Flattening Filter Free VMAT Research Articles

Dosimetric evaluation of accelerated partial breast irradiation (APBI) using volumetric modulated arc therapy (VMAT) technique

Purpose: Five-fraction radiation therapy treatment regimens for accelerated partial breast irradiation (APBI) using intensity modulated radiotherapy (IMRT) have become increasingly used after several phase three trials demonstrated similar or reduced toxicity compared with whole breast irradiation. Improving the treatment efficiency of this technique could significantly improve delivery accuracy and tolerability, especially for treatment plans utilizing deep inspiration breath hold (DIBH) technique. The aim of this study was to determine the optimal technique for APBI, with a focus on volumetric modulated arc therapy technique (VMAT) and flattening-filter free (FFF) delivery. Methods: Ten APBI cases, five cases each left and right breast, were randomly selected. Each case was contoured following guidelines included in the APBI-IMRT-Florence Trial and planned to use both non-coplanar static-field IMRT with standard flattened-filter (FF) beam and VMAT techniques with both FF and FFF beam. For each VMAT plan, two partial arcs were used with patient-specific start and stop angles. Arc extents emulated opposed tangential fields and were chosen based on target and organ at risk (OAR) locations. Evaluated OARs included the heart, ipsilateral and contralateral lung, and normal breast. All plans were generated using the Eclipse treatment planning system (Version 15.6). A prescription dose of 30Gy delivered in 5 fractions was used for all cases. Plans were compared and evaluated using several dose metrics as well as treatment time. Results: For targets, VMAT exhibited similar coverage (V95%) but higher Dmax compared to IMRT (105% (IMRT) v.s .111% (VMAT), p=0.002. VMAT provided similar OAR avoidance compared to IMRT for the heart (Dmean, V3Gy, V0.5Gy), ipsilateral lung (V10Gy), contralateral lung (V5Gy) and contralateral breast Dmax within 10cGy/2% (p~0.005). VMAT contributed slightly lower dose outside the target in the ipsilateral breast, with an average difference in V15Gy of approximately 8% (37% (IMRT) v.s. 29% (VMAT)). Finally, because IMRT delivery is non-coplanar, and typically requires at least 5 treatment fields, co-planar VMAT reduced the total treatment significantly. VMAT with FF beam energies also used fewer monitor unites compared to IMRT with FF beam energies. The utilization of FFF beam energies for VMAT further improved delivery time compared to IMRT. Overall, VMAT with FFF beam energies reduced treatment time by approximately 20% compared to IMRT with FF beam. Conclusions: Co-planar partial-arc FFF VMAT technique produced equivalent target coverage, improved efficiency, better normal tissue sparing, and shorter delivery time compared to non-coplanar IMRT technique. Shorter delivery time also assists in the reduction of patient motion associated with breath hold. VMAT technique with FFF delivery is a suitable replacement for IMRT in APBI.

Flattening Filter-Free Volumetric-Modulated Arc Radiotherapy for Left-Sided Whole-Breast, Partial-Breast, and Postmastectomy Irradiations.

Purpose:Unflattened photon beams exhibit many benefits over traditional flattened beams for radiotherapy (RT), but comprehensive evaluations of dosimetric results and beam-on time using flattening filter-free (FFF) beams for all types of breast irradiations are still lacking. The purpose of this study was to investigate if FFF RT can maintain equal or better dose coverage than standard flattened-beam RT while reducing doses to organs at risk (OARs) and beam-on time for various types of breast cancer irradiations.Methods and Materials:FFF volumetric-modulated arc therapy (FFF-VMAT) and standard VMAT (STD-VMAT) treatment plans were created for 15 whole-breast irradiation (WBI) patients with 50 Gy/25 fractions, 13 partial-breast irradiation (PBI) patients with 38.5 Gy/10 fractions, and 9 postmastectomy irradiation (PMI) patients with 50 Gy/25 fractions. Planning target volume (PTV) coverage and dose to OARs were evaluated.Results:Both techniques produced clinically acceptable plans for all three types of irradiations. For WBI, FFF-VMAT plans exhibited similar PTV and OARs evaluation metrics as STD-VMAT. For PBI, FFF-VMAT plans showed significantly lower mean and maximum doses for ipsilateral and contralateral lungs, contralateral breast, and heart. For PMI, FFF-VMAT plans showed significantly lower mean dose and V5 for contralateral breast but significantly higher Dmean, Dmax, and V20 for ipsilateral lung and significantly higher Dmean, V22.5, and V30 for heart. FFF-VMAT techniques significantly reduced beam-on time than STD-VMAT for all cases.Conclusion:This work has shown that FFF beams are most beneficial for small-field irradiation such as PBI, and breast cancer patients could potentially benefit from the shortened beam-on time.

