MV Flattening Filter Free Beams Research Articles

The effect of beam interruption during FFF-VMAT plans for SBRT.

To investigate the dosimetric effect of intended beam interruption during volumetric modulated arc therapy (VMAT) with flattening filter free (FFF) beam for exploring the possibility of deep inspiration breath hold stereotactic body radiation therapy (SBRT). A total of ten SBRT plans with 6 and 10 MV FFF beams were retrospectively selected. All plans consisted of four partial arcs, except one plan with six partial arcs. We delivered the plans using a Varian Truebeam™ with three different scenarios; without interruption (0int), with one intentional interruption (1int), or with two intentional interruptions (2int), per each partial arc. The treatment log files were exported from the treatment console, and the variations in delivered MU were evaluated at the beam interruption angles. The dose distributions were also measured using a 3D cylindrical diode array detector, ArcCHECK™. The 2D global gamma evaluations were performed, compared to the planned dose distribution, with 3%/3 and 4%/2mm passing criterion. The dose difference (DD) was also determined between uninterrupted and interrupted data with 3, 2, 1, and 0.5% of global maximum dose. The interruption caused a total increase of 0.14 ± 0.05% and 0.25 ± 0.08% of the total planned MU, ranging from 1746 to 3261 MU, at the interrupted angles in 1int and 2int, respectively. All global gamma passing rates satisfied our clinical threshold of 90%, and the differences of passing rates were less than 0.3% on average with both criterions. All measured 1int and 2int data were within 3% DD from 0int measured data. For 6 MV FFF beams, the average passing rate with 2, 1, and 0.5% DD were 99.9 ± 0.2%, 92.3 ± 12.0%, and 81.9 ± 24.9%, respectively, between 0int and 1int, and 99.8 ± 0.4%, 92.1%12.4%, and 80.7 ± 26.5%, respectively, between 0int and 2int. For 10 MV FFF beams, the average passing rate with 2, 1, and 0.5% DD were 100.0 ± 0.2%, 95.4 ± 9.4% and 87.0 ± 19.8%, respectively, between 0int and 1int, and 99.9 ± 0.3%, 95.4 ± 9.7%, and 87.2 ± 21.3% between 0int and 2int. The dosimetric impact of beam interruption was investigated with small field and high dose rate FFF-VMAT SBRT plans. The delivered dose distributions with up to 12 interruptions per plan were still clinically acceptable. Only minimal changes were observed in Gamma, DD, and log file analysis.

Feasibility of Stereotactic Body Radiation Therapy for Non–small Cell Lung Cancer Using a Linear Accelerator-Based Magnetic Resonance Image Guided Radiation Therapy System

To describe a linear accelerator (linac) based magnetic resonance image guided radiation therapy system (MR-IGRT) and demonstrate the feasibility of delivering stereotactic body radiation therapy (SBRT) for non-small cell lung cancer (NSCLC) treatment. The linac-based MR-IGRT (MRL) system currently being installed at our institution integrates a 0.35T MRI with a flattening-filter free (FFF) linac delivering 6MV at 600 cGy/min. A novel double-focused and double-stack multileaf collimator (MLC) with an effective leaf width of 4 mm provides modulation for step-and-shoot IMRT delivery. The ability of a similar Cobalt-based system for real time, real anatomy gating has already been demonstrated, and a number of patients with NSCLC were treated. Here, we investigate the feasibility of planning and delivery with the linac-based system for SBRT, primarily focusing on plan quality as compared to conventional linacs (CL) and delivery times. Four MRL plans were generated for NSCLC patients who were previously planned and treated on a CL using 6MV FFF beams with a total dose of 54 Gy in 3 fractions. MRL plans were compared to clinical plans (CP) by evaluating conformity numbers with 100% (CN100), 95% (CN95) of prescription dose (Rx), heterogeneity indices (HI), maximum dose and dose-volume histograms of organs-at-risks (OARs), and delivery times. All MRL plans achieved PTV coverage and OAR sparing within clinical constraints and were comparable to clinically-treated plans as indicated in Table 1. Three plans showed an average of 8% and 12% improvement on PTV conformity CN100 and CN95, respectively. There is no clear trend for treatment time comparison but in general they only differ by 3 mins for a 10-min long treatment. The MRL is capable of generating clinically acceptable SBRT plans targeting NSCLC. With real-time MR gating, novel double-stack MLC design and fast FFF delivery, the system is able to achieve a more conformal and precise dose to the target therefore having great potential for further lung tumor dose escalation and reduction of treated volumes. We expect that a higher dose rate version will be available soon to further reduce the treatment time.Abstract 3536IDPlanTumor LocationVPTV Rx (%)VPTV 95%Rx (%)HICN100CN95Max Cord (Gy)Max Esophagus (Gy)Max Heart (Gy)Max Carina (Gy)Mean Lung-ITV (Gy)V20 Lung-ITV (%)Treatment time (min)1MRLLeft upper lobe97.999.61.160.920.844.4910.5521.828.843.193.110.2CP97.399.91.230.860.785.779.6818.519.223.023.07.22MRLLeft lower lobe99.11001.160.800.641.268.3310.970.452.412.45.7CP98.71001.150.840.690.456.2210.010.281.872.27.83MRLRight upper lobe99.299.91.160.740.665.138.670.730.781.210.711.8CP99.81001.170.720.581.418.160.310.421.081.09.04MRLLeft lower lobe97.299.71.260.890.8313.8714.0712.8912.676.216.26.9CP96.6991.200.780.709.5218.0115.6816.576.319.310.0 Open table in a new tab

