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@#<b>Objective</b> To investigate the dosimetric differences between volumetric modulated arc therapy (VMAT) with a flattening filter (FF) and flattening filter-free (FFF) VMAT in fractionated stereotactic radiotherapy for brain metastases. <b>Methods</b> Seventeen patients with brain metastases were divided into FF-VMAT group (VMAT plans with the FF mode) and FFF-VMAT group (VMAT plans with the FFF mode). The two groups were compared in terms of target volume dose parameters (<i>D</i><sub>98%</sub>, <i>D</i><sub>2%</sub> and <i>D</i><sub>mean</sub>), the conformal index (<i>CI</i>), the gradient index (<i>GI</i>), the gradient, normal brain tissue dose parameters (<i>V</i><sub>5Gy</sub>, <i>V</i><sub>10Gy</sub>, <i>V</i><sub>12Gy</sub> and <i>D</i><sub>mean</sub>), monitor units, and beam-on time. <b>Results</b> Compared with the FF-VMAT group, the FFF-VMAT group had significantly lower <i>GI</i> (3.33 ± 0.37 <i>vs</i> 3.27 ± 0.35, <i>P</i> = 0.001), a significantly lower gradient [(0.85 ± 0.20) cm <i>vs</i> (0.84 ± 0.19) cm, <i>P</i> = 0.002], a significantly shorter beam-on time [(177.05 ± 62.68) s <i>vs</i> (142.71 ± 34.59) s, <i>P</i> = 0.001], and significantly higher <i>D</i><sub>2%</sub> [(65.69 ± 2.15) Gy <i>vs</i> (66.99 ± 2.03) Gy, <i>P</i> = 0.001] and <i>D</i><sub>mean</sub> [(58.77 ± 1.60) Gy <i>vs</i> (59.95 ± 1.43) Gy, <i>P</i> <0.001]. There were no significant differences in the <i>CI</i>, the<i> D</i><sub>98% </sub>of the target volume, the <i>V</i><sub>5Gy</sub>, <i>V</i><sub>10Gy</sub>, <i>V</i><sub>12Gy</sub> and <i>D</i><sub>mean</sub> of the normal brain tissue, and monitor units between FFF-VMAT and FF-VMAT. <b>Conclusion</b> FFF-VMAT can better protect the normal tissue around the target volume, reduce the beam-on time, and improve treatment efficiency.
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@#<b>Objective</b> To compare the dosimetric differences of volumetric modulated arc therapy (VMAT) with flattening filter(FF) and flattening filter-free(FFF) modes in hippocampal avoidance whole brain radiotherapy. <b>Methods</b> We included 15 patients with hippocampal-sparing whole brain radiotherapy, and designed two radiotherapy plans of FF-VMAT and FFF-VMAT for each patient. On the premise of meeting clinical dose requirements, the two plans’ dosimetry, total number of monitor units, and beam-on time were compared. <b>Results</b> There were no significant differences in the target coverage, conformity index, and dose gradient of the FF-VMAT and FFF-VMAT plans (<i>P</i> > 0.05). The <i>D</i><sub>max</sub>, <i>D</i><sub>100%</sub>, and <i>D</i><sub>mean</sub><sub> </sub>to the hippocampal tissue were significantly lower with FFF-VMAT [(15.13 ± 0.38) Gy, (7.12 ± 0.34) Gy, and (9.76 ± 0.43) Gy, respectively)] than with FF-VMAT (16.46 ± 0.56) Gy, (7.72 ± 0.28) Gy, and (10.54 ± 0.48) Gy, respectively)] (<i>P</i> < 0.05). The <i>D</i><sub>max</sub><sub> </sub>to the left and right lenses and the <i>D</i><sub>mean</sub><sub> </sub>to the left and right eyeballs with FFF-VMAT were (7.26 ± 0.43) Gy, (6.29 ± 1.13) Gy, (11.01 ± 0.94) Gy, and (9.78 ± 1.13) Gy, respectively, which were significantly lower than FF-VMAT’s corresponding doses of (8.09 ± 0.66) Gy, (7.80 ± 0.74) Gy, (11.38 ± 1.09) Gy, and (11.05 ± 0.90) Gy, respectively (<i>P</i> < 0.05). The doses to other organs at risk including the optic nerve and optic chiasm were all controlled within the safe dosage ranges, with no significant differences between the two plans (<i>P</i> > 0.05). The FFF-VMAT plan had a significantly greater number of monitor units and a significantly shorter beam-on time than the FF-VMAT plan (<i>P</i> < 0.05). <b>Conclusion</b> Both FF-VMAT and FFF-VMAT can meet the clinical requirements, with FFF-VMAT having better hippocampus and lens protection, shorter beam-on time, and higher treatment efficiency.
