Gantry Speed Research Articles

The impact of continuously‐variable dose rate VMAT on beam stability, MLC positioning, and overall plan dosimetry

A recent control system update for Elekta linear accelerators includes the ability to deliver volumetric‐modulated arc therapy (VMAT) with continuously variable dose rate (CVDR), rather than a number of fixed binned dose rates (BDR). The capacity to select from a larger range of dose rates allows the linac to maintain higher gantry speeds, resulting in faster, smoother deliveries. The purpose of this study is to investigate two components of CVDR delivery — the increase in average dose rate and gantry speed, and a determination of their effects on beam stability, MLC positioning, and overall plan dosimetry. Initially, ten VMAT plans (5 prostate, 5 head and neck) were delivered to a Delta4 dosimetric phantom using both the BDR and CVDR systems. The plans were found to be dosimetrically robust using both delivery methods, although CVDR was observed to give higher gamma pass rates at the 2%/2 mm gamma level for prostates (p < 0.01). For the dual arc head‐and‐neck plans, CVDR delivery resulted in improved pass rates at all gamma levels (2%/2 mm to 4%/4 mm) for individual arc verifications (p < 0.01), but gave similar results to BDR when both arcs were combined. To investigate the impact of increased gantry speed on MLC positioning, a dynamic leaf‐tracking tool was developed using the electronic portal imaging device (EPID). Comparing the detected MLC positions to those expected from the plan, CVDR was observed to result in a larger mean error compared to BDR (0.13 cm and 0.06 cm, respectively, p < 0.01). The EPID images were also used to monitor beam stability during delivery. It was found that the CVDR deliveries had a lower standard deviation of the gun‐target (GT) and transverse (AB) profiles (p < 0.01). This study has determined that CVDR may offer a dosimetric advantage for VMAT plans. While the higher gantry speed of CVDR appears to increase deviations in MLC positioning, the relative effect on dosimetry is lower than the positive impact of a flatter and more stable beam profile.PACS numbers: 87.56.bd; 87.55.km; 87.55.Qr

SBRT Treatment Planning With VMAT Versus IMRT: Impact of Duodenal Sparing on Tumor Coverage in Patients With Locally Advanced Pancreatic Adenocarcinoma

Stereotactic body radiation therapy (SBRT) achieves excellent local control rates for locally advanced pancreatic cancer (LAPC), but may increase the risk of late duodenal toxicity. Volumetric modulated arc therapy (VMAT) is a modified method of delivering intensity modulated radiation therapy (IMRT) with a rotating gantry rather than multiple fixed beams. Dynamic modulation of angular dose rate through changes in multileaf collimator motion, LINAC dose rate, and gantry speed may facilitate duodenal sparing with VMAT.

Treatment planning and dosimetric comparison study on two different volumetric modulated arc therapy delivery techniques

AimTo compare and evaluate the performance of two different volumetric modulated arc therapy delivery techniques. BackgroundVolumetric modulated arc therapy is a novel technique that has recently been made available for clinical use. Planning and dosimetric comparison study was done for Elekta VMAT and Varian RapidArc for different treatment sites. Materials and methodsTen patients were selected for the planning comparison study. This includes 2 head and neck, 2 oesophagus, 1 bladder, 3 cervix and 2 rectum cases. Total dose of 50Gy was given for all the plans. All plans were done for RapidArc using Eclipse and for Elekta VMAT with Monaco treatment planning system. All plans were generated with 6MV X-rays for both RapidArc and Elekta VMAT. Plans were evaluated based on the ability to meet the dose volume histogram, dose homogeneity index, radiation conformity index, estimated radiation delivery time, integral dose and monitor units needed to deliver the prescribed dose. ResultsRapidArc plans achieved the best conformity (CI95%=1.08±0.07) while Elekta VMAT plans were slightly inferior (CI95%=1.10±0.05). The in-homogeneity in the PTV was highest with Elekta VMAT with HI equal to 0.12±0.02Gy when compared to RapidArc with 0.08±0.03. Significant changes were observed between the RapidArc and Elekta VMAT plans in terms of the healthy tissue mean dose and integral dose. Elekta VMAT plans show a reduction in the healthy tissue mean dose (6.92±2.90)Gy when compared to RapidArc (7.83±3.31)Gy. The integral dose is found to be inferior with Elekta VMAT (11.50±6.49)×104Gycm3 when compared to RapidArc (13.11±7.52)×104Gycm3. Both Varian RapidArc and Elekta VMAT respected the planning objective for all organs at risk. Gamma analysis result for the pre-treatment quality assurance shows good agreement between the planned and delivered fluence for 3mm DTA, 3% DD for all the evaluated points inside the PTV, for both VMAT and RapidArc techniques. ConclusionThe study concludes that a variable gantry speed with variable dose rate is important for efficient arc therapy delivery. RapidArc presents a slight improvement in the OAR sparing with better target coverage when compared to Elekta VMAT. Trivial differences were noted in all the plans for organ at risk but the two techniques provided satisfactory conformal avoidance and conformation.