Hypofractionated Stereotactic Body Radiation Therapy With Simultaneous Integrated Boost With Flattening Filter-Free Volumetric Modulated Arc Therapy for Spinal Metastases

A novel approach could be the use of hypofractionated stereotactic body radiation therapy with simultaneous integrated boost (HSBRT-SIB) with flattening filter-free volumetric modulated arc therapy (FFF-VMAT) for spinal metastases. The aim of the present study is to report the clinical results of the treatment modality.The patients with spinal metastases (n ≤ 3) were treated in our institution with HSBRT-SIB using FFF-VMAT. The radiographically visible tumor was contoured as gross tumor volume (GTV). Clinical target volume (CTV) was defined according to international consensus guidelines. 25 Gy/5F to the planning target volume (PTV) and 35 Gy/5F to GTV were generated on the treatment planning system with FFF-VAMT. The primary endpoints were the tumor local control and pain control; the secondary ones were the overall survival and toxicity. Toxicity was assessed according to the Common Terminology Criteria for Adverse Events (CTCAE) v4.0.31 spinal metastases in 27 patients were treated with HSBRT-SIB from December 2017 to December 2019. There were 18 male and 9 female patients (median age, 59 years; range 24 to 79 years). The primary tumors were lung cancer (n = 13), hepatocellular carcinoma (n = 3), nasopharyngeal carcinoma (n = 2), colon cancer (n = 2), breast cancer (n = 1), prostate cancer (n = 1) and others (n = 5). 21 treatments included at least two vertebrae. 22 and 10 had paraspinal and epidural involvement, respectively. 7 had pathological fractures. With a median follow-up of 13 months (range 3-37 months), 1 lesion had a local failure, and 1 lesion had a relapse at 11 months. The local control rates at 1 year were 89.1%. The overall survival at 1 year were 55.6%. 85.1% of the patients had a complete pain remission and 14.9% had a partial pain remission. No adverse events ≥3 grade were observed.The use of HSBRT-SIB for spinal metastases is safe, and provides excellent local control and pain control.X. Liu: None. X. Li: None. P. Zhang: None. H. Yan: None. Y. Peng: None. N. Zou: None.

Comparison of Moderate Hypofractionated Volumetric-Modulated Arc Therapy Plans With and Without Flattening Filter for Localized Prostate Cancer.

Background/AimThe aim of this study was to compare volumetric-modulated arc therapy (VMAT) radiation plans between conventional VMAT with flattening filter (cFF-VMAT) and flattening filter-free VMAT (FFF-VMAT) for localized prostate cancer.Materials and methodsTen patients with localized prostate cancer who underwent cFF-VMAT at Yokosuka General Hospital Uwamachi, Yokosuka, Japan, from July 2020 to October 2020 were enrolled. Dose-volume histogram (DVH) parameters of the target volume, normal organs, monitor units (MU), and beam-on time (BOT) were compared between cFF-VMAT and FFF-VMAT plans.ResultsNo significant difference was observed for DVH parameters for the target volume. No significant difference was observed in all parameters for the bladder and rectum between the cFF-VMAT and FFF-VMAT groups. The mean values of MU were 686 ± 52 and 784 ± 80 in cFF-VMAT and FFF-VMAT, respectively (p < 0.001). The mean BOT was 97.0 ± 6.6 s and 72.9 ± 1.4 s for cFF-VMAT and FFF-VMAT, respectively (p < 0.001).ConclusionDVH parameters of the target volume and normal organs were not significantly different between the cFF-VMAT and FFF-VMAT plans. In FFF-VMAT, MU was significantly higher, and the BOT was significantly shorter than those in cFF-VMAT.