Equivalent (uniform) square field sizes of flattening filter free photon beams

Various types of treatment units, such as CyberKnife, TomoTherapy and C-arm linear accelerators (LINACs) are operated using flattening filter free (FFF) photon beams. Their reference dosimetry, however, is currently based on codes of practice that provide data which were primarily developed and tested for high-energy photon beams with flattening filter (WFF). The aim of this work was to introduce equivalent uniform square field sizes of FFF beams to serve as a basis of a unified reference dosimetry procedure applicable to all aforementioned FFF machines. For this purpose, in-house determined experimental data together with published data of the ratio of doses at depths of 20 cm and 10 cm in water (D20,10) were used to characterize the depth dose distribution of 6 and 10 MV WFF and FFF beams. These data were analyzed for field sizes ranging from 2 × 2 cm2 to 40 × 40 cm2. A scatter function that takes the lateral profiles of the individual beams into account was fitted to the experimental data. The lateral profiles of the WFF beams were assumed to be uniform, while those of the FFF beams were approximated using fourth or sixth order polynomials. The scatter functions of the FFF beams were recalculated using a uniform lateral profile (the same as the physical profile of the WFF beams), and are henceforth denoted as virtual uniform FFF beams (VUFFF). The field sizes of the VUFFF beams having the same scatter contribution as the corresponding FFF beams at a given field size were defined as the equivalent uniform square field (EQUSF) size. Data from four different LINACs with 18 different beams in total, as well as a CyberKnife beam, were analyzed. The average values of EQUSFs over all investigated LINACs of the conventional 10 × 10 cm2 reference fields of 6 MV and 10 MV FFF beams for C-arm LINACs and machine-specific reference fields for CyberKnife and TomoTherapy were 9.5 cm, 9 cm, 5.0 cm and 6.5 cm respectively. The standard deviation of the mean of these EQUSFs was below 0.1 cm. It has been shown that with the introduction of a VUFFF beam, EQUSFs can be consistently defined for a variety of energies and collimations. These EQUSFs can form the basis for a unified reference dosimetry protocol for all different types of FFF machines.

Preclinical studies for implementing flattening filter-free beams in hypofractionated volumetric-modulated arc therapy (VMAT) for prostate cancer radiotherapy

AbstractBackgroundVolumetric-modulated arc therapy (VMAT) has emerged as one of the most favourable techniques for radiotherapy treatment in recent years because of its conformal dose distribution to the planning target volume (PTV), lower doses to adjacent normal organs at risk (OARs) and faster and easier dose delivery. A typical conventional VMAT protocol for low-intermediate risk prostate cancer uses a flattened 6 MV photon beam to deliver 78 Gy in 39 fractions, however, a recent Radiation Therapy Oncology Group study investigated prostate cancer radiotherapy with a hypofractionated dose scheme of 36·25 Gy in 5 fractions. One advantage of flattening filter-free (FFF) beams in radiotherapy is the higher doses in the central region on the dose profile and much higher dose delivery rates.Methods and materialsThis paper reports the investigation of preclinical studies for implementing FFF beams in hypofractionated VMAT for prostate cancer radiotherapy. All treatment planning were accomplished using Varian EclipseTM treatment planning system version 11 and delivered on Varian Truebeam linear accelerators. The studies compared the biological-effective dose–volume histograms and dose–volume histograms of PTV and OARs for 20 patients using conventional and hypofractionated dose schemes. The study also evaluated the 6 and 10 MV FFF by comparing 6 and 10 MV VMAT plans with the FFF beams. The treatment time was investigated using plans with 6 MV beams and doses of 2, 4, 5, 6, 7·25 Gy/fraction and plans with 10 MV FFF with a dose of 7·25 Gy/fraction. We also investigated an angular monitor unit (MU) quantity (MU/deg) and its threshold value for RapidArcTM plans, beyond which FFF beams can be considered superior to flattened beams in terms of treatment time increased caused by higher dose per fraction.ResultsThe results show that the hypofractionated plans resulted in greater biological equivalent doses to PTV and lower doses to OARs. The 10 MV FFF plans have statistically lower mean doses to all the OARs, whereas PTV homogeneity index remains the same compared with other beam energies. The mean body integral dose for the 20 patients is 8·7% lower using 10 MV FFF compared with 6 MV FFF mainly because of the higher energy and less required MUs with the 10 MV FFF beam. The hypofractionated scheme with 10 MV FFF plan has the same treatment time as that of the 6 MV plan at 2 Gy/fraction, as the higher dose delivery rates at 10 MV FFF can compensate for the higher prescribed dose per fraction without the need of extra treatment time.ConclusionIn this study, we observed that the 10 MV FFF beam is better for hypofractionated prostate cancer VMAT plan delivery. The threshold value of MU/deg is found to be 2·083 MU/deg based on our machine configurations.

