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Objective:To investigate the workflow, feasibility and advantages of respiratory navigator-guided stereotactic body radiation therapy (SBRT) of liver malignancies on the magnetic resonance linear accelerator (MR-linac).Methods:Clinical data of 10 patients with liver cancer treated with respiratory navigator-guided SBRT on the MR-linac from September to December 2021 were analyzed retrospectively. All patients underwent CT and MR simulated localization, and plain, enhanced and 4D CT scan, and T 1 3D MR and T 2 3D MR images were collected. The expiratory 4D CT was chosen to design the reference plan. The T 2 3D navigator MR image (end-exhalation) was collected before treatment, the target position was adjusted or the target shape was modified in combination with the real-time monitoring 2D MR image and appropriate online adaptive planning process was selected. Then, the ability of CT, T 2 3D and T 2 3D navigator MR images to display the tumor was evaluated by naked eye. The changes of target volume were calculated. Dosimetric differences between the adaptive and reference plans were compared. The efficacy and adverse reactions of patients were evaluated. Results:In the free breathing state, the T 2 3D navigator MR image was significantly better than T 2 3D MR image to clearly display the tumor and its boundary. The adaptive plans of adapt-to-position (ATP) and adapt-to-shape (ATS) adopted by 10 patients was 37 times and 22 times respectively. The tumor subsided significantly in 3 patients. The average target conformal index (CI) of the adaptive plans was no different from that of the reference plans, but the gradient index (GI) was higher ( P<0.05), especially in the ATS plans. Compared to the reference plans, the normal liver V 5 Gy, V 10 Gy and D mean were almost the same, but the average MU was increased with a significant difference in the ATP adaptive plans ( P<0.05). The average of MU, segments and normal liver D mean and V 10 Gy in the ATS adaptive plans were lower than those in the reference plans, and the liver V 5 Gy was slightly increased. Seven patients were evaluated after 1 month and 3 months. The local control of lesions was promising. Toxicities were mild and no grade 3 or higher toxicities were observed. Conclusion:Respiratory navigator on MR-linac improves the visual clarity of tumors and online MR images, and shows its advantages to guide the adaptive precision radiotherapy of liver tumors, especially in SBRT.
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Objective:To study the impact on dose accuracy for the treatment planning by manually assigning accurate electron density for CT image-based tumor tissues and organs at risk.Methods:Twenty cases of retrospective postoperative cervical cancer radiotherapy plans were selected. The body electron density of the corresponding organs was derived from the ICRU 46 report and assigned in the treatment planning system (Monaco5.11, Sweden), including the bladder, rectum, intestine, kidney, spinal cord, femoral head, and ilium. The original plans were double-arc volumetric modulated arc therapy plan (360° VMAT), using Monte Carlo algorithm, the calculation grid was 0.3 cm × 0.3 cm × 0.3 cm, and the minimum subfield width was 0.6 cm. Keep the original plan fluence unchanged and recalculate the dose to generate a new plan. The two-dimensional dose distribution and dose-volume histogram (DVH) were used to compare the differences between the two plans. The difference was compared between the two group plans by using the dosimetry parameters and DVH two dimension curve.Results:For the planning of assigning bulk electron density (Plan RED), the deviation of the patient′s target dose parameters and the original plan (Plan ref) was <2%, and the average deviation of all target regions D2, D98, Dmean was < 0.7%, only 2 of the 180 data were between 2% and 3%. The average deviation of V20, V30, D1 cm 3, Dmean of the bladder, rectum, and small intestine, the original Plan ref was less than 0.6%, and 4 out of 240 data had values > 2%. Plan RED′s average hop count was 0.9% higher than Plan ref, and the total number of subfields remains unchanged. The planned dose generated by manually assigning the electron density in Plan RED was higher than that in Plan ref, but met the clinical requirements. The two-dimensional curves of the DVH diagram for targets and OARs almost completely overlapped, and there was no obvious difference in the dose distribution diagram of the same cross section. The statistical result of all parameters showed that the difference in planned dose parameters between the two groups was not statistically significant( P>0.05). Conclusions:The overall deviation of dose accuracy between Plan RED and Plan ref is <2%, which meets the clinical requirements and provides a reference for realizing MRI-only treatment planning.