Is it advantageous to use deep inspiration breath hold (DIBH) over free breathing for FAST-Forward dose fractionation scheme in treating carcinoma of left-sided breast? A dosimetric study

Abstract Aim: To study the effect of deep inspiration breath hold (DIBH) on Fast-Forward trial for left-sided breast radiotherapy dosimetrically using tangential field-in-field (FiF), flattening filtered volumetric-modulated arc therapy (FF-VMAT) and flattening filter free volumetric-modulated arc therapy (FFF-VMAT) in comparison with free breathing (FB). Methods: Computed tomography images were acquired on 15 patients with carcinoma of left breast in FB and DIBH. Planning target volume (PTV) and organs at risk were contoured on both image sets. Dose of 26 Gy in five daily fractions was prescribed to PTV. FiF, FF-VMAT and FFF-VMAT plans were created in treatment planning system on both FB and DIBH. PTV V95%, V107%, D0·1 cc, CI and HI, heart V1·5 Gy, V7 Gy, lung left V8 Gy, monitor units (MU) and beam ON time were used for evaluation. Different technique analysis in same breathing condition and FB versus DIBH for same planning technique were performed. Results: Mean of all 15 patients was reported as mean ± 1 standard deviation. PTV V95% was 97·55 ± 0·10 (FiF), 95·75 ± 0·66 (FF-VMAT) and 96·15 ± 0·46 (FFF-VMAT) in FB while 97·34 ± 0·50 (FiF), 96·03 ± 0·71 (FF-VMAT) and 95·86 ± 0·63 (FFF-VMAT) in DIBH. Heart V7 Gy was 8·53 ± 4·26 (FiF), 8·86 ± 2·20 (FF-VMAT) and 9·27 ± 2·46 (FFF-VMAT) in FB while 6·30 ± 2·98 (FiF), 5·23 ± 2·20 (FF-VMAT) and 4·68 ± 2·01 (FFF-VMAT) in DIBH. p-value of heart V7 Gy between FB and DIBH was 0·278 (FiF), 0·009 (FF-VMAT) and 0·003 (FFF-VMAT). Beam ON time for FFF-VMAT was reduced by 65% (FF-VMAT) and 11% (FiF). Conclusion: Conformal dose to PTV was achieved better with VMAT plans. FFF-VMAT was delivered in less time compared to FF-VMAT and FiF for 26 Gy in five fractions. Heart dose can be significantly minimised with DIBH for VMAT plans.

Markerless Real-Time 3-Dimensional kV Tracking of Lung Tumors During Free Breathing Stereotactic Radiation Therapy

PurposeAccurate verification of tumor position during irradiation could reduce the probability of target miss. We investigated whether a commercial gantry-mounted 2-dimensional (2D) kilo-voltage (kV) imaging system could be used for real-time 3D tumor tracking during volumetric modulated arc therapy (VMAT) lung stereotactic body radiation therapy (SBRT). Markerless tumor tracking on kV fluoroscopic images was validated using a life-like moving thorax phantom and subsequently performed on kV images continuously acquired before and during free-breathing VMAT lung SBRT. Methods and MaterialsThe 3D-printed/molded phantom containing 3 lung tumors was moved in 3D in TrueBeam developer mode, using simulated regular/irregular breathing patterns. Planar kV images were acquired at 7 frames/s during 11 Gy/fraction 10 MV flattening filter free VMAT. 2D reference templates were created for each gantry angle using the planning 4D computed tomography inspiration phase. kV images and templates were matched using normalized cross correlation to determine 2D tumor position, and triangulation of 2D matched projections determined the third dimension. 3D target tracking performed on cone beam computed tomography projection data from 18 patients (20 tumors) and real-time online tracking data from 2 of the 18 patients who underwent free-breathing VMAT lung SBRT are presented. ResultsFor target 1 and 2 of the phantom (upper lung and middle/medial lung, mean density –130 Hounsfield units), 3D results within 2 mm of the known position were present in 92% and 96% of the kV projections, respectively. For target 3 (inferior lung, mean density –478 Hounsfield units) this dropped to 80%. Benchmarking against the respiratory signal, 13/20 (65%) tumors (10.5 ± 11.1 cm3) were considered successfully tracked on the cone beam computed tomography data. Tracking was less successful (≤50% of the time) in 7/20 (1.2 ± 1.5 cm3). Successful online tracking during lung SBRT was demonstrated. Conclusions3D markerless tumor tracking on a standard linear accelerator using template matching and triangulation of free-breathing kV fluoroscopic images was possible in 65% of small lung tumors. The smallest tumors were most challenging.