Surface dose measurements in and out of field: Implications for breast radiotherapy with megavoltage photon beams

This study examines the difference in surface dose between flat and flattening filter free (FFF) photon beams in the context of breast radiotherapy. The surface dose was measured for 6MV, 6MV FFF, 10MV, 10MV FFF and 18MV photon beams using a thin window ionisation chamber for various field sizes. Profiles were acquired to ascertain the change in surface dose off-axis. Out-of-field measurements were included in a clinically representative half beam block tangential breast field. In the field centres of FFF beams the surface dose was found to be increased for small fields and decreased for large fields compared to flat beams. For FFF beams, surface dose was found to decrease off-axis and resulted in lower surface dose out-of-field compared to flat beams.

Accounting for the ion recombination factor in relative dosimetry of flattening filter free photon radiation

Flattening filter free (FFF) linear accelerators (linacs) are increasingly being used to deliver external beam radiotherapy. Advantages of FFF include lower out-of-field dose to the patient due to reduced head scatter, and shorter treatment times due to increased dose rate. The ion recombination factor (kion) is known to increase linearly with dose per pulse in pulsed photon beams. This variation is neglected when performing relative dosimetry in flattened beams, however could be significant in the higher dose rates encountered with FFF. kion has been derived from theory and measured using Jaffe plots and two voltage analysis for three ionisation chambers at a range of depths and points off-axis in 6 and 10 MV FFF beams produced by an Elekta Versa HD linac. The chambers under investigation were the PTW 30013 ‘Farmer’ (0.60 cc), the Exradin A1SL (0.053 cc) and the IBA CC04 (0.04 cc). The effect of neglecting ion recombination corrections in commissioning data was quantified using a 1D gamma analysis (1%/2 mm) on depth dose curves and beam profiles. The largest variation in kion was for a Farmer chamber in a 10 MV FFF beam, where kion varied by up to 1.2% ± 0.15% (95% CL) between different points in a water tank. The Farmer variation with depth was comparable to that measured by Kry et al (2012 J. Appl. Clin. Med. Phys. 13 318–325). Other chambers exhibited negligible changes in kion. It was concluded that neglecting ion recombination in relative measurements at 6 MV FFF would have a negligible effect on dosimetric accuracy. Furthermore kion could be safely neglected when acquiring relative data with small volume chambers for commissioning 10 MV FFF, although may need to be accounted for if relative data are acquired with a Farmer chamber and when routine energy checks are performed.

P21. Treatment planning for liver stereotactic body radiation therapy using FFF photon beams