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Objective:To explore a new technique for lung dose reduction in esophageal cancer radiotherapy based on the dose distribution characteristics of the half-field combined with intensity-modulated radiotherapy (IMRT) technique.Methods:A three-dimensional water tank was used to measure the dose distribution at the edge of the symmetrical field and half-field, which was then compared and analyzed. Twenty patients with the middle and lower thoracic esophageal cancer receiving radiotherapy with prescription doses of 50.4-60.0 Gy were selected. Based on the Varian Vital beam linear accelerator and Eclipse planning system, flattening filter (FF) technique symmetrical field and half-field beam design and the flattening filter-free (FFF) technique symmetrical field and half-field design were adopted to compare and analyze various dose data and treatment MU numbers for the target area and the endangered organs. The field settings were chosen in the front 1 and back 4 mode.Results:Compared with the symmetrical field plan, the half-field plan significantly improved the irradiated dose to the lung with a statistically significant difference ( P<0.05), and the half-field FFF was slightly better than the half-field FF mode. Compared with the total lung V 5Gy, V 20Gy, V 30Gy, and D mean dosimetric parameters (Gy), the FF symmetric field and FFF half-field were (49.64±5.39)% vs.(42.70±5.53)%, (15.99±3.93)% vs.(13.32±3.06)%, (9.24±2.77)% vs.(8.50±2.62)%, and (10.45±1.76)% vs.(9.50±1.53)%, respectively. There was a significant reduction in the volume dose values for all structures of the lung (all P<0.05). For other comparative data, the irradiated dose for the heart of the symmetrical field was better than that of the half-field mode ( P<0.05), and the differences in conformity index (CI) and homogeneity index (HI), number of treated MU, and spinal cord associated with the target area were not statistically significant (all P>0.05). Conclusions:During radiotherapy for esophageal cancer, target area coverage and dose volume data of the lung are the main parameters affecting the efficacy and side effects. IMRT treatment based on the half-field mode can give full play to the advantages of half-field and IMRT, and significantly improve the irradiated dose to the lung, which can offer an additional clinical option.
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Objective@#To compare relevant dosimetric parameters of non-coplanar volumetric modulated arc therapy (VMAT) in treating brain tumors in conventional flattening filter (FF) or flattening filter-free (FFF) delivery mode, aiming to explore the appropriate evaluation method of accelerator for stereotactic radiosurgery (SRS).@*Methods@#Clinical data of 10 patients with single cranial tumor were retrospectively analyzed. All patients received non-coplanar VMAT at a prescription dose of 25 Gy in 1 fraction. Dosimetric parameters including conformity index (CI), heterogeneity index (HI), gradient index (GI50, GI25), gradient, volume of the brain tissue receiving larger than 10 Gy and 12 Gy(V10 and V12) and beam-on time were statistically compared between two treatment plans by paired sample t-test.@*Results@#When FFF-VMAT was compared with FF-VMAT in SRS for intracranial tumors, Paddick gradient index GI50 was 2.91±0.34 vs.3.07±0.35, 6.91±0.28 vs.7.35±0.27 for GI25, (0.57±0.07) cm vs.(0.61±0.08) cm for gradient, respectively (all P<0.05), whereas CI did not significantly differ (P>0.05). For the normal brain tissues, the average dose was (160.64±43.64) cGy vs.(174.27±53.98) cGy, (45.35±30.32)% vs.(48.37±30.88)% for V10 and (36.69±25.15)% vs.(39.48±25.37)% for V12, respectively (all P<0.05).@*Conclusions@#Non-coplanar VMAT in FFF delivery mode can improve dose distribution and normal brain tissue sparing in the treatment of intracranial single tumors. Meanwhile, supplement of GI index and gradient index can implement comprehensive evaluation of the SRS planning.