Single Fraction (30 Gy) Stereotactic Body Radiation Treatment of Peripheral Lung Lesions using Flattening Filter Free Volumetric Modulated Arc Therapy (FFF-VMAT): Early Clinical Outcomes

To present early clinical outcomes utilizing stereotactic body radiation therapy (SBRT) to mid lung or peripheral lung lesions, delivering a dose of 30 Gy in a single fraction with flattening filter free volumetric modulated arc therapy (FFF-VMAT). Twenty-four lesions were identified from twenty-three patients with either early stage non-small cell lung cancer (lesions n = 17) or isolated, limited, thoracic progression from a known metastatic cancer diagnosis (lesions n = 2 head and neck, n = 2 rectal, n = 3 lung). Patients were prescribed a single dose of 30 Gy, delivered using FFF VMAT with noncoplanar 6MV beams and cone beam CT image guidance. Four-dimensional (4D) CT based treatment plans were generated using an advanced Acuros-based dose calculation engine for heterogeneity corrections. Plan quality and delivery efficiency were reported. Outcomes evaluated include local control, radiation-induced pneumonitis, and rib toxicity. Median follow-up time was 9 months (range: 1 to 38 months). Mean planning target volume (PTV) was 17.6 ± 13.9 cc. All plans met RTOG requirements for target coverage and dose to organs at risk (OAR). Average values of the conformity index, gradient index, maximum dose at 2 cm away from the PTV, normal lung V20Gy, and normal lung V10Gy were 1.06 ± 0.1, 5.4 ± 1.4, 49.5 ± 4.5%, 0.8 ± 0.5% and 3.2 ± 1.9 %, respectively. All OAR doses were well below RTOG compliance criteria. Average beam on time was 6.6 ± 1.3 min. Four patients were lost to follow up. One patient has initial post-treatment imaging pending. During the follow up period, all patients demonstrated local control of the treated lesion. Six patients developed new distant metastases, 4 of whom had prior stage 4 disease. All patients developed asymptomatic radiographic changes surrounding the treated lesion, though there were no reports of grade 2+ pneumonitis or chest wall pain and no radiographic evidence of rib fracture. Stereotactic delivery of 30 Gy in a single fraction using cone-beam CT image guidance with a non-coplanar FFF-VMAT plan is a fast, safe, and effective treatment modality for managing small mid-lung or peripherally located lung lesions. Local control is excellent and no symptomatic adverse events were noted over the follow up period. This is a promising strategy for rapidly delivering a meaningful oncologic dose to patients unable to tolerate a multi-fraction regimen due to discomfort, medical comorbidities, or socio-economic barriers hindering treatment compliance. Longer clinical follow up in a larger patient cohort is ongoing to further validate these outcomes.

Standardized flattening filter free volumetric modulated arc therapy plans based on anteroposterior width for total body irradiation.

In this work, the feasibility of using flattening filter free (FFF) beams in volumetric modulated arc therapy (VMAT) total body irradiation (TBI) treatment planning to decrease protracted beam‐on times for these treatments was investigated. In addition, a methodology was developed to generate standardized VMAT TBI treatment plans based on patient physical dimensions to eliminate plan optimization time. A planning study cohort of 47 TBI patients previously treated with optimized VMAT ARC 6 MV beams was retrospectively examined. These patients were sorted into six categories depending on height and anteroposterior (AP) width at the umbilicus. Using Varian Eclipse, clinical 40 cm × 10 cm open field arcs were substituted with 6 MV FFF. Mid‐plane lateral dose profiles in conjunction with relative arc output factors (RAOF) yielded how far a given multileaf collimator (MLC) leaf must move in order to achieve a mid‐plane 100% isodose for a specific control point. Linear interpolation gave the dynamic MLC aperture for the entire arc for each patient AP width category, which was subsequently applied through Python scripting. All FFF VMAT TBI plans were then evaluated by two radiation oncologists and deemed clinically acceptable. The FFF and clinical VMAT TBI plans had similar Body–5 mm D98% distributions, but overall the FFF plans had statistically significantly increased or broader Body–5 mm D2% and mean lung dose distributions. These differences are not considered clinically significant. Median beam‐on times for the FFF and clinical VMAT TBI plans were 11.07 and 18.06 min, respectively, and planning time for the FFF VMAT TBI plans was reduced by 34.1 min. In conclusion, use of FFF beams in VMAT TBI treatment planning resulted in dose homogeneity similar to our current VMAT TBI technique. Clinical dosimetric criteria were achieved for a majority of patients while planning and calculated beam‐on times were reduced, offering the possibility of improved patient experience.