Introduction Since April 2015, patients are treated in our department with liver stereotactic body radiation therapy on a linear accelerator equipped with flattening filter free (FFF) photon beams. The energy and the use of respiratory gating are adapted for each patient. This study presents the results for the first 14 patients. Material and methods. Three gold fiducials are implanted inside or in the vicinity of the lesion 7–10 days prior planning imaging. The minimum 4 h fasting patient undergoes first a 4D CT scan with systematic audio coaching and a thoracic thermoplastic mask (Orfit Industries, Wijnegem, Belgium). An MRI scan in treatment position is acquired and registered to the CT images based on the fiducials. The radiation oncologist outlines the GTV on each of the six respiratory phases. A 6 phases ITV (entire breathing cycle) and a 3 phases ITV (expiratory phases) are then created. The corresponding PTVs are created by adding a 5 mm margin. When the ratio PTV 6/ PTV 3 is higher than 1.5, the treatment is achieved with respiratory gating. The beam used is a conformal dynamic arc of 180 to 200° amplitude, conformed to the PTV. 10 MV FFF beams were preferably used as the dose rate is higher than for 6 MV FFF beams (2400 and 1200 MU/min respectively). For organs at risk, constraints from Timmerman et al. [1] were used. Results For the past year, 14 patients have been treated (8 with 3 fractions of 18 Gy, 3 with 5 fractions of 11 Gy and 3 with 5 fractions of 10 Gy), without any restriction on PTV size (from 11 to 273 cm 3 ). The prescription isodose is 80%. 5 patients were treated in free breathing (PTV 6) and 9 with respiratory gating (PTV 3). Sometimes it has been necessary to treat with respiratory gating in order to fulfill the constraints to the organs at risk. The systematic comparison for the first 10 patients between the two FFF energies showed that the dosimetric objectives were fulfilled similarly for both energy except when the PTV was close to an organ at risk. In this case, better results were obtained with 6MV FFF beam. Conclusion Despite the fact that the liver is a very mobile organ, it has been possible to treat some patients in free breathing, without compromising the dosimetric objectives, thus allowing a better patient comfort (treatment time 3 times faster). The 10 MV FFF with its 2400 MU/min dose rate was most often used, allowing treatment times lower than 10 min even with respiratory gating and the highest prescribed dose.

Gafchromic EBT-XD film: Dosimetry characterization in high-dose, volumetric-modulated arc therapy.

Radiochromic films are important tools for assessing complex dose distributions. Gafchromic EBT‐XD films have been designed for optimal performance in the 40–4,000 cGy dose range. We investigated the dosimetric characteristics of these films, including their dose‐response, postexposure density growth, and dependence on scanner orientation, beam energy, and dose rate with applications to high‐dose volumetric‐modulated arc therapy (VMAT) verification. A 10 MV beam from a TrueBeam STx linear accelerator was used to irradiate the films with doses in the 0–4,000 cGy range. Postexposure coloration was analyzed at postirradiation times ranging from several minutes to 48 h. The films were also irradiated with 6 MV (dose rate (DR): 600 MU/min), 6 MV flattening filter‐free (FFF) (DR: 1,400 MU/ min), and 10 MV FFF (DR: 2,400 MU/min) beams to determine the energy and dose‐rate dependence. For clinical examinations, we compared the dose distribution measured with EBT‐XD films and calculated by the planning system for four VMAT cases. The red channel of the EBT‐XD film exhibited a wider dynamic range than the green and blue channels. Scanner orientation yielded a variation of ∼3% in the net optical density (OD). The difference between the film front and back scan orientations was negligible, with variation of ∼1.3% in the net OD. The net OD increased sharply within the first 6 hrs after irradiation and gradually afterwards. No significant difference was observed for the beam energy and dose rate, with a variation of ∼1.5% in the net OD. The gamma passing rates (at 3%, 3 mm) between the film‐ measured and treatment planning system (TPS)‐calculated dose distributions under a high dose VMAT plan in the absolute dose mode were more than 98.9%.PACS number(s): 87.56 Fc

SU‐F‐T‐260: Using Portal Image Device for Pre‐Treatment QA in Volumetric Modulated Arc Plans with Flattening Filter Free (FFF) Beams

Purpose:To implement and validate a method of using electronic portal image device (EPID) for pre‐treatment quality assurance (QA) of volumetric modulated arc therapy (VMAT) plans using flattering filter free (FFF) beams for stereotactic body radiotherapy (SBRT).Methods:On Varian Edge with 6MV FFF beam, open field (from 2×2 cm to 20×20 cm) EPID images were acquired with 200 monitor unit (MU) at the image device to radiation source distance of 150cm. With 10×10 open field and calibration unit (CU) provided by vendor to EPID image pixel, a dose conversion factor was determined by dividing the center dose calculated from the treatment planning system (TPS) to the corresponding CU readout on the image. Water phantom measured beam profile and the output factors for various field sizes were further correlated to those of EPID images. The dose conversion factor and correction factors were then used for converting the portal images to the planner dose distributions of clinical fields. A total of 28 VMAT fields of 14 SBRT plans (8 lung, 2 prostate, 2 liver and 2 spine) were measured. With 10% low threshold cutoff, the delivered dose distributions were compared to the reference doses calculated in water phantom from the TPS. A gamma index analysis was performed for the comparison in percentage dose difference/distance‐to‐agreement specifications.Results:The EPID device has a linear response to the open fields with increasing MU. For the clinical fields, the gamma indices between the converted EPID dose distributions and the TPS calculated 2D dose distributions were 98.7%±1.1%, 94.0%±3.4% and 70.3%±7.7% for the criteria of 3%/3mm, 2%/2mm and 1%/1mm, respectively.Conclusion:Using a portal image device, a high resolution and high accuracy portal dosimerty was achieved for pre‐treatment QA verification for SBRT VMAT plans with FFF beams.