Patient specific QA for RapidArc

Abstract: RapidArc is a volumetric arc therapy which was recently introduced to Ireland by St. Lukes Radiation Oncology Network. RapidArc is characterised by continuous MLC and gantry motion in combination with varying gantry speed, dose rate and MLC speed. This approach leads to significantly shortened treatment times and dose conformity which is comparable to that achieved by static IMRT. Because of the complexity of RapidArc treatment plans, it is vital to implement an efficient and accurate protocol for routine patient specific QA. We decided to use the ArcCHECK phantom for this purpose. ArcCHECK is a commercial diode array designed specifically for rotational measurements. Our ArcCHECK measurements were validated by comparison with Gafchromic film measurements as this is a standard high resolution patient verification method. Specifically, our work involved delivering a number of prostate RapidArc treatment plans to ArcCHECK. Independent validation of ArcCHECK measurements with Gafchromic film, in two orientations was performed for a subset of these plans. Gafchromic film analysis was performed using the ‘triple channel dosimetry’ approach. This is a new method of evaluating radiochromic film dosimetry data which improves the integrity of the dose information in the film by removing disturbances in scanned images. As a result of following this method, a high level dosimetric accuracy was evident in our film measurements. Dose distributions were analysed using gamma criteria of 3% dose and 3 mm distance to agreement. A pass rate of at least 95% was achieved using data from ArcCHECK and Gafchromic film respectively. This work combines the construction of an efficient protocol for using ArcCHECK for patient specific QA of RapidArc treatment plans with an implementation of the ‘triple channel dosimetry’ approach for Gafchromic film analysis. Thus we have formulated a protocol for patient specific QA of RapidArc treatment plans and incorporated Gafchromic film as a suitable backup system.

Sci-Fri PM: Delivery - 05: A complete set of VMAT machine-specific QA tests using EPID.

During a VMAT (Volumetric Modulated Arc Therapy) radiation delivery, various machine parameters (gantry position and speed, MLC leaves position, dose rate, etc) have to be highly synchronized and coordinated. To ensure that all of the machine components are effectively operating together requires unique machine-specific quality assurance (QA) tests. The purpose of this work is to describe a complete set of VMAT machine-specific QA tests using Electronic Portal Imaging Device (EPID). To address the need for VMAT machine-specific QA tests, Varian has recommended 6 EPID-based tests for commissioning and QA of RapidArc™ capable machines. Recent studies have identified that these Varian tests do not define a complete set of tests needed for adequate commissioning and machine-specific QA of RapidArc machines. The goal of this work is twofold: a) to report and discuss the results of the original 6 Varian's EPID-based tests, and b) to extend the original set by adding 2 more EPID-based tests, which results in a more complete and comprehensive set of tests. We have demonstrated that the Varian EPID-based QA tests can be used to verify that the synchronization of Dose Rate, Gantry Speed and MLC Leaves speed is generally to within 1.5% during a RapidArc delivery. Also, the gantry rotation does not have any significant effect on MLC leaves positional accuracy during a RapidArc delivery. Finally, we have introduced a new EPID-based QA test, which can be used to verify that the angular accuracy of the gantry position during a RapidArc delivery is to within ∼1 degree.