Flattening filter free VMAT for a stereotactic, single-dose of 30 Gy to lung lesion in a 15-min treatment slot.

Cone‐beam CT‐guided single dose of lung stereotactic body radiotherapy (SBRT) treatment with a flattening filter free (FFF) beam and volumetric modulated arc therapy (VMAT) is a safe and highly effective treatment modality for selective small lung lesions. Four‐dimensional (4D) CT‐based treatment plans were generated using advanced AcurosXB algorithm for heterogeneity corrections. 6X‐FFF beam produced highly conformal radiosurgical dose distribution to the target and reduced lung SBRT fraction duration to less than 10 min for a single dose of 30 Gy, significantly improving patient comfort and clinic workflow. Early follow‐up CT imaging results (mean, 8 months) show high local control rates (100%) with no acute lung or rib toxicity. Longer clinical follow‐up in a larger patient cohort managed in this fashion is underway to further validate this treatment approach.

Interaction between CIEDs and modern radiotherapy techniques: Flattening filter free-VMAT, dose-rate effects, scatter radiation, and neutron-generating energies

Background and purposeProviding evidence for radiotherapy (RT)-induced effects on cardiac implantable electric devices (CIEDs) with focus on flattening filter free-volumetric modulated arc therapy (FFF-VMAT) at 6 and 10 MV as well as 3D-conformal radiotherapy (3D-CRT) at 18 MV. Materials and methods68 CIEDs (64 implantable cardioverter-defibrillators (ICDs) and 4 cardiac pacemakers (PMs)) were located on the left chest position on a slab phantom and irradiated under telemetrical surveillance either directly, or distant to 3D-CRT or FFF-VMAT, dose-rate 2500 cGy/min, and target dose of 150 Gy. Devices were placed within, close by (2.5 cm and 5 cm), and distant (35 cm) to the radiation field. Scatter radiation (SR) and photon neutrons (PN) were recorded. CIEDs were investigated in following groups: 1a) 18 MV 3D-CRT − 4 ICDs/4 PMs out of radiation field, 1b) 18 MV open field − 4 ICDs/4 PMs within radiation field, 2) 6 MV FFF-VMAT, 15 ICDs in 35 cm distance to VMAT, 3) 10 MV-FFF VMAT, 15 ICDs in 35 cm distance to VMAT, 4) 6 MV FFF-VMAT, 15 ICDs in 2.5 cm distance to VMAT, 5) 10 MV FFF-VMAT, 15 ICDs in 2.5 cm distance to VMAT. ResultsNo incidents occurred at 6 MV FFF. 10 MV FFF-VMAT and 18 MV 3D-CRT resulted in data loss, reset, and erroneous sensing with inhibition of pacing (leading to inadequate defibrillation) in 8/34 ICDs and 2/4 PMs which were not located within radiation. Direct radiation triggered instantaneous defibrillation in 3/4 ICDs. Conclusions6 MV FFF-VMAT is safe even at high dose-rates of 2500 cGy/min, regardless whether CIEDs are located close (2.5 cm) or distant (35 cm) to the radiation beam. CIEDs should never be placed within radiation and energy should always be limited to 6 MV. At 6 MV, VMAT at high dose-rates can be used to treat tumors, which are located close to CIEDs.

Case Report: Anaplastic Astrocytoma Treated with Postoperative Radiotherapy Using Flattening Filter-Free Volumetric Arc Therapy (FFF-VMAT) during Pregnancy—Acceptable Fetal Dosimetry

Background: Postoperative irradiation for brain tumor in pregnant women is a matter of concern. Aim: We aimed to assess the safety of radiotherapy for brain tumors in pregnancy. We here report a successful treatment for anaplastic astrocytoma during pregnancy: surgery + postoperative irradiation. We wish to emphasize how we devised irradiation procedure to achieve both therapeutic effectiveness and safety to the fetus/infant. Case Presentation: A 34-year-old pregnant woman suffered with brain anaplastic astrocytoma. Tumor resection under craniotomy was performed with success. We decided to conduct postoperative radiotherapy at 25 weeks of gestation to reduce the risk of recurrence. We used a flattening filter-free volumetric arc therapy (FFF-VMAT) technique, which can achieve lower out-of-field dose than VMAT with a flattening filter or helical tomotherapy. We prescribed 60 Gy over 30 fractions. During actual beam delivery, surface and rectal dose to the patient (mother) were measured. The total fetal dose was estimated at 0.006 - 0.018 Gy, which is under the threshold set by the ICRP. A male healthy infant was born vaginally at the 37th week of pregnancy. The patient (mother) and the infant are healthy at the time of writing. Conclusion: FFF-VMAT is a good choice for brain tumors during pregnancy.