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Objective To compare relevant dosimetric parameters of non-coplanar volumetric modulated arc therapy (VMAT) in treating brain tumors in conventional flattening filter (FF) or flattening filter-free (FFF) delivery mode,aiming to explore the appropriate evaluation method of accelerator for stereotactic radiosurgery (SRS).Methods Clinical data of 10 patients with single cranial tumor were retrospectively analyzed.All patients received non-coplanar VMAT at a prescription dose of 25 Gy in 1 fraction.Dosimetric parameters including conformity index (CI),heterogeneity index (HI),gradient index (GI50,GI25),gradient,volume of the brain tissue receiving larger than 10 Gy and 12 Gy (V10 and V12) and beam-on time were statistically compared between two treatment plans by paired sample t-test.Results When FFF-VMAT was compared with FF-VMAT in SRS for intracranial tumors,Paddick gradient index GI50 was 2.91±0.34 vs.3.07±0.35,6.91±0.28 vs.7.35±0.27 for GI25,(0.57±0.07) cm vs.(0.61±0.08) cm for gradient,respectively (all P＜0.05),whereas CI did not significantly differ (P＞0.05).For the normal brain tissues,the average dose was (160.64±43.64) cGy vs.(174.27± 53.98) cGy,(45.35± 30.32) ％ vs.(48.37± 30.88) ％ for V10 and (36.69±25.15) ％ vs.(39.48±25.37) ％ for V12,respectively (all P＜0.05).Conclusions Non-coplanar VMAT in FFF delivery mode can improve dose distribution and normal brain tissue sparing in the treatment of intracranial single tumors.Meanwhile,supplement of GI index and gradient index can implement comprehensive evaluation of the SRS planning.
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Aim: The aim of this study is to commission and validate the portal dosimetry (PD) system using an indirect method for flattening filter free (FFF) photon beam of the upgraded c-series linear accelerator. Background: Varian Medical System clinacs with amorphous-silicon portal imager panel (aSi-1000) do not have PD for FFF beams. Recently, our c-series linear accelerator was upgraded to deliver 6MV FFF (6MVFFF) photon beam with the highest dose rate of 1400 monitor unit (MU)/min. The study, therefore, focuses on the commissioning and validation of PD for the 6MVFFF beam. Materials and Methods: An indirect method was implemented to predict the portal dose for FFF beam in Eclipse as the treatment planning system does not have direct prediction algorithm for FFF beam (version. 11). Dosimetrical characteristics of aSi-electronic portal imaging device (EPID) were evaluated for 6MVFFF beam and validation of PD for 6MVFFF beam was performed for open fields along with pretreatment quality assurance of intensity-modulated radiation therapy (IMRT), volumetric-modulated arc therapy (VMAT), and stereotactic radiosurgery (SRS) techniques for 30 patients planned with 6MVFFF beam. Results: ASi-EPID saturates between 100 and 130 cm source to detector distance (SDD) for 6MVFFF beam and resolved at more than 140 cm SDD. The squared correlation coefficient (R2) for MU linearity was found to be 1 (R2 = 1), and instantaneous dose response linearity at different SDD's was found to be 0.999 (R2 = 0.999) for the 6MVFFF beam. Maximum gamma area index (GAI) for 3% dose difference and 3 mm distance-to-agreement criteria for IMRT, VMAT, and SRS/stereotactic radiotherapy plans was 97.9% ± 0.3%, 96.3% ± 0.5%, and 98.2% ± 0.2%, respectively. Conclusion: The results reveal that this novel method can be used to commission portal dosimetry for 6MVFFF beam as it is a convenient, faster, and accurate method
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Objective To establish a novel method of describing the off-axis ratio (OAR) characteristics of the flattening filter-free (FFF) beam.Methods The OAR curves at a depth of 1.5,5,10 and 20 cm were measured for Varian Edge,Elekta VersaHD and Tomotherapy using the water tank.The second derivatives of the OAR in the positive and negative directions were calculated.The center of the line connecting the maximum and minimum second derivatives was defined as the field edge.The distance between the left and right field edges was defined as the dosimetric field size.The OAR curve within the 80％ of dosimetric field size was fitted using the gaussian function and the fitting parameters were adopted to describe the shape of OAR.Results The calculated field size error was less than 0.11 cm and the central axis position error was less than 0.05 cm.The fitting correlation coefficient was greater than O.998.The fitting maximum error of OAR curve did not significantly alter with the depth,whereas slightly increased over the increased field size.The maximum error for a field size of 10,20,30 and 40 cm was 0.49％,0.67％,1.25％ and 2.52％,respectively.Conclusions A method which can independently and accurately describe the OAR characteristics of FFF photon beam is established for the first time,which can calculate the field size of FFF beam and fit the OAR curve of FFF beam using the gaussian function.