Poster - Thur Eve - 10: Long term stability of VMAT quality assurance parameters using an EPID.

The rapidly growing use of volumetric modulated arc therapy (VMAT) treatments in radiation therapy calls for a quantitative, automated, and reliable quality assurance (QA) procedure that can be used routinely in the clinical setting. In this work, we present a series VMAT QA procedures used to assess dynamic multi-leaf collimator (MLC) positional accuracy, variable dose-rate accuracy, and MLC leaf speed accuracy. The QA procedures were performed using amorphous silicon electronic portal imaging devices (EPID) to determine the long term stability of the measured parameters on two Varian linear accelerators. The measurements were repeated weekly on both linear accelerators for a period of three months and the EPID images were analyzed using custom Matlab software. The results of the picket fence tests indicate that MLC leaf positions can be identified to within 0.11 mm and 0.15 mm for static gantry delivery and VMAT delivery respectively. In addition, the dose-rate, gantry speed and MLC leaf speed tests both show very good stability over the measurement period. The measurements thus far, suggest that a number of the dosimetry tests may be suitable for quarterly QA for Varian iX and Trilogy linacs. However, additional measurements are required to confirm the frequency with which each test is required for safe and reliable VMAT delivery at our centre.

Quality assurance of dynamic parameters in volumetric modulated arc therapy

The purpose of this study was to demonstrate quality assurance checks for accuracy of gantry speed and position, dose rate and multileaf collimator (MLC) speed and position for a volumetric modulated arc treatment (VMAT) modality (Synergy S; Elekta, Stockholm, Sweden), and to check that all the necessary variables and parameters were synchronous. Three tests (for gantry position-dose delivery synchronisation, gantry speed-dose delivery synchronisation and MLC leaf speed and positions) were performed. The average error in gantry position was 0.5° and the average difference was 3 MU for a linear and a parabolic relationship between gantry position and delivered dose. In the third part of this test (sawtooth variation), the maximum difference was 9.3 MU, with a gantry position difference of 1.2°. In the sweeping field method test, a linear relationship was observed between recorded doses and distance from the central axis, as expected. In the open field method, errors were encountered at the beginning and at the end of the delivery arc, termed the "beginning" and "end" errors. For MLC position verification, the maximum error was -2.46 mm and the mean error was 0.0153 ±0.4668 mm, and 3.4% of leaves analysed showed errors of >±1 mm. This experiment demonstrates that the variables and parameters of the Synergy S are synchronous and that the system is suitable for delivering VMAT using a dynamic MLC.

A new column-generation-based algorithm for VMAT treatment plan optimization

We study the treatment plan optimization problem for volumetric modulated arc therapy (VMAT). We propose a new column-generation-based algorithm that takes into account bounds on the gantry speed and dose rate, as well as an upper bound on the rate of change of the gantry speed, in addition to MLC constraints. The algorithm iteratively adds one aperture at each control point along the treatment arc. In each iteration, a restricted problem optimizing intensities at previously selected apertures is solved, and its solution is used to formulate a pricing problem, which selects an aperture at another control point that is compatible with previously selected apertures and leads to the largest rate of improvement in the objective function value of the restricted problem. Once a complete set of apertures is obtained, their intensities are optimized and the gantry speeds and dose rates are adjusted to minimize treatment time while satisfying all machine restrictions. Comparisons of treatment plans obtained by our algorithm to idealized IMRT plans of 177 beams on five clinical prostate cancer cases demonstrate high quality with respect to clinical dose–volume criteria. For all cases, our algorithm yields treatment plans that can be delivered in around 2 min. Implementation on a graphic processing unit enables us to finish the optimization of a VMAT plan in 25–55 s.

SU-D-BRA-03: Simultaneous MV-KV Imaging for Intra-Fractional Motion Management during Volume Modulated Arc Therapy (VMAT) Delivery on the Varian TrueBeam.

