target significantly for IMRT.MLC transmission varied with the field size,depth of measurement and off axis position.

Objective:To investigate the accuracy and feasibility of 3D-printing individualized template-guided and CT-guided 192Ir interstitial brachytherapy in the central recurrent gynecologic tumors by comparing pre-plan and intraoperative physical dosimetric parameters. Methods:This study involved 38 patients with central recurrent gynecologic tumors who underwent 3D printing individual template (3D-PIT)-assisted and CT-guided 192Ir interstitial brachytherapy in the Department of Radiation Oncology of the Peking University Third Hospital from Jan 2018 to Dec 2019.The prescription doses for the target tumor areas were 10-36 Gy to be delivered at 5-6 Gy/fraction for 2-6 fractions.The pre-plan and intraoperative dosimetric parameters were compared, including the minimum prescription doses delivered to 90% and 100% of target volume( D90, D100)and the mean percentage of volume receiving 100% of the prescription doses ( V100). Meanwhile, the doses delivered to 2 cm 3 ( D2 cm 3) of organs at risk (bladders, rectums, and colons) were analyzed.The quality parameters of the brachytherapy were studied, including conformity index (CI), homogeneity index (HI), and external index (EI) of the target volume.Perioperative complications were also observed. Results:A total of 194 treatments were included.During the treatment, 5-13 (median 6) needles were inserted, with a prescription dose of 5-6 Gy per fraction.There were no statistical differences between pre-plan and intraoperative D90, D100, V100, CI, HI, and EI as well as the D2 cm 3 of bladders and colons at risk ( P>0.05). In contrast, for the D2 cm 3 of rectums, the intraoperative dose was slightly higher than the pre-plan dose, showing a statistical difference ( t=-0.335, P=0.027). Conclusions:The 3D-PIT-assisted and CT-guided 192Ir interstitial brachytherapy at a high dose rate is accurate and feasible in the treatment of recurrent gynecologic tumors, meeting the pre-plan dose requirement.

In recent years,the application of machine learning in the field of radiotherapy has been gradually increased along with the development of big data and artificial intelligence technology.Through the training of previous plans,machine learning can predict the results of plan quality and dose verification.It can also predict the multi-leaf collimator (MLC) positioning error and linear accelerator performance.In addition,machine learning can be applied in the quality assurance of intensity-modulated radiotherapy to improve the quality and efficiency of treatment plan and implementation,increase the benefits to the patients and reduce the risk.However,there are many problems,such as difficulty in the selection,extraction and calculation of characteristic value,requirement for large training sample size and insufficient prediction accuracy,which impede its clinical translation and application.In this article,research progress on the application of machine learning in the quality assurance of IMRT was reviewed.

Objective To compare five different detectors in output factor(OF)measurement for the CyberKnife(CK)system,and to select a suitable detector. Methods OFs for 12 different sizes of CK collimators were measured by EBT3films and 5 different commercially available detectors,consisting of diode detectors PTW 60017 and PTW 60018,ionizing chamber detectors PTW 31010 and PTW 30013,and diamond detector PTW 600019.OF was compared between different detectors and different measurement orientations. Results When the size of collimator was larger than 30 mm,the OF deviation among five detectors was less than 1%.When the size of collimator was smaller than 30 mm,however,the OF deviation among five detectors became large and obviously increased with the decrease in the size of collimator. With a OF deviation less than 2%,PTW 60019 achieved the best agreement with films. Compared with films,diode detectors gave slightly higher OFs,while ionizing chamber detectors gave much smaller OFs. The OF measurement was also affected by measurement orientation. PTW 60019 gave a smaller OF in the direction parallel to the central axis than in the direction perpendicular to the central axis of the radiation field,while PTW 31010 had an opposite result. Conclusions When the size of collimator is larger than 30 mm,PTW 31010,PTW 60017,PTW 60018,and PTW 60019 can be directly used for the OF measurement. When the size of collimator is smaller than 30 mm,correction is needed for the OF measurement using the above detectors. PTW 30013 is not suitable for the OF measurement in the small radiation field.