SU‐F‐T‐629: Effect of Multi‐Leaf Collimator (MLC) Width On Plan Quality of Single‐Isocenter VMAT Intracranial Stereotactic Radiosurgery for Multiple Metastases

Purpose:Single‐isocenter VMAT has been shown able to create high quality plans for complex intracranial multiple metastasis SRS cases. Linacs capable of the technique are typically outfitted with an MLC that consists of a combination of 5 mm and 10 mm leaves (standard) or 2.5 mm and 5 mm leaves (high‐definition). In this study, we test the hypothesis that thinner collimator leaves are associated with improved plan quality.Methods:Ten multiple metastasis cases were identified and planned for VMAT SRS using a 10 MV flattening filter free beam. Plans were created for a standard (std) and a high‐definition (HD) MLC. Published values for leaf transmission factor and dosimetric leaf gap were utilized. All other parameters were invariant. Conformity (plan and individual target), moderate isodose spill (V50%), and low isodose spill (mean brain dose) were selected for analysis.Results:Compared to standard MLC, HD‐MLC improved overall plan conformity (median: Paddick CI_HD = 0.83, Paddick CI_std = 0.79; p = 0.004 and median: RTOG CI_HD =1.18, RTOG CI_std =1.24; p = 0.01), improved individual lesion conformity (median: Paddick CI_HD,i =0.77, Paddick CI_std,i =0.72; p < 0.001 and median: RTOG CI_HD,i = 1.28, RTOG CI_std,i =1.35; p < 0.001), improved moderate isodose spill (median: V50%_HD = 37.0 cc, V50%_std = 45.7 cc; p = 0.002), and improved low dose spill (median: dmean_HD = 2.90 Gy, dmean_std = 3.19 Gy; p = 0.002).Conclusion:For the single‐isocenter VMAT SRS of multiple metastasis plans examined, use of HD‐MLC modestly improved conformity, moderate isodose, and low isodose spill compared to standard MLC. However, in all cases we were able to generate clinically acceptable plans with the standard MLC. More work is need to further quantify the difference in cases with higher numbers of small targets and to better understand any potential clinical significance.This research was supported in part by Varian Medical Systems.

Commissioning an Elekta Versa HD linear accelerator.

The purpose of this study is to report the dosimetric aspects of commissioning performed on an Elekta Versa HD linear accelerator (linac) with high‐dose‐rate flattening filter‐free (FFF) photon modes and electron modes. Acceptance and commissioning was performed on the Elekta Versa HD linac with five photon energies (6 MV, 10 MV, 18 MV, 6 MV FFF, 10 MV FFF), four electron energies (6 MeV, 9 MeV, 12 MeV, 15 MeV) and 160‐leaf (5 mm wide) multileaf collimators (MLCs). Mechanical and dosimetric data were measured and evaluated. The measurements include percent depth doses (PDDs), in‐plane and cross‐plane profiles, head scatter factor (Sc), relative photon output factors (Scp), universal wedge transmission factor, MLC transmission factors, and electron cone factors. Gantry, collimator, and couch isocentricity measurements were within 1 mm, 0.7 mm, and 0.7 mm diameter, respectively. The PDDs of 6 MV FFF and 10 MV FFF beams show deeper dmax and steeper falloff with depth than the corresponding flattened beams. While flatness values of 6 MV FFF and 10 MV FFF normalized profiles were expectedly higher than the corresponding flattened beams, the symmetry values were almost identical. The cross‐plane penumbra values were higher than the in‐plane penumbra values for all the energies. The MLC transmission values were 0.5%, 0.6%, and 0.6% for 6 MV, 10 MV, and 18 MV photon beams, respectively. The electron PDDs, profiles, and cone factors agree well with the literature. The outcome of radiation treatment is directly related to the accuracy in the dose modeled in the treatment planning system, which is based on the commissioned data. Commissioning data provided us a valuable insight into the dosimetric characteristics of the beam. This set of commissioning data can provide comparison data to others performing Versa HD commissioning, thereby improving patient safety.PACS number(s): 87.56.bd

Cadaveric Verification of VMAT Dose Distributions for Spine SBRT Treatments With Titanium Implants

Cadaveric Verification of VMAT Dose Distributions for Spine SBRT Treatments With Titanium Implants

Amplitude gating for a coached breathing approach in respiratory gated 10 MV flattening filter-free VMAT delivery.