To evaluate a MV-kV intra-fractional imaging technique for use during volume modulated arc therapy (VMAT) with the Varian TrueBeam. MV-kV image pairs were acquired intra-fractionally during VMAT delivery. kV images (11 fps) were acquired throughout delivery using a standard pre-programmed imaging template. MV images (9.5 fps) were acquired simultaneously by deploying the EPID and passively collecting the resulting images using Varian proprietary software, iTools Capture. Localization accuracy was evaluated by imaging a Rando phantom implanted with 3 fiducials while moving the couch according to XML- programmed trajectories simulating typical prostate and respiratory motion. VMAT delivery was done using a single 360 degree arc in TrueBeam Developer mode. The effect on accuracy of total MU and gantry speed was studied. To improve image quality, MV frame averaging was performed and the MV and kV images were then registered to their corresponding DRRs using in-house registration software. From these 2D registrations, the 3D position at each MV-kV acquisition point was determined. Between 130 and 390 MV-kV pairs were acquired for each delivery. The mean difference between planned couch and measured fiducial 3D positions with prostate motion was less than 0.03 cm in each direction (SD 0.03 cm). Neither gantry speed nor MU significantly impacted accuracy. for respiratory motion, the mean difference between planned and measured position was less than 0.04 cm. Standard deviation averaged 0.06 cm but increased to 0.12 cm with large instantaneous motion and less MV dose per frame. MV frame averaging and inaccuracies in MV image gantry angle determination also affected accuracy, particularly with significant motion. With high quality MV imaging, MV-kV localization techniques can be highly accurate, even in the presence of significant motion. As clinical MV-kV methods become available, such techniques can provide an efficient and accurate method for monitoring intra-fractional motion. This work was partially supported through a research agreement with Varian Medical Systems, Palo Alto, CA.

SU-E-T-559: Evaluation of Flatening Filter Free (FFF) Beams in Radiotherapy of Head and Neck Cancer.

To investigate th applicability of the flattening filter free (FFF) to fractionated radiotherapy of head and neck cancer. Twelve patients previously treated with IMRT were selected. A SIB protocol was chosen so that 66/54 Gy were prescribed to the gross tumors/elective nodal volumes in 30 fractions. Photon beams of 6X(6X-FFF) were used for both IMRT (@ 9 gantry angles) and VMAT (single and dual full arcs). Consistent optimization and acceptance criteria were used for all plans, with prescription dose to cover 90% of PTV66 and hot spots limited to 110% and within PTV66. Both plan quality and deliver-ability were compared. All plans met clinical objectives. The IMRT plan quality is similar or slightly better than VMAT, except that PTV66 conformity index is significantly better in 2-arc VMAT (1.18) than IMRT (1.32). The total MUs for IMRT are 1680/2090 for 6X/6X-FFF, a 24% increase. However, the total beam-on time (BOT) under treatment automation is 430/400 seconds for 6X(@300 MU/min)/6X-FFF (@400 MU/min), a 6% decrease. Increasing dose rate to the maximum (600/1400 for 6X and 6X-FFF) can reduce BOT by up to 50%, but at a cost of increased MU (up to 40%) and degraded plan quality. Significant reduction in MU and BOT are realized in VMAT, with ∼600 MU for both 6X and 6X-FFF and either single or dual-arcs. The BOT for VMAT is governed by the gantry speed and approximately 1 min for single arc and 2 min for dual arcs. Plan quality is similar between IMRT and 2-arc VMAT for both 6X and 6X-FFF. Total MU for IMRT and delivery time is significantly higher for IMRT than VMAT, but not much different between flat and non-flat beams. For standard fractionated radiotherapy of HN cancer, there is no advantage for the non-flat beam. The recommended choice is 2-arc VMAT.