Breathing-motion induced interplay effects for stereotactic body radiotherapy of liver tumours using flattening-filter free volumetric modulated arc therapy

The purpose of this study was to investigate breathing-motion induced interplay effects for stereotactic body radiotherapy (SBRT) of liver tumours treated with flattening-filter free (FFF) volumetric modulated arc therapy (VMAT).Ten patients previously treated with liver SBRT were included in this study. All patients had four-dimensional computed tomography (4DCT) scans acquired prior to treatment. The 4DCT was sorted into 8–10 phases covering an equal time interval. A FFF VMAT plan was created for one fraction in the mid-ventilation phase for each patient. To generate dose distributions including both interplay effects and dose blurring, a sub-plan was calculated for each phase. The total dose distributions were accumulated to the mid-ventilation phase using the deformed vector fields (DVF) from deformable image registration between the corresponding CT and the mid-ventilation phase CT. A blurred dose distribution, not including interplay effects, was also obtained by distributing the delivery of the whole plan uniformly on all phases, and was similarly accumulated to the mid-ventilation phase. To isolate interplay effects, this blurred dose distribution was subtracted from the total dose distribution with interplay effects. The near minimum dose (D98%), mean dose (Dmean), heterogeneity index (HI), and the near minimum dose difference (ΔD98%) between the accumulated dose distributions with and without interplay effects were calculated within the gross tumour volume (GTV) for each patient.Comparing the accumulated dose distributions with and without interplay effects, the D98% decreased for nine of the ten patients and the HI increased for all patients. The median and minimum differences in D98% were −2.1% and −5.0% (p = 0.006), respectively, and the median HI significantly increased from 6.2% to 12.2% (p = 0.002). The median ΔD98% was −4.0% (range −7% to −1.5%).In conclusion, statistically significant breathing-induced interplay effects were observed for a single fraction of FFF VMAT liver SBRT, resulting in heterogeneous dose distributions within the GTV.

Comparison of Flattening Filter and Flattening Filter-Free Volumetric Modulated Arc Radiotherapy in Patients with Locally Advanced Nasopharyngeal Carcinoma.

BackgroundThis study aimed to investigate the therapeutic role of flattening filter-free (FFF) mode in volumetric modulated arc therapy (VMAT) compared with flattening filter (FF) mode in patients with locally advanced nasopharyngeal carcinoma (NPC).Material/MethodsTen previously treated patients with NPC underwent treatment re-planning with FFF and FF VMAT. Radiotherapy dose distribution on planning target volume (PTV), organs at risk (OAR), target conformity index (CI), total monitor units (MUs), and therapeutic time were compared.ResultsMaximum and mean radiotherapy dose in PTV and PGTV (primary lesions of NPC and cervical lymph node metastases) in FFF VMAT planning were significantly increased compared with FF VMAT planning, but PTV and OAR showed no significant differences. The CI value of PTV in FFF VMAT planning was significantly reduced compared with FF planning (P<0.05). No differences were found for the maximum radiotherapy dose in the spinal cord and left and right optic nerve, and the mean radiotherapy dose in the brainstem, left and right parotid gland (P>0.05). The maximum dose in the brainstem in the FFF planning was significantly higher compared with FF planning (P>0.05). The maximum radiotherapy dose in left and right crystalline lens (P<0.05) in FFF planning was significantly reduced compared with FF planning. The total hop count in FFF planning was significantly increased compared with FF planning (P<0.05).ConclusionsBoth 6 MV X-ray FFF mode and FF mode in the treatment of patients with NPC showed that FFF VMAT planning provided improved protection for OAR.