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Objective To compare the neck skin dose between fixed-field dynamic intensity-modulated radiation therapy (dlMRT),volumetric modulated arc therapy (VMAT),and helical tomotherapy (HT) in the treatment of early-stage nasopharyngeal carcinoma.Methods A total of 16 early-stage nasopharyngeal carcinoma patients undergoing radiotherapy were enrolled as subjects.The neck skin was delineated by contraction of the outer edge of neck by 3 mm.Dose planning was made by the traditional method (TP group)and a new method (NP group),in which the neck skin was considered as the organ at risk.Dmean and V5-V70 for the neck skin were recorded.The paired t-test was used to analyze the differences between two plans in each radiotherapy method.An analysis of variance was used to compare the same plan between the three radiotherapy methods.Results The HT group had significantly higher Dmean and V5-V70 for the neck skin than the dIMRT group and the VMAT group (P=0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.02).Using dIMRT,the D and V10-V60 for the neck skin were reduced by 7％,8％,22％,25％,38％,59％,and 85％ in the NP group than in the TP group (P=0.00,0.00,0.00,0.00,0.00,0.00,0.00).Using VMAT,the D and V20-V40 for the neck skin were reduced by 4％,19％,29％,and 34％ in the NP group than in the TP group (P=0.02,0.01,0.02,0.01).Using HT,the V30-V60 for the neck skin were reduced by 20％,29％,50％,and 67％ in the NP group than in the TP group (P=0.00,0.00,0.00,0.00,0.03).Conclusions In the treatment of early-stage nasopharyngeal carcinoma,HT causes a higher radiation dose to the neck skin than dIMRT and VMAT,while dIMRT and VMAT have similar neck skin doses.The neck skin dose can be significantly reduced with the neck skin as the organ at risk.
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Objective To measure and analyze the neutron dose equivalent rate produced by an IORT accelerator with 9 and 12 MeV electron energyies,and compare them with those from a Siemens Primus linear accelerator with the same electron energy,in order to provide data reference for the risk of secondary cancer induced by radiotherapy.Methods Using the neutron detector LB6411,the neutron dose equivalent rates produced by the IORT accelerator of 9 and 12 MeV were measured on some key locations,such as the head of the accelerator,cylinder bottom,patient plane with electron energies 9 and 12 MeV.The similar measurements were also performed on the same locations on a Siemens conventional accelerator.The data were collected and analyzed and the result wer compared between the two accelerators.Results Neutron dose equivalent rates from the IORT accelerator with 9 MeV energy were (51.8±3.1),(45.5 ±1.5),(70.5 ±4.9) and (68.2±3.3) μ Sv/h near the head of the accelerator,cylinder bottom,patient plane,with 5.9％,5.4％,17.8％ and 21.5％ lower than at 12 MeV,respectively.The dose equivalent rates at the similar locations from the Siemens Primus accelerator were (277.3 ±1.2),(285.1 ±1.6),(185.1 ±1.8) and (182.8 ±2.4) μSv/h at 9 MeV,with 48.8％,47.6％,48.7％,52.2％ lower than those at 12 MeV,respectively.At the energy of 12MeV,the neutron equivalent dose rate from the IORT was lower by a factor of about 10 than for Siemens Primus accelerator.Conclusions The neutron dose equivalent rates generaged by both the IORT and the Siemens Primus are higher at 12 MeV than at 9 MeV,which would lead to an increased risk of secondary cancer to patients.The traditional medical accelerator produces much higher neutron dose equivalent rates than the intraoperative electron accelerator,for which the appropriate shielding should be takn.