SU-E-T-36: Comprehensive End to End Data Transfer Integrity Check of Delivery Parameters Using TrueBeam Trajectory Log Files Analysis

Purpose: Traditional manual quality assurance checks of a patient treatment plan beam parameters transfer involve visually checking the information printed from the treatment planning system against the record‐and verify system for one‐to‐one correspondence. To provide the efficiency and accuracy of data transfer check, we have utilized the Varian TrueBeam log files which contain the complete delivery parameters. Methods: The approved plans are transferred to the Record and Verify (R&V) system, and the beams are delivered in the Truebeam Linac. The expected and actual status of TrueBeam delivery parameters are recorded during the delivery. Compared to traditional dynalog files from Varian, TrueBeam log files offer complete delivery parameters, which enable complete data transfer QA. An automatic program was designed to read the planned parameters from TPS and delivered parameters from Linac workstation. The program can verify the delivered parameters for all the external deliveries, including 3D conformal, EDW delivery, IMRT, VMAT, and electrons treatments. Results: The patient ID, beam ID, photon or electron energy, beam MU, gantry angle, couch angle, cough positions, collimator angle, jaw positions, EDW wedge angle and orientations, MLC positions, number of segments, gantry speed, dose rate, applicator size are all checked. The data transfer and/or delivery errors are flagged in the report for each type of delivery. Conclusion: The end‐to‐end data transfer check using TrueBeam log files provides an efficient, reliable and complete beam parameter plan check process for variety of radiation delivery technique. It is more efficient than human chart checking and improves the robustness and quality of chart checking.

SU‐C‐213CD‐06: Quantitative Assessment of 4D Cone‐Beam CT Based Image Guidance for Patient Positioning in Precision Radiotherapy

Purpose: To evaluate the precision of 4D cone‐beam CT as an image guidance tool for stereotactic body radiotherapy using intensity modulated arc or fixed gantry radiotherapy in sites with significant intrafraction tumor motionMethods: 4D cone‐beam CT (4D‐CBCT) is a recent innovation that has the potential to significantly improve the precision of highly conformai radiotherapy treatments. The performance of a commercial Elekta Synergy X‐ray Volume Imager (XVI) 4D‐CBCT system was quantitatively analyzed using a motor driven respiration phantom for a series of uniform and irregular breathing patterns. The quality of image guidance was assessed based on the precision of the 4D‐CBCT registration obtained from the XVI system with respect to the known motion of the respiration phantom (Quasar ModusQA). The quality of the registration was evaluated with various scan acquisition settings for field size, projection arc, gantry speed, etc., to develop an optimized image guidance protocol for 4D treatment sites. Results: The results indicated that the 4D‐CBCT registration shifts were in good overall agreement with the actual phantom motion. After rigid registration corrections using static regions of the phantom to eliminate systematic shifts, the respiration amplitudes were accurately reflected to within 1–3 mm for most breathing patterns, which is well within the expected uncertainty of registration algorithms as well as the amplitude and phase variability of the motor control. However, position differences in individual phase comparisons were found to be as high as 8–10 mm. We also observed that the larger field settings (m20) for the 4D‐CBCT acquisition rendered comparable accuracy to smaller vendor recommended fields (S20). Conclusions: The accuracy of 4D‐CBCT based patient positioning for treatment sites with significant respiratory motion was verified along with its potential to facilitate greater precision for dose convergence in hyperfractionated radiotherapy as well as to evaluate the feasibility of respiration gated radiotherapy.

SU-E-T-382: An Analysis of VMAT Monthly Quality Assurance.

The purpose of this study is to quantify the change in monthly VMAT quality assurance (QA) and determine the tests to maintain consistent delivery with a baseline. VMAT monthly QA has been performed for over 14 months on two Elekta Synergy LINACs. A baseline was established at acceptance and the monthly QA results were compared to those initial values. Films were used to test the dependence on varying dose rate, gantry speed, rotational direction, and MLC speed. These parameters were tested independently and then together in a test called the synchronicity test. Ion chamber readings test a DMLC field with a varying dose rate and MLC speed. Introducing intentional errors into the tested fields allowed the detectable limits of the QA to be determined. The monthly QA has consistently matched the baseline within a 3% dose limit on film. Analyzing the synchronicity film with a gamma test using a 5%/0.5mm tolerance showed a monthly pass rate of over 99%. The DMLC test has been identical for the entire course of VMAT QA. Furthermore, intentional changes in the MLC speed were noticed on the synchronicity test in the form of a smaller gamma pass rate as the MLC error was increased. There is a monthly agreement with all films testing individual parameters, ion chamber DMLC readings, as well as agreement with the synchronicity test. This collection of data has lead to the conclusion that only the synchronicity test and DMLC readings need to be performed on a monthly basis. If those tests fail, then individual parameters need to be tested to determine the singular cause of the error. Having a single test used as a red flag increases the efficiency of the monthly QA and is being implemented with an EPID to eliminate the use of film.