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Objective To compare the dosimetric characteristics of the VMAT plans between the flattening-filter-free (FFF) and flattening filter (FF) modes for nasopharyngeal carcinoma and analyze the feasibility of VMAT plans in the FFF model applied in clinical practice.Methods Ten patients diagnosed with stage Ⅱ nasopharyngeal carcinoma between 2016 and 2017 were recruited in this investigation.For the CT image and clinical target volume (CTV) of the identical patient,FFF-and FF-mode VMAT plans were established with the identical prescription dose of 6 975 cGy and the identical modified parameters on the RayStation platform (6 MV X-ray).The dosimetric characteristics,the number of machine hops and the time of beam were recorded and statistically compared between the FFF and FF modes.Results The minimum dose D98％ target,target coverage,conformal index (CI) did not significantly differ between the FF and FFF plans (P=0.262-0.975),whereas the maximum dose of D2％,the average dose of D and target homogeneity index (HI) of the FF plans were slightly advantageous to those of the FFF plan (P=0.004-0.038).The acceptable dose of bilateral lenses,the volume of the left parotid gland with an acceptable dose of＞ 30 Gy and the average dose of the left temporal lobe in the FFF mode were declined compared with those in the FF mode (P=0.002-0.025).The volume of normal tissues (Body) within the dose range of 2-30 Gy in the FFF mode was less than that in the FF mode (P=0.000-0.003).In the FFF mode,the mean number of machine hops was 1.188 times (P=0.000) and the average time of beam was 91.2％ of the FF plan (P=0.024).Conclusions Both the FF and FFF plans qualify the dosimetric requirements in clinical settings.FF plan is advantageous in terms of the dose distribution in the target area,whereas FFF plan yields better OAR protection and low-dose irradiation.In spite of the increasing quantity of machine hops,the time of beam is declined in the FFF plan.Consequently,FFF mode probably contributes to enhance the clinical efficacy.
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Objective To quantitatively assess the plan quality of conventional fractionated volumetric modulated arc therapy ( VMAT ) plans for liver cancer treatments using flattening filter-free (FFF) and flattening filter (FF) modes based on plan quality metric (PQM). Methods A total of ten patients with liver tumor were selected. The patients were planned in FF and FFF mode of 6 MV X-ray respectively. The dose distribution of the planning target volume ( PTV) and the organ at risk ( OAR) were evaluated, and the monitor units ( MUs) and the beam on time ( BoT) were compared. According to the clinical requirements, a PQM with 16 submetrics was defined to evaluate the plan quality of the two modes. Results The maximal dose of FFF plan was less than that of FF plan (t =3.828, P <0.05). The normalized volume of 5 Gy ( V5 ) and mean dose ( Dmean ) of the normal liver of FFF plan were lower than those of the FF plan (t=2.716, 3.007, P<0.05). The average MU of FFF plan [(574 ± 130) MU] was higher than that of FF plan [(518 ± 81) MU](t= -2. 782,P<0. 05), while the average BoT of FFF plan [(108 ±36)s] was significantly lower than that of FF plan [(160 ±29) s](t=6.767,P<0.05). The score of FFF plan was higher than that of FF plan (t= -2.746, P<0.05). Conclusions FFF mode can better protect the OAR at low dose levels. The MU of FFF mode plan were higher, but the beam on time was significantly lower than FF mode plan. With reference to the PQM evaluation criteria, the overall plan quality of the FFF mode was slightly higher.