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Objective To investigate the potential dosimetric advantages of half jaw volumetric modulated arc therapy ( H-VMAT) applied to the Oropharyngeal Cancer, comparing with full jaw VMAT (F-VMAT) and intensity modulated radiotherapy ( IMRT ). Methods Planning CT images of 10 oropharyngeal cancer patients were retrospectively chosen and transferred to Eclipse treatment planning system v. 11. 0 (Varian Medical Systems, Pala Alto, USA), based on which H-VMAT, W-VMAT, and IMRT plans were created. Two full arcs (360°) were adopted for VMAT planning, and the 7 beams were equally distributed for IMRT planning. The optimization constraints remained the same for the three kinds of plans. The dosimetric parameters such as D2 , D98 , D50 , HI, and CI were evaluated for PGTV, PCTV1, PCTV2, PGTVln, and PCTVln. In addition, the maximum dose (Dmax) and D1 cc(minimum dose received by 1cc) of the brainstem and spinal cord were analyzed respectively. The mean dose ( Dmean ) to the parotids, oral cave, larynx, and cervical normal tissues were also reviewed. The monitor units ( MU) for all treatment plans were recorded. Results Comparisons of the three planning techniques showed that H-VAMT improved the HI and CI of the targets (except PCTV2) significantly (HI: F =3. 959, 6. 764, 10. 581, 6. 770, 13. 040, P<0. 05;CI:F=6. 594, 4. 138, 0. 842, 4. 031, 5. 388, P<0. 05);reduced Dmax(F=4. 509, 20. 331, P<0. 05) and D1 cc for brainstem and spinal cord (F=27. 432, 26. 314, P<0. 05) significantly;reduced Dmean(F=4. 279, 29. 498, 19. 295, P<0. 05) to the normal tissues of the mouth, throat and neck significantly. The V50 of the mouth and throat were slightly lower in IMRT plans (F=8. 140, P<0. 05). IMRT was slightly better than W-VMAT in sparing oral cavity and larynx, but the dose distribution was the worst. The H-VMAT plans showed the best dose distribution in the cervical normal tissues, especially for the lower and posterior parts, where IMRT plans displayed high dose curves. Conclusions H-VMAT is dosimetrically superior than W-VMAT and IMRT for oropharyngeal cancer, which could be considered for clinical applications.
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Objective To compare dosimetric parameters between automatic three-dimensional conformal radiotherapy (a3DCRT),inverse three-dimensional conformal radiotherapy (i3DRT),inverse intensity-modulated radiotherapy (iIMRT).Methods Ten lung cancer patients with a single target and 10 intracranial tumor patients also with a single target,who were treated in our radiotherapy center from 2014 to 2015,were included in the study.Their image data were transferred to RayStation 4.5 via network,and then the treatment plans for a3DCRT,i3DCRT,and iIMRT were designed for the 20 patients.The dosimetric parameters for planning target volume (PTV) and organs at risk (OAR) were compared between the three plans using multiple dependent variables and two dependent variables.Results For lung cancer patients,iIMRT achieved better results in D98％,D50％,D2％,conformity index (CI),and homogeneity index (HI) than i3DCRT and a3DCRT (P=O.007,0.001,0.002,0.000,and 0.000),and the CI of a3DCRT was superior to that of i3DCRT (P=O.000);there were no significant differences in heart D33,spinal cord Dmax and D1 cm3,and both lungs between the three plans (P=O.702,0.237,0.163,0.739,0.908,0.832,0.886,0.722,0.429,0.840,and 0.702).For intracranial tumor patients,there were no significant differences in dosimetric parameters between the three plans,except that the CI of iIMRT and a3DCRT was superior to that of i3DCRT (P=O.648,0.783,0.256,0.931,0.002,and 0.034);there were no significant differences in whole brain irradiation dose between the three plans (P=0.446,0.755,0.772,0.0266,0.440,0.290,and 0.939).Conclusions For the single target in patients with lung cancer and intracranial tumors,a3DCRT can improve the CI of PTV compared with i3DCRT,and shows no significant dosimetric disadvantage for OARs compared with iIMRT.Considering the simplicity and low cost of 3DCRT,a3DCRT holds promise as a novel radiotherapy technique.
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Objective To compare the effective dose deposited in patients between helical tomotherapy (HT) and multi-ISO radiotherapy (M-ISO) in carniospinal irradiation (CSI).Methods Nine children with craniospinal irradiation were selected .For these patients , new plans were designed with HT and M-ISO centers planning method on the treatment planning system ( TPS) .The effective dose of the nine patients from 18 treatment plans were calculated ,and the difference of the effective dose between HT and M-ISO was compared using paired t-test.Results The plans designed in two groups were both satisfied all clinical requirements .For the planning target volume ( PTV ) , no statistically significant difference was found in D95% between two groups ( P>0.05 ) , while D98%, D2% and homogeneity index (HI) in HT group were superior to M-ISO group (t=2.762, 2.413, 4.563, P<0.05), D50%, Dmean and CI in M-ISO group were superior to HT group (t=5.259, 3.685, 7.815, P<0.05).HT and M-ISO had different advantages in the protection of the OARs .The effective dose of patients in M-ISO group was superior to HT group (t=5.921, P<0.05).Conclusions HT and M-ISO have different advantages in CSI.The low dose area has greater influence on the effective dose in HT group compared to M-ISO group. The low dose area should be concerned while designing the treatment planning for CSI .