The purpose of this study was to investigate amplitude gating combined with a coached breathing strategy for 10 MV flattening filter‐free (FFF) volumetric‐modulated arc therapy (VMAT) on the Varian TrueBeam linac. Ten patient plans for VMAT SABR liver were created using the Eclipse treatment planning system (TPS). The verification plans were then transferred to a CT‐scanned Quasar phantom and delivered on a TrueBeam linac using a 10 MV FFF beam and Varian's real‐time position management (RPM) system for respiratory gating based on breathing amplitude. Breathing traces were acquired from ten patients using two kinds of breathing patterns: free breathing and an interrupted (~5 s pause) end of exhale coached breathing pattern. Ion chamber and Gafchromic film measurements were acquired for a gated delivery while the phantom moved under the described breathing patterns, as well as for a nongated stationary phantom delivery. The gate window was set to obtain a range of residual target motion from 2–5 mm. All gated deliveries on a moving phantom have been shown to be dosimetrically equivalent to the nongated deliveries on a static phantom, with differences in point dose measurements under 1% and average gamma 2%/2 mm agreement above 98.7%. Comparison with the treatment planning system also resulted in good agreement, with differences in point‐dose measurements under 2.5% and average gamma 3%/3 mm agreement of 97%. The use of a coached breathing pattern significantly increases the duty cycle, compared with free breathing, and allows for shorter treatment times. Patients' free‐breathing patterns contain considerable variability and, although dosimetric results for gated delivery may be acceptable, it is difficult to achieve efficient treatment delivery. A coached breathing pattern combined with a 5 mm amplitude gate, resulted in both high‐quality dose distributions and overall shortest gated beam delivery times.PACS number: 87.55.Qr

A comparison between direct TMR measurements and TMRs calculated from PDDs using BJR Supplement 25 data for flattened and unflattened photon beams.

This study assessed the validity of the conversion from percentage depth dose (PDD) to tissue maximum ratio (TMR) using BJR Supplement 25 data for flattened and flattening filter free (FFF) beams. PDD and TMR scans for a variety of field sizes were measured in water using a Sun Nuclear Corporation 3D SCANNER™ on a Varian TrueBeam linear accelerator in 6 MV, 10 MV and 6 MV FFF beams. The BJR Supplement 25 data was used to convert the measured PDDs to TMRs and these were compared with the directly measured TMR data. The TMR plots calculated from PDD were within 1% for the 10 MV and 6 MV flattened beams, for field sizes 3 cm × 3 cm to 40 cm × 40 cm inclusive, at depths measured beyond the depth of maximum dose. The disagreement between the measured and calculated TMR plots for the 6 MV FFF beam increased with depth and field size to a maximum of 1.7% for a 40 cm × 40 cm field. The results found in this study indicate that the BJR Supplement 25 data should not be used for field sizes larger than 20 cm × 20 cm at depths greater than 15 cm for the 6 MV FFF beam. It is advised that PDD to TMR conversion for FFF beams should be done with phantom scatter ratios appropriate to FFF beams, or the TMR should be directly measured if required.

SU‐E‐T‐719: Surface Dose Evaluation of the Equivalent Quality Flattening Filter‐Free Photon Beams

Purpose:To provide data on the relative surface dose of photon beams of Versa HD linear accelerator designed to delivered the Flattening Filter Free (FFF) beam having beam quality equivalent to the corresponding Flattened beam.This study involves the comparative evaluation of the entrance dose of FFF and Flattened photon beams in the buildup region.Methods:Surface dose was measured using Markus parallel plate chamber with the thin flat entrance window thickness of 0.025mm of water equivalent material in solid water phantom. Data were collected for square fields of 2×2cm2, 3×3cm2,4×4cm2,5×5cm2,7×7cm2,10×10cm2 and 20×20cm2 created in Monaco Treatment Planning System.The surface dose was define for this study as the relative dose at depth of 0.5mm with respect to the dose at dmax.The same Markus chamber was used to investigate the dose in the buildup region for all the square fields.The buildup depths for 6MV flattened and FFF beams were 0 to 18mm and for 10MV Flattened and FFF beams were 0 to 24mm. All values were determined in relation to the depth at dose maximum dmax.Results:The relation of surface dose for both Flattened and FFF beams with field sizes were linear. The surface dose for smaller field size upto 10×10cm2 for both FFF beams were higher than Flattened beams but the variation was marginal. The difference was 0.3 percentage at 6MV and 1.9 percentage at 10MV for a 10×10cm2 field but for the larger field the relative surface dose was lower for FFF beam with difference of 3.6 percentage at 6MV and 4.3 percentage at 10MV for a 20×20cm2 field.Conclusion:The equivalent quality FFF Photons produce higher surface dose for smaller field sizes upto 10×10cm2 but lower surface dose for large field sizes than the conventional flattened beam. The percentage variation was marginal for smaller field sizes and may not be clinically significant.