TU‐G‐BRA‐06: Motion Adaptive Rotational Intensity Modulated Radio Therapy (MA‐RIMRT) with Image Guidance

Purpose: Commercial intensity‐modulated‐arc‐therapy (IMAT) implementations rely on a 100% beam‐on duty‐cycle to achieve high efficiency. With moving targets, real‐time tracking or gating may be applied. Tracking is technically challenging and gating reduces efficiency by lowering the duty‐cycle. By combining burst‐mode rIMRT (in which all dose is administered while the gantry is within a few degrees of a beam optimization point [OP]), with a motion pre‐emption strategy and intrafractional imaging, we demonstrate an efficient implementation of motion‐adaptive rIMRT. Methods: 1. An rIMRT plan is created with 36–72 OPs. A gantry rotating at 6 degrees/s takes 3–6s to reach successive OPs. This co‐incides with the natural breathing period. 2. A Fourier‐descriptor‐based trajectory model is fitted to target features identified in pre‐treatment CBCT projections. The model predicts feature location at any combination of gantry angle and breathing phase. 3. The model sets the gantry speed to place the gantry at the next beam OP just in time as the target arrives at the planned position. In‐line projection images are acquired immediately before and during beam delivery. Treatment ensues only if the target position is correct (verification completes within 50ms, so maximum misdelivery possible at 2000MU/min is within 2MU). In cases of irregular breathing, the system gracefully defaults to gated‐IMRT. Results: For a MA‐rIMRT delivery of 750MU at 2000MU/min, treatment duration increases from 154s (regular rIMRT) to 199s (5s respiration cycle). Conclusions: With predictable patient breathing, MA‐rIMRT delivery times are competitive with conventional IMAT. High treatment accuracy is easily achieved, since there is no tracking lag and since the beam shape is static during delivery. Contemporary linacs rely on non‐realtime communication systems (typically Ethernet) to control time‐critical elements such as the MLC. Since such variable‐delay systems pose formidable problems in terms of predictive control, our approach offers a pragmatic alternative to tracking. Siemens Medical Solutions USA

SU-E-T-196: Commissioning for Volumetric Modulated Radiation Therapy on Varian Clinac 21EX.

Recently the volumetric arc therapy (VMAT) technology such as RapidArc is widely distributed in Japan. These technologies are normally provided by the high spec linear accelerator such as Trilogy, Novalis Tx, Synergy, et al. The specific DICOM-file is generally used for commissioning of these technologies. On the other hand, we had to apply RapidArc into historic linear accelerator. This title expresses an experience how we performed the commissioning of RapidArc with the old linear accelerator. Two Varian's linear accelerators "Clinac 21EX" equipped with Millenium multi-leaf collimator and a Varian's treatment planning system "Eclipse ver.8.9" were used for this study. The commissioning for RapidArc was performed in energy 4,6,10,15 MV (Max-DR: 250, 600, 400, 600 MU/min). Commissioning procedure composed two categories: the general machine QA for DMLC-IMRT procedure and the specific RapidArc QA procedure. In RapidArc QA procedure, we modified DICOM-file to apply into the potential spec of Clinac 21EX optimally. The specific MLC-motion sequence and the gantry rotation speed were created by the dedicated programs (Shaper and DicomEdit, Varian) for RapidArc QA procedure. Each tolerance value was defied by the data from daily/monthly QA and the paper by Ling et al. As the results of the general machine QA procedure, the variance of radiation output during static/dynamic gantry rotation was less than 1%. The deference of fence tests during static/dynamic gantry rotation and RapidArc were less than 1 mm in each. However, the results of the RapidArc QA were worse than the latest machine (especially variable gantry speed) and it was careful to define tolerance level. The procedure of commissioning for RapidArc on historic linear accelerator was proposed. Several minor revisions for DICOM-file should be required for suitable commissioning and it may ensure the tolerance limit for gantry/MLC-leaf motion speeds.