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Objective The purpose of this study is to investigate the method to reduce the radiation dose to the neck skin in the Tomotherapy treatment plans for early-stage nasopharyngeal carcinoma.Methods The 17 patients with early-stage nasopharyngeal carcinoma that have been treated by the Tomotherapy were selected randomly for this skin sparing study.The neck skin sparing region was generated as an internal margin of 3 mm from the out body contour,excluding the intercrossed area with the targets.Candidate patients were planned using TP and NP method respectively:the TP group was planned with the traditional method.The new neck skin region was considered as an organ at risk (OAR) for planning dose constrain in NP group.The dosimetric metrics of targets and OARs,monitor units (MU) and delivery time were compared as the end points of these two groups.Results The two treatment plan groups satisfied the clinical requirement.There were no significant differences for D98,D95 and D2 of the targets (P ＞ 0.05).The Dmax of brainstem,D1cc of spinal cord,D of right parotid were higher in NP group than in theTPgroup (t =2.47,2.34,2.77,P＜0.05).The Dmax of left mandible joint was lower than TP group(t =2.30,P ＜ 0.05).The V30,V40,V50 and V60 of the skin were considerably lower than TP group (t =8.37,6.02,5.82,4.89,P ＜ 0.05).The mean MU and mean delivery time per fraction of NP group were 6.3％ and 8.1％ less than that of TP group respectively.Conclusions The neck skin region should be delineated as an OAR to be spared in the Tomotherapy treatment planning for early-stage nasopharyngeal carcinoma.This method can reduce the skin radiation dose effectively,alleviate the skin reaction,and improve the life quality of patients in radiotherapy.
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Objective To investigate the dose and mechanical properties of medical electron linear accelerators in grass-roots radiotherapy units in Sichuan Province,China via sampling and inspection.Methods A total of eight radiotherapy units in Sichuan Province were selected by sampling,and the tests were performed for the dose and mechanical properties of the medical electron linear accelerators in use.Among these accelerators,there were 5 imported accelerators and 3 domestic accelerators.The test items and methods were determined according to the requirements in GB15213-94.Results Among the 14 test items,the items related to the flatness,symmetry,and overlap of radiation field.The other tests of dose accuracy and mechanical precision achieved good results.Conclusions There is a need to strengthen the daily quality control work for dose and mechanical accuracy of medical electron linear accelerators in grass-roots radiotherapy units in Sichuan Province and perfect the allocation of professional equipment and personnel and training of related personnel.With the support of Sichuan Radiotherapy Quality Control Center,quality control supervision and guidance should cover the whole province.
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Objective To study the impact of setup error caused by computed tomography ( CT) images with different resolutions in the Sentinel system on clinical treatment. Methods A phantom was scanned by large?aperture positioning CT with two different resolutions ( CT1:0. 5 mm × 0. 5 mm × 1. 0 mm, FOV 256 mm, Matrix 512, thickness 1 mm;CT3:1. 0 mm×1. 0 mm×3. 0 mm, FOV 500 mm, Matrix 512, thickness 3 mm) . The CT images were transferred to the planning system. The radiation fields were designed and transferred to MOSAIQ and Sentinel systems. Ten fixed setup errors were applied to a six degree of freedom couch. The Sentinel system was used to position the two groups of CT images and generate the setup errors. The comparison of two datasets was made by paired t?test. Cone?beam CT was used for independent verification. Results The setup errors in x?, y?, and z?directions were significantly smaller on CT1 than on CT3(0.19±0. 11 vs. 0.33±0. 16 mm, P=0. 061;0.59±0. 79 vs. 1.07±1. 09 mm, P=0. 008;0.67±0. 75 vs. 1.16±1. 30 mm, P=0. 043). There were no significant differences in rotational errors in x?, y?, or z?directions between the two datasets ( P=0. 494;P=0. 182;P=0. 298) . Conclusions The Sentinel system has a higher setup accuracy in the 0. 5 mm×0. 5 mm×1. 0 mm mode than in the 1. 0 mm×1. 0 mm×3. 0 mm mode. However, the later mode is still an acceptable choice in clinical treatment.
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Objective To explore the application value of multi-ISO center planning method in intensity-modulated radiotherapy for carniospinal irradiation.Methods Ten patients treated with craniospinal irradiation with helical tomotherapy were selected.For these patients, new plans were designed with multi-ISO centers planning method on the treatment planning system (TPS) named Eclipse 11.0.Dose distribution to the tumor, OARs and normal tissue, the treatment time and the monitor units (MUs) of the two plans were compared.Results The plans designed in two groups satisfied all clinical requirements.For the tumor target (PTV) , the difference of D95％ between two groups was not statistical significant, while D9s ％ , D2 ％ and HI in HT group were superior to M-ISO group (t =2.822,2.333,4.743, P ＜0.05) , D50％ , D and CI in M-ISO group were superior to HT group (t =5.259,3.685,8.835 ,P ＜ 0.05).The dose of OARs such as cochlea, parotid, submandibular gland, thyroid gland and kidney in HT group was lower than M-ISO group (t =4.365,5.416,2.674,3.077,2.782,2.607,4.659,P ＜0.05) , and the dose of pancreas and small bowel was higher than M-ISO group (t =5.265,5.935, P ＜ 0.05).Differences were not significant for V5 of normal tissue between two groups;while V10, V20 and V36 of normal tissue in M-ISO group were lower than HT group (t =3.57,3.701,2.602, P ＜ 0.05).M-ISO group reduced 41.0％ of the treatment time by average and reduced 94.1％ MUs by average.Conclusions Intensity-modulated radiotherapy for carniospinal irradiation with multi-ISO centers planning method not only met the requirements of clinical dosimetry, but also shorten the treatment time, reduced the damage to the machine.Multi-ISO centers planning method might be promoted as a new design scheme.
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Objective To evaluate the role of equivalent uniform dose (EUD) in planning optimization of intensity-modulated radiotherapy (IMRT) for prostate cancer.Methods Ten patients with prostate cancer were randomly selected who treated with IMRT.For these patients,the treatment plans were designed with dose-volume objectives.Based on these plans,new plans were designed through replacing the dose-volume objectives with maximum EUD for rectum,bladder and small bowel,while keeping the dosevolume objectives unchanged.Comparison was made between the new plans and the former cones by paired t-test.Results The conformity index of planning target volume was better with EUD optimization compared to dose-volume (1.00 ± 0.04 ∶ 0.94 ± 0.04,t =3.80,P =0.04).The D53,D30 and Dicm3 for rectum was better with EUD optimization compared to dose-volume (24.4 ± 2.7 ∶ 25.5 ± 2.6,t =-3.82,P =0.004,34.1 ±4.3∶39.1±2.1,t=-3.80,P=0.004 and51.4±1.0∶51.8±0.9,t=-2.42,P=0.039),with V10,V20 for bladder and V10,V20,V30,V40 for small bowel also better with EUD optimization (92.2 ±6.2∶99.4±1.1,t=-4.28,P=0.002;70.7±5.7∶78.7±6.3,t=-3.10,P=0.013 and 62.2±30.2∶74.7 ±30.0,t =-4.18,P =0.002;34.3 ±26.3∶46.5 ±30.9,t =-5.46,P =0.000;17.1 ±17.0∶25.1 ±22.6,t=-3.52,P=0.007;10.6± 11.5∶ 15.6± 16.1,t=-2.64,P=0.030).Conclusions The conformity index of planning target volume is better with EUD optimization compared to dose-volume.And the dose to rectum,bladder and small bowel can be reduced through optimization with EUD optimization compared to dose-volume.
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Objective The aim of this study was to evaluate the role of multicriteria optimization (MCO) in planning of intensity-modulated radiotherapy (IMRT).Methods Twenty IMRT patients (ten with prostate and ten with lung cancers) were randomly selected.For these patients,the treatment plans were designed with direct machine parameter optimization (DMPO).Based on these plans,new plans were designed with MCO,while keeping the setting conditions unchanged.Comparison was made between the two plans including the dose distribution,the dose volume histogram,the time of optimization and number of monitor unit (MU),but were play by pairing-t test.Results The plan designed in both optimizations satisfied all clinical requirements.For the same or better target coverage,rectum,bladder and small bowel were better with MCO compared with DMPO,MCO reduced 58％ of the time for optimization by average while MU increased 32％ by average for prostate cancer.For lung cancer,the whole lung,heart and spinal cord were better with MCO compared with DMPO,MCO reduced 59％ of the time for optimization by average while MU increased 11％ by average.Conclusions In comparison with DMPO,MCO reduces the dose of organs at risk,shorten the time of optimization.