SU-E-T-265: Development of Dose-To-Water Conversion Models for Pre-Treatment Verification with the New AS1200 Imager

Purpose: To develop and evaluate models for dose verification of flattened (FF) and flattening filter free (FFF) beams for the new Varian aS1200 backscatter-shielded electronic portal imaging device (EPID). Methods: The model converts EPID images to incident energy fluence using deconvolution of EPID scatter kernels and fluence to dose in water using convolution with dose-to-water kernels. Model parameters were optimized using non-transmission EPID images of varying jaw defined field sizes for energies of 6 and 10 MV FF and FFF beams. Energy fluence was obtained from the Acuros planning system and reference dose profiles and output factors were measured at depths of 5, 10, 15 and 20 cm in a water phantom. Images for 34 IMRT fields acquired at 6 and 10 MV FF energy were converted to dose at 10 cm depth in water and compared to treatment planning system dose plane calculations using gamma criteria. Results: Gamma evaluations for the IMRT fields had mean (1 standard deviation) pass rates of 99.4% (0.8%) and mean gamma scores of 0.32 (0.06) with 2%, 2 mm criteria and 10% of maximum dose threshold. Conclusion: The developed model has been shown to be highly accurate for pre-treatment verification with the new aS1200 imager which does not display support-arm backscatter artefact and has improved dosimetric properties. Further investigation of FFF modes is in progress. The model is currently being evaluated at sites for potential clinical release.

Safety of high-dose-rate stereotactic body radiotherapy.

Background and purposeFlattening filter free (FFF) beams with high dose rate are increasingly used for stereotactic body radiotherapy (SBRT), because they substantially shorten beam-on time. The physical properties of these beams together with potentially unknown radiobiological effects might affect patient safety. Therefore here we analyzed the clinical outcome of our patients.Material and methodsBetween 3/2010 and 2/2014 84 patients with 100 lesions (lung 75, liver 10, adrenal 6, lymph nodes 5, others 4) were treated with SBRT using 6 MV FFF or 10 MV FFF beams at our institution. Clinical efficacy endpoints and toxicity were assessed by Kaplan-Meier analysis and CTCAE criteria version 4.0.ResultsMedian follow-up was 11 months (range: 3–41). No severe acute toxicity was observed. There has been one case of severe late toxicity (1%), a grade 3 bile duct stricture that was possibly related to SBRT. For all patients, the 1-year local control rate, progression free survival and overall survival were 94%, 38% and 80% respectively, and for patients with lung lesions 94%, 48% and 83%, respectively.ConclusionsNo unexpected toxicity occurred. Toxicity and treatment efficacy are perfectly in range with studies investigating SBRT with flattened beams. The use of FFF beams at maximum dose rate for SBRT is time efficient and appears to be safe.

Interplay Effect During Lung SBRT With VMAT Delivered by Flattening Filter–Free Beams