SU-E-T-152: Development of a Phantom for Commissioning and Patient Specific QA for Volumetric Modulated Arc Therapy (VMAT).

To design a cost effective phantom for commissioning tests and patient specific quality assurance in VMAT techniqueMethods: The phantom is made of PMMA with the physical density of 1.18 gm/cc. It consists of 20 numbers of 20cm diameter discs connected together in a horizontal stack, making a cylinder of length 20cm. The centers of the discs are drilled with 2.1cm hole which accommodates chamber sleeve of near 2.1cm diameter, which is made of Delrin with the physical density of 1.414gm/cc. The cylindrical geometry of the phantom is ideal for rotational therapy QA like VMAT or Rapid arc. The entire phantom is self supporting and hence no supporting bar or holders are in the path of beam geometry. The chamber sleeve itself acts as the supporting rod for these discs and it can be inserted in to the disc assembly from distance of-9cm to +10cm from the origin and hence any axial point from -9cm to +10cm with respect to isocenter can be used for point dose measurement. The other side of the chamber sleeve is filled by spacer rod for sustaining the discs in place. The films can be inserted, transversely in any XZ plane from -9cm to +10cm from the origin between the discs next to the chamber center. Simultaneous irradiation for the chamber and the film is done for VMAT plans without collapsing the gantry angles to nominal angle of zero. Evaluated the results using RIT film analysis software for gamma index of 3%, 3mm. Good agreement is found with more than 95% of pixels passing 3% 3mm criteria. The point dose verification showed less than 3% variation with TPS planned dose. Most of the commercial QA systems needs gantry to be collapsed to zero degree to perform QA for modern RT techniques. Especially for techniques like VMAT, collapsing gantry to zero will fail to validate the influence of gantry speed on dose delivery.

EP-1425 PRE-TRIAL PLANNING QA FOR THE IDEAL-CRT TRIAL: DOSE ESCALATION IN NSCLC

using the linac control system (LCS) software 'Elekta RTD 7'. Later the LCS was replaced by Integrity 1.1 which allows Elekta users to introduce continuous variable dose rate (CVDR) on their linac instead of dose rate variation in discrete steps. Starting from the settings used in the clinical plan, different plans were generated by changing TPS and LCS VMAT-parameters. All plans were delivered to investigate their influence on linac behaviour during delivery. Parameters varied in the TPS were 1) minimum and maximum allowed dose rate, 2) constant or continuous variable gantry speed and 3) maximum gantry speed change per segment. Parameters varied in the LCS were 1) discrete dose rate variation or CVDR, 2) maximum allowed gantry speed and 3) gantry inertia compensation distance (ICD). All plans were generated using the same dose and dose-volume objectives. Differences in target coverage and organ at risk sparing were negligible, and the resulting dose distributions met in all instances the planning criteria. Plan efficiency was analyzed by recording the delivery time. Linac behaviour was characterized by comparing the actual gantry angle with the requested gantry angle. Results: Introduction of CVDR improved delivery efficiency and linac behaviour most. When using maximum allowed gantry speed, occasionally the delivery was still inhibited by the LCS. Reducing both the maximum allowed gantry speed from the initial 6°/s to 5°/s and changing to CVDR resulted in delivery without interruptions keeping the delivery time the same. Changes made to the other variables did not result in faster delivery or behaviour changes of the gantry rotation. The only exception was increasing the ICD value which resulted in a smoother gantry rotation motion but also in a prolonged delivery time. Conclusions: Introduction of CVDR minimized most of the irregular gantry rotation