P140] Flattening filter-free radiotherapy for paediatric brain cancer: Impact on the dose to selected out-of-field organs

Purpose Removal of the flattening filter has been shown to alter the out-of-field dose during radiotherapy. In this study, the dose to selected out-of-field organs is compared for conventional flattened and flattening filter free (FFF) volumetric-modulated arc therapy (VMAT) of paediatric brain cancer. Methods Out-of-field doses to the thyroid, breast and testes were measured using thermoluminescence dosimeters inserted in two anthropomorphic phantoms resembling a 1-year and 5-year old child. Coplanar VMAT plans were prepared for 6 MV and 6 MV FFF photon beams, for two different PTV sizes (2 cm and 5 cm diameter), simulating post-operative radiotherapy of an ependymoma in the posterior fossa. The phantoms were subject to three VMAT fractions of 1,8 Gy each, delivered using a Varian TrueBeam linear accelerator. Some additional irradiations were performed for the 1-year phantom with the head of the phantom removed, to investigate the contribution from internal scatter to the total out-of-field dose. Results For the testes, measured doses showed a statistically significant decrease (Student’s t-test, significance level p 0,05) of 21–42% for FFF compared with conventional plans, for both phantoms as well as both PTV sizes. Doses to the breast showed a significant decrease of approximately 12% for the 5-year phantom for the small PTV, but other breast doses did not show a significant difference with and without the flattening filter. Doses to the thyroid showed a significant increase of approximately 18% for the 1-year phantom for the small PTV, with other dose differences being insignificant. However, doses to all three organs were significantly reduced (19–37%) when removing the head of the 1-year phantom, minimizing the contribution from internal scatter. Conclusions The results show that FFF VMAT of paediatric brain cancer leads to reduced doses to organs far from the field compared to VMAT with conventional flattened beams, while doses to organs close to the field seem to increase. The latter finding can be explained by an increase in internal scatter, which is possibly caused by the softer energy spectrum of the 6 MV FFF beam for the investigated linear accelerator model compared to conventional 6 MV.

Dosimetric validation study of a flattening filter free SABR treatment by use of a normoxic PAG gel dosimeter with MRI readout

This paper describes clinical studies of the normoxic PAG gel dosimeter with MRI readout for validation of complex flattening filter free (FFF) treatment delivery techniques. Recently our department implemented the Elekta FFF technology for stereotactic ablative radiotherapy (SABR) treatments using dynamic conformal arc (DCAT) and volumetric arc therapy (VMAT). In this study 6MV FFF DCAT and VMAT plans were analysed using 3D gel dosimetry techniques and compared against standard clinical dosimetry QA tools. Some challenges with this validation study included high dose per fraction for SABR, MRI access time and R2-dose calibration errors. A study of the dependency of the dose response on dose rate revealed insignificant dose rate dependence of the normoxic PAG dosimeter up to 20 Gy/min for both photon energies (6 MV and 10 MV).

Stereotactic ablative radiation therapy for brain metastases with volumetric modulated arc therapy and flattening filter free delivery: feasibility and early clinical results.

For selected patients with brain metastases (BMs), the role of stereotactic radiosurgery (SRS) or fractionated stereotactic radiotherapy (SFRT) is well recognized. The recent introduction of flattening filter free (FFF) delivery during linac-based SRS or SFRT allows shorter beam-on-time, improving patients' comfort and facility workflow. Nevertheless, limited experiences evaluated the impact of FFF linac-based SRS and SFRT in BMs treatment. Aim of the current study was to analyze SRS/SFRT linac-based FFF delivery for BMs in terms of dosimetric and early clinical results. Patients with life expectancy >3months, number of BMs <5, diameter <3cm, and controlled or synchronous primary tumor received SRS/SFRT. The prescribed total dose and fractionation, based on BMs size and proximity to organs at risk, ranged from 15Gy in 1 fraction to 30Gy in 5 fractions. A FFF volumetric modulated arc therapy (VMAT) plan was generated with one or two coplanar partial arcs. Toxicity was assessed according to CTCAE v4.0. From April 2014 to February 2016, 45 patients (89 BMs) were treated with SRS/SFRT linac-based FFF delivery. The mean beam-on-time was 140s for each lesion (range 90-290s) and the average brain Dmean was 1Gy (range 0.1-4.8Gy). At the time of analysis, local control was reported in 93.2% (83/89BMs). With a median follow-up time of 12months (range 1-27months), the median overall survival was 14months and the 6-month overall survival was 77%. Finally, the median intracranial disease control was 11months. Acute and late toxicities were acceptable without severe events (no adverse events ≥G2 were recorded). These preliminary results highlighted the feasibility and safety of linac-based SRS/SFRT with FFF mode for BMs patients. A longer follow-up is necessary to confirm the efficacy of this treatment modality in BM patients.