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Objective To evaluate the secondary cancer risk to various organs due to radiation treatment for early left-sided breast cancer using 6 MV X-ray flattening filter free (FFF) and flattening filter (FF) modes.Methods Two techniques,6 MV FFF and 6 MV FF X-rays modes,were used to develop the two tangential fields d-IMRT plans for 20 early left-sided breast cancer patients respectively.For all the patients,the dose to surrounding tissues was minimized as low as possible,the 95％ volume of the planning target volume (PTV) and clinical target volume (CTV) was consistent with the prescribed dose.The beam parameters and the plan optimization parameters of FFF modes were in consonance with FF modes.The radiation doses and volumes of the planning target volumes,organs at risk and normal tissue were detected by dose-volume histogram.And then,the excess absolute risk(EAR) of contralateral breast,ipsilateral lung,contralateral lung,thyroid and liver were calculated using the radiotherapy-induced secondary primary cancer risks models.Results The treatment plans of two groups met the requirements of clinical.The FFF d-IMRT techniques resulted in a systematic reduction of the organ equivalent dose (OED) (t =2.18-9.72,P ＜ 0.05),and the EAR (t =2.11-9.99,P ＜ 0.05) of contralateral breast,ipsilateral lung,contralateral lung,thyroid and liver was compared to the FF IMRT techniques,especially for the contralateral breast.Conclusions Both FFF and FF d-IMRT plans can achieve comparable target dose coverage in patients with early left-sided breast cancer,while the FFF d-IMRT techniques could reduce the excess absolute risk of contralateral breast,ipsilateral lung,contralateral lung,thyroid and liver.These results are of clinical importance,especially for the early-stage patients with an overall good prognosis.
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Objective To explore the Monte Carlo calculation methods for the absolute dose calibration and output factor of 6 MV flattening-filter ( FF) and flattening-filter free ( FFF) photon beams based on TrueBeam accelerator .Methods The BEAMnrc code was used to model the LINAC head of FF and FFF modes.The BEAM_up covers the components from the target to the monitor chamber , and BEAM_down includes the structures beneath the chamber , the dose deposit to the monitor chamber contributed by the incidence electrons and scattered particles from the secondary collimators were calculated respectively .The incidence electron-induced dose at certain depths on the central axis were simulated by means of the DOSXYZnrc code .By means of dose calibration equation , the calibration factor for the standard field (10 cm ×10 cm) and the output factors for various fields (1 cm ×1 cm-40 cm ×40 cm) were computed respectively .Results For the 6 MV FF and FFF beams under the standard 10 cm ×10 cm field, 1 MU equals to 7.747 ×1013 ±3.099 ×1011 and 3.248 ×1013 ±1.624 ×1011 electrons to the target , respectively , which deposited 21.53 and 35.01 cGy to the monitor chamber of the virtual accelerator respectively .The difference between the simulated and calculated output factors were 0.72%±1.4%and 0.56%±0.78%for FF and FFF , respectively .Conclusions The model generated and measured output factors agree well , indicating the good accuracy of the dose calculation by the model , which would provides basis for further clinical dosimetric studies .
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Objective To study the dosimetric characteristics of flattened and flattening filter free (FFF) beams with 6 MV and 10 MV photon energy using a novel dual-energy medical linear accelerator (Elekta Versa HDTM),to identify the dosimetric characteristics and advantages of FFF beams,and to provide a basis for their clinical application.Methods The percentage depth dose (PDD),profiles/dose rate of off-axis ratio (OAR),field size,penumbral width,dose out of the fields,collimator scatter factor (Sc),and total scatter factor (Sc,p) were compared between flattened and FFF beams.Results (1) After beam energy matching,the FFF beams had the same beam energy with the flattened beams.The matching error of PDD at a depth of 10 cm was less than 1％ between fields.(2) The FFF beams had a smaller variation in dose rate of OAR with the depth than the flattened beams.(3) The FFF beams had smaller variations in field size and penumbral width than the flattened beams.Moreover,the penumbral width of the FFF beams increased with the increasing field size or depth.The FFF beams had a lower dose out of the fields than the flattened beams.(4) The FFF beams had smaller variations in Sc and Sc,p with the field size and depth than the flattened beams.Conclusions Removal of the flattening filter can substantially improve the dose rate,shorten radiotherapy time,and reduce leakage and scattering of the head.The dosimetric advantages make the FFF beams appropriate for clinical treatment.