Objective To evaluate four detectors for the off-axis ratio profile measurements of a CyberKnife system, and provide reference and suggestions for selecting and using the correct detectors. Methods Profiles were acquired by using four detectors, PTW-60017, PTW-60018, PTW-60019 and IBA-SFD, at different depths for different collimator sizes, with the detector stem being oriented both perpendicular and parallel to the central beam axis. The differences of profiles and the influence of detector orientation on measurement result were analyzed. Results All full width at half maximum ( FWHM) of field measured by four detectors in parallel orientation was larger than that in actual field size. The deviation was increased with the size of collimator and measurement depth, with the maximum deviation of 1. 9 mm. The maximum deviation of FWHM among four detectors was 0. 2 mm. The penumbra was the smallest for IBA-SFD, and the largest for PTW-60019. The maximum deviation of penumbra was 0. 3 mm. The IBA-SFD tended to over-respond in the out-of-field region when the collimator size was larger than 30 mm. Both FWHM and penumbra in perpendicular orientation were smaller than those in parallel orientation for PTW-60017, PTW-60018 and PTW-60019, especially at 5 mm collimator. However, the trend was opposite for IBA-SFD. With the increase of collimator aperture, the difference between the right and left penumbra acquired by four detectors was increased, with more obvious stem effects. Conclusions Similar profiles were acquired by four detectors, but the detector characteristics and effects of detector orientations should be considered.

Objective To compare the differences of gross tumor volume (GTV) and dose distribution between MRI-CT fusion imaging and CT-based imaging and investigate the dose difference in the therapeutic regime.Methods Ten patients diagnosed with primary tumors of the cervical vertebra between 2013 and 2014 were enrolled.Prior to radiotherapy,the imaging data of MRI examination (GE Discovery MR 750 3.0T) were collected,transfered into the Eclipse system and subject to fusion with CT images.GTV delineation,organ at risk (OAR) delineation and dose distribution discrepancy induced by target volume differences were analyzed and statistically compared between the MRI-CT fusion and CT-based images.GTV delineation and dose parameters among different radiologists between two approches were analyzed by analysis of variance (ANOVA) and paired t-test.Non-normally distributed variables were analyzed by Wilcoxon rank-sum test.The reliability of intraclass correlation coefficient (ICC) was assessed.Results The GTVMRI-CT volume was larger compared with the GTVCT volume.The volume overlap index was 0.84±0.17.The cordMRI-CT volume was significantly less than the cordCT volume (P=0.001).For 5 radiologists,the ICCMRI-CT was larger than ICCCT.The DmaxMRI-CT of the spinal cord was (46.00± 1.09) Gy,significantly less than (52.39±1.34) Gy for the DmaxCT(P=0.014).Conclusions It is unlikely to miss the target delineation on MRI-CT fusion imaging.MRI-CT fusion imaging can minimize the discrepancy of interobserver radiologists and cause dosimetric advantages.

Objective To evaluate the accuracy and reliability of the CyberKnife VSI system for stereotactic radiotherapy.Methods First,the mechanical accuracy of the robotic manipulator system,the repeatability of the Iris variable aperture collimator system,the couch position accuracy of the patient positioning system,the tacking precision of the target locating system and the dosimetric beam characteristics of the linac system for each subsystem of CyberKnife VSI system were tested.Finally,the total beam delivery precision of the CyberKnife VSI system was evaluated.Results The mean positioning deviations of the mechanical arm movement of the robotic manipulator system were less than 0.1 mm,and the maximum positioning deviation of single note was ≤ 0.29 mm.The repeatability of the aperture sizes for the Iris variable aperture collimator system was ≤ 0.28 mm.The couch position accuracy of the patient positioning system was＜0.2 mm and the tracking precision of the target locating system was less than 0.5°.The linac beam-laser beam axes coincidence between the Iris and fixed collimators was better than 0.4 mm.The 6-MV beam parameters,such as beam quality and profile,were found within the acceptance limits.The deviations of output reproducibility,linearity and constancy versus linac orientation were less than 1.0％.The transmission factors of two types of collimators were lower than 0.4％.End-to-end test demonstrated that the maximum deviation of the total delivery precision of CyberKnife VSI system was 0.87 mm.Conclusion The CyberKnife VSI system is accurate and reliable for stereotactic radiotherapy.