Optimal Patient Selection for Stereotactic Body Radiation Therapy: A Practical Approach in a High-Volume Community-Based Private Setting

Establishment of an objective and documentable selection process of patients for free-breathing stereotactic body radiation therapy (SBRT) in a high-volume community-based private setting. A vendor-based respiratory gating system and wide-bore multi-slice CT simulation acquire 4-dimensional image data sets throughout full respiratory cycles. SBRT is delivered on modern linear accelerators including SBRT-dedicated systems, with respiratory gating control and multiple volumetric modulated arc therapy (VMAT) techniques. Flattening Filter Free (FFF) mode of RT delivery is available as needed. Use of cone-beam CT image guidance and 6 degrees of freedom patient repositioning are also available to optimize target alignment. Patients were observed closely at time of CT simulation to select those with optimal and stable respiratory patterns with or without the need for supplemental oxygen. Selection of appropriate patients for SBRT is needed to assure optimal CT simulation image capture, prediction for subsequent treatment deliverability, and reproducibility of high dose SBRT. Typical SBRT is up to 10 Gy per fraction for 5 fractions. Through real-time evaluation of respiratory patterns, appropriate patients can be selected for SBRT. Compliance rate of selected patients during simulation and subsequent SBRT is 100% without significant interruption during SBRT delivery. Use of respiratory gating control effectively prevents out-of-range SBRT delivery during fits of coughing and dyspnea. Use of FFF mode and VMAT via multiple arcs significantly reduces SBRT delivery time and optimizes normal tissue protection. The selection ”process“ is also recorded via screen capture of the respiratory cycles with a corresponding graphical representation of steady breathing patterns. These graphics can then be inserted into the CT simulation documentation as part of the permanent electronic health record. Appropriate selection of patients for SBRT via objective analysis and recorded respiratory patterns ensures optimal conditions for clinical setup, tumor targeting, and reproducible delivery of SBRT. This process is also recorded in the electronic health records of each patient for permanent documentation, future reference, and chart review.

SU-F-T-338: Flattening Filter Free Photon Beams Can Achieve the Same Plan Quality as Conventional Flattened Beams for Prostate Radiotherapy

Purpose: Some modern linear accelerators are equipped with one low energy flat beam and two flattening filter free (FFF) beams at high and low energies. The purpose of this study is to investigate whether the high energy FFF beam can produce the same plan quality as the conventional low energy flat beam, using a volumetric modulated arc (VMAT) technique for prostate patients. Methods: Ten prostate cancer patients were selected with a prescription of 78Gy. For each patient, three plans were created: (a) double arc flat 6MV plan used clinically; (b) double arc 10MV FFF plan; (c) single arc 10MV FFF plan. Each plan was prescribed so that at least 95% of the PTV received the prescription dose. The following dosimetric endpoints were evaluated: volume receiving 78Gy (V78) of the CTV and PTV, PTV conformality index (CI, ratio of prescription isodose volume to the PTV volume), bladder volume receiving 70Gy (V70) and 60Gy (V60), rectum volume receiving 70Gy (V70) and 50Gy (V50), dose to 10cc of the rectum, and volume of both femoral heads receiving 50Gy (V50). Total monitor units for each plan were recorded. Results: No significant difference was found for all dosimetric endpoints between all plans (p>0.05). Compared to the 6MV plans, monitor units were higher with the double arc 10MV FFF plans and lower with the single arc 10MV FFF plans, 29% and 4% respectively. Conclusion: Both single arc and double arc 10MV FFF VMAT can achieve equivalent plan quality as 6MV flat beam double arc treatment plans. With the gantry speed restriction, a high dose rate of 2400MU/min may allow the optimizer to use more MUs than actually needed. Single arc 10MV FFF VMAT plans are a reasonable alternative to double arc 6MV flat beam VMAT plans.

PO-0855: Flattening Filter Free VMAT for extreme hypofractionation of prostate cancer

PO-0855: Flattening Filter Free VMAT for extreme hypofractionation of prostate cancer

EP-1763: Experimental analysis of interplay effects in flattening filter free VMAT treatment techniques

EP-1763: Experimental analysis of interplay effects in flattening filter free VMAT treatment techniques