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INTRODUCTION: Linear accelerator (Linac) based stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SRT) using volumetric modulated arc therapy (VMAT) has been used for treating small intracranial lesions. Recent development in the Linacs such as inbuilt micro multileaf collimator (MLC) and flattening filter free (FFF) beam are intended to provide a better dose conformity and faster delivery when using VMAT technique. This study was aimed to compare the dosimetric outcomes and monitor units (MUs) of the stereotactic treatment plans for different commercially available MLC models and beam profiles. MATERIALS AND METHODS: Ten patients having 12 planning target volume (PTV)/gross target volume’s (GTVs) who received the SRS/SRT treatment in our clinic using Axesse Linac (considered reference arm gold standard) were considered for this study. The test arms comprised of plans using Elekta Agility with FFF, Elekta Agility with the plane beam, Elekta APEX, Varian Millennium 120, Varian Millennium 120HD, and Elekta Synergy in Monaco treatment planning system. Planning constraints and calculation grid spacing were not altered in the test plans. To objectively evaluate the efficacy of MLC‑beam model, the resultant dosimetric outcomes were subtracted from the reference arm parameters. RESULTS: V95%, V100%, V105%, D1%, maximum dose, and mean dose of PTV/GTV showed a maximum inter MLC ‑ beam model variation of 1.5% and 2% for PTV and GTV, respectively. Average PTV conformity index and heterogeneity index shows a variation in the range 0.56–0.63 and 1.08–1.11, respectively. Mean dose difference (excluding Axesse) for all organs varied between 1.1 cGy and 74.8 cGy (mean dose = 6.1 cGy standard deviation [SD] = 26.9 cGy) and 1.7 cGy–194.5 cGy (mean dose 16.1 cGy SD = 57.2 cGy) for single and multiple fraction, respectively. CONCLUSION: The dosimetry of VMAT‑based SRS/SRT treatment plan had minimal dependence on MLC and beam model variations. All tested MLC and beam model could fulfil the desired PTV coverage and organs at risk dose constraints. The only notable difference was the halving of the MU for FFF beam as compared to the plane beam. This has the potential to reduce the total patient on couch time by 15% (approximately 2 min).
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Objective To compare the dosimetric difference of stereotactic body radiation therapy (SBRT) plans for spine metastasis using 6 MV X-ray flattening filter free (FFF) and flattening filter (FF) modes.Methods A total of nine previously treated patients with a total of twelve spine metastasis were retrospectively included and replanned using FFF-SBRT and FF-SBRT, respectively.The dose-volume histograms (DVH), target conformity index (CI), monitor unites (MUs) and treatment time of target volume, organs at risk and normal tissues were compared between the plans.Results Both FFF-SBRT and FF-SBRT met the clinical objectives.Dose distribution of target volume, organs at risk and normal tissues were similar.MUs of the FFF-SBRT were significantly higher than that of FF-SBRT (t =-5.20, P ＜0.0l), while the treatment time was almost half of the latter (t =17.27, P ＜ 0.01).Conclusions Two plan modes are both clinically acceptable.FFF-SBRT plans are delivered in less time and with better efficiency.Trial registration Chinese clinical trial registry, ChiCTR-TRC-14004281.
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Objective To find the best model parameters through Monte Carlo simulation of 6 MV flattening-filter-free (FFF) beams in TrueBeam accelerator, and establish the foundation for the further study of the clinical dosimetry on 6 MV FFF X-rays.Methods Using the BEAMnrc and DOSXYZnrc codes, the percentage depth dose (PDD) and the off-axis ratio (OAR) curves of field ranges from 4 cm ×4 cm to 40 cm × 40 cm were simulated for 6 MV FFF X-ray by adjusting the incident beam energy, radial intensity distribution and angular spread, respectively.The simulation results and measured data were compared, where the optimal Monte Carlo model input parameters were acquired.Results The simulation was most comparable to the measurement when the incident electron energy, full width at half maximum (FWHM) and the spread angle were set as 6.1 MeV, 0.75 mm and 0.9°, respectively.The deviation of 1 mm (position)/1％ (local dose) could be met by the PDD of all tested field sizes and by the OAR when the fields sizes were no larger than 30 cm ×30 cm.The OAR of 40 cm ×40 cm field sizes fulfilled criteria of 1 mm (position)/1.5％ (local dose).Conclusions Monte Carlo simulation agrees well with the measurement and the proposed model parameters, which can be used for further clinical dosimetry studies of 6 MV FFF X-rays.