Volumetric modulated arc therapy (VMAT) delivered by flattening filter free (FFF) beams can significantly reduce treatment time for stereotactic body radiation therapy (SBRT). However, high dose rate raises the concern regarding the impact of the interplay effect between MLC leaf sequences and target motion. We have utilized four-dimensional (4D) dose calculations to investigate the interplay effect on the delivered dose in lung SBRT cases treated using FFF beams. Ten lung cancer patients with tumor motion amplitude of 10-30 mm were investigated. For each patient, a respiratory-correlated 4D computed tomography data set was acquired. Single arc SBRT VMAT plans were generated using the treatment planning system module for 10 MV FFF beam on a linear accelerator. The delivered dose was estimated at two dose rates: conventional dose rate of 400 MU/min and the maximum FFF dose rate of 2300 MU/min. Based on the dose rate, beam-on timing and respiratory cycle, the planned segment MUs were assigned to different respiratory phases to simulate MLC interplay effect. The fractional dose was evaluated in 10 scenarios corresponding to the treatment starting at 0%, 10%, 20%, 90% respiratory phases, respectively. For each scenario the 4D dose was evaluated on the CT at end of exhalation. We also calculated the 4D dose with no interplay effect where the MUs for each segment were equally distributed over all breathing phases. Following parameters were used to analyze the interplay effect in single fraction: standard deviation (SD) of the minimum dose Dmin to the GTV; dose deviation (DE) for GTV between 4D dose and the original planning dose; DE for GTV between 4D doses with and without interplay effects. For all patients, dose delivered at 2300 MU/min FFF mode resulted in larger interplay effects between MLC and tumor motion than that delivered at 400 MU/min. SD for Dmin of GTV was 1.7% of the prescription dose for 2300 MU/min, and 0.15% for 400 MU/min. Compared to the original planning dose, maximum DE of GTV dose delivered at 2300 MU/min was 4.9%, and the DE was less than 3% for 94% of GTV. Compared to the 4D dose calculated without interplay effect, the maximum DE was 3.0%, and the DE is less than 2% for over 99% of GTV. Lung SBRT VMAT treatment delivered by high dose rate FFF beams is more susceptible to the MLC interplay effect compared with that delivered at conventional dose rate. However the dosimetric impact of interplay on the target volume is unlikely to be clinically significant.

Poster — Thur Eve — 31: Dosimetric Effect of Respiratory Motion on RapidArc Lung SBRT Treatment Delivered by TrueBeam Linear Accelerator

Volumetric modulated arc therapy (VMAT) allows fast delivery of stereotactic radiotherapy. However, the discrepancies between the calculated and delivered dose distributions due to respiratory motion and dynamic multileaf collimators (MLCs) interplay are not avoidable. The purpose of this study is to investigate RapidArc lung SBRT treatment delivered by the flattening filter‐free (FFF) beam and flattened beam with Varian TrueBeam machine. CIRS Dynamic Thorax Phantom with in‐house made lung tumor insertion was CT scanned both in free breathing and 4DCT. 4DCT was used to determine the internal target volume. The free breathing CT scan was used for treatment planning. A 5 mm margin was given to ITV to generate a planning target volume. Varian Eclipse treatment planning was used to generate RapidArc plans based on the 6 MV flattened beam and 6MV FFF beam. The prescription dose was 48 Gy in 4 fractions. At least 95% of PTV was covered by the prescribed dose. The RapidArc plans with 6 MV flattened beam and 6MV FFF beam were delivered with Varian TrueBeam machine. The dosimetric measurements were performed with Gafchromic XR‐RV3 film, which was placed in the lung tumor insertion. The interplay between the dynamic MLC‐based delivery of VMAT and the respiratory motion of the tumor degraded target coverage and created undesired hot or cold dose spots inside the lung tumor. Lung SBRT RapidArc treatments delivered by the FFF beam of TrueBeam linear accelerator is superior to the flattened beam. Further investigation will be performed by Monte Carlo simulation.

Sci-Sat AM: Stereo - 06: Dosimetric Comparison of 3D Conformai, Flattened and Flattening Filter-Free TrueBeam RapidArc Planning for Lung SBRT

The major advantages of the VMAT SBRT plans compared to the conventional 3D conformai plan include faster delivery and improved target dose conformity. This study quantifies the dosimetric differences among 3D conformai plan; flattened beam and flattening filter-free (FFF) beam RapidArc Plans for lung SBRT. Five early stage lung cancer patients with various tumor positions and sizes previously treated with 3D non-coplanar SBRT were randomly selected. 4DCT was used for each patient to determine the internal target volume. Abdominal compression was applied to minimize respiratory motion for SBRT patients. For treatment planning, a 5 mm margin was given to the ITV to generate a planning target volume. The prescription dose was 48 Gy in 4 fractions and normalized to 95% of the PTV. Organs at risk (OAR) included spinal cord, esophagus, heart, trachea, bilateral lung, and great vessels. Optimization constraints were set to meet the criteria of the RTOG-0915 protocol. All VMAT plans were optimized with the RapidArc technique using two full arcs in Eclipse treatment planning system. The RapidArc SBRT plans with flattened 6MV beam and 6MV FFF beam were generated and dosimetric results were compared with the previous treated 3D non-coplanar plans. RapidArc plans demonstrated better conformity tomore » target, sharper dose fall-off in normal tissues and lower dose to normal lung and other OARs than the 3D conformai plans. RapidArc SBRT for FFF beam showed comparable target conformity, adequate tumor dose, and clinically acceptable DVHs of OARs to flattened beams and significantly reduced treatment delivery time.« less