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<p><b>OBJECTIVE</b>To evaluate the radiation output and stability of linac-integrated kV cone beam CT unit.</p><p><b>METHODS</b>Air kermas in radiographic mode were measured with 0.6 cc ion chamber and Unidos electrometer for Synergy-integrated XVI kV cone beam CT unit. Air kermas vs image frames were measured in fluoroscopic mode. Output stability and depth doses were measured.</p><p><b>RESULTS</b>The air kerma increased quadratically with the increased tube voltage, while increasing linearly with the tube current, exposure time, and number of frames. The radiation output stability and its change with the gantry angle were within +/-1%. The percentage depth dose increased with higher tube voltage.</p><p><b>CONCLUSION</b>The radiation output of XVI is stable. The radiation outputs change considerably with the preset parameters. Parameters should be optimally chosen to reduce the patient dose.</p>
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Cone-Beam Computed Tomography , Particle Accelerators , Radiation DosageABSTRACT
Objective To evaluate the patient dose from Hi-ART MV helical CT imaging in image-guided radiotherapy.Methods Weighted CT dose index (CTDI_W) was measured with PTW TM30009 CT ion chamber in head and body phantoms,respectively,for slice thicknesses of 2,4,6 mm with scanned range of 5 cm and 15 cm.Dose length products (DLP) were subsequently calculated.The CTDI_W and DLP were compared with XVI kV CBCT and ACQSim simulator CT for routine clinical protocols.Results An inverse relationship between CTDI and the slice thickness was found.The dose distribution was inhomogeneous owing to the attenuation of the couch.CTDI and DLP had close relationship with the slice thickness and the scanned range.Patient dose from MVCT was lower than XVI CBCT for head,but larger for body scan.Results CTDI_W can be used to assess the patient dose in MV helical CT due to its simplicity for measurement and reproducibility.Regular measurement should be performed in QA & QC program.Appropriate slice thickness and scan range should be chosen to reduce the patient dose.
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Objective To evaluate the variations of effective doges with the preset scanning parameters from linac-integrated kV cone beam CT( CBCT). Methods Weighted CT dose index(CTDIW) were measured with PTW TM30009 CT ion chamber in head phantom and body phantom, respectively, for different combinations of tube voltage, mAs, collimator and gantry rotation range. Dose length products(DLP) were derived from CTDIW and effective doses(E) were calculated by the DLP and EDLP. Results CTDIW and effective dose had the quadratical relationship with tube voltage, depending linearly on product of tube current and exposure time. Effective dose had close relationship with the collimator and the gantry rotation range. Both the DLP and ED for CBCT were lower than the reference dose level recommended for conventional CT. Conclusions Effective dose from CBCT has a close relationship with the scanning parameters. Optimal imaging parameters should be chosen according to the patient's anatomy to reduce patient dose.
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Objective To evaluate the penumbra of a new multileaf collimator equipped with Elekta Synergy accelerator. Methods The penumbra were derived from beam profiles measured in air and water using PinPoint ion chamber with PTW MP3 water phantom. Variations of penumbra with X-ray beam energy, depth in water, and leaf position were investigated. Results The penumbra in air for 6 MV X-ray was 2 mm less than that at depth of maximal dose in water. The penumbra of leaf side was 1 mm less than that of the leaf end. The penumbra had close relationship with beam energy, depth in water and leaf position. Penumbra was increased with beam quality and water depth. The leaf position had great influence on the penumbra. Conclusions The penumbra of the multileaf collimator is related to its original design and radiation delivery technique. Special considerations should be taken into during treatment planning.Regular measurement should be performed to guarantee the delivery quality.
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easurement should be performed in QA & QC program. Optimal image parameters should be chosen to reduce the scanning range and patient dose.
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Objective To evaluate the dosimetric effect of carbon fiber tabletop on the patient doses in radiotherapy. Methods The transmission factors of couch and inserts were measured in air and solid water phantom using 0.6 cm3 ion chamber and PTW 2D ion chamber array for 6 MV ,10 MV and 18 MV X-ray,respectively. Absorbed doses at depth of maximum dose ,5 cm and 10 cm in solid water were measured with the 2D ion chamber array. Absorbed doses fluctuations with different gantry angles and air gaps between phantom and couch were also measured. Results The posterior field measurement showed that the reductions of absorbed doses at the depth maximum dose,5 cm and 10 cm were within 5%. The ratios of the absorbed doses with to without couch increased with the oblique incident angles and varied slightly with the air gap at depth of 5 cm. The transmission factors of inserts were less than those of couch owing to its thinner thickness. Conclusion The carbon-fiber tabletop affects the absorbed doses and dose distributions of the target, and this effect changes with the gantry angle and air gap. Special considerations should be taken during treatment planning.