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Objective To compare the dosimetric differences of volumetric modulated arc therapy (VMAT) for preoperative radiotherapy of rectal cancer using 6MV X-ray flattening filter-free (FFF) and flattening filter (FF) modes.Methods FF-VMAT and FFF-VMAT plans were designed for 15 rectal cancer patients with preoperative radiotherapy by planning treatment system (Eclipse 10.0),respectively.Prescription dose of PTV was 50 Gy in 25 fractions.When the plans were normalized to 50 Gy to 95 ％ of PTV,the dose volume histogram (DVH),target and risk organ doses,conformity indexes (CI),homogeneity indexes (HI),low dose volume of normal tissue (B-P),monitor units (MU) and treatment time (TT) were compared between the two kinds of plans.Results FF-VMAT provided the lower Dmean,V105,HI (P ＜ 0.05),and higher CI (P ＜ 0.05) compared with FFF-VMAT.Small intestine (D5),Bladder (D5,Dmean V40,V50),L-femoral head (V40),R-femoral head (Dmean) were lower in FF-VMAT than those in FFF-VMAT (P ＜ 0.05).FF-VMAT had higher B-P (V5) compared with FFF-VMAT (P ＜ 0.05).FF-VMAT reduced the monitor units (MU) by 21％ (382±53 vs 483±26,P ＜ 0.05),as well as the treatment time (TT) in FF-VMAT was no significant difference compared with that in FFF-VMAT [(148±4) s vs (146±3) s,P ＞ 0.05)].Conclusions The qualities of FF-VMAT and FFF-VMAT plans were comparable and both clinically acceptable.When comparing two plans,FF-VMAT showed better target coverage and some of OARs sparing.The MUs of FFF-VMAT were higher than those of FF-VMAT,yet were not delivered within the same time.
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Objective To compare the dosimetric effects of systematic MLC leaf position errors on flattening filter (FF) and flattening filter-free (FFF) IMRT for nasopharyngeal carcinoma (NPC).Methods In totaly of ten patients with NPC were enrolled in the current study.FF IMRT and FFF IMRT plans were designed for each patient, and in-house software was developed to modify the corresponding MLC files.Briefly, three kinds of systematic MLC error presented with 0.5 and 1 mm magnitudes error were simulated.The modified MLC files were re-imported into IMRT plans for dose recalculation, and differences in the dosimetry trends between FF-IMRT and FFF-IMRT plans were analyzed using a dosevolume histogram.Results Upon closed MLC, the average changes in D95％ of PTV and D of parotid glands in FFF-IMRT plans were more sensitive than in FF-IMRT plans (t =3.298-4.793, P ＜ 0.05).Otherwise, when MLC are shifted in the same direction, changes in D95％of PTV, D of PGTV, PTV as well as D of left parotid gland in FFF-IMRT plans were also more sensitive (t =2.372-6.844, P ＜ 0.05), whereas average changes in D of right parotid gland presented with less sensitivity (t =-4.049,-3.378, P ＜ 0.05).Conclusions For out-of-field or large organs, FFF-IMRT plans are more sensitive to leaf position error than FF-IMRT plans.
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Objective To compare the dosimetric difference of volumetric modulated arc therapy (VMAT) plans for the loco-regionally advanced nasopharyngeal carcinoma (NPC) patients using 6 MV X-ray flattening filter-free (FFF) and flattening filter (FF) modes.Methods Ten previously treated patients with loco-regionally advanced nasopharyngeal carcinoma were retrospectively included and replanned using FFF and FF VMAT,respectively.Meeting all clinical criteria,the dose-volume histograms (DVH),dose distribution of target volume and organs at risk (OARs),target conformity index (CI),total monitor unites (MUs) and treatment time were compared across the plans.Results VMAT plans suing either FFF and FF modes can meet the clinical objectives.The maximum and mean target dose of FFF VMAT plans were significantly higher than that of FF VMAT plans (t =-0.31,-O.35,P ＜ O.05).Yet the planning target volume (PTV) CI of FF mode was slightly better than of FFF mode (t =5.42,P ＜0.05).The maximum doses of lenses in FFF VMAT plans were lower than that of FF VMAT plans (t =25.87,17.45,P ＜ 0.05),and other OARs displayed no significant difference.The mean total MUs of FFF and FF VMAT plans were (699 ± 16) and (628 ± 12) MUs respectively.The mean treatment time of two modes were consistent (about 2 min).Conclusions The plan qualities of FFF and FF VMAT plans were comparable and both clinically acceptable.The OARs at the edge of radiation field,such as lens,could be spared better using FFF VMAT mode.The total MUs of FFF VMAT plans were higher than FF VMAT plans,yet were delivered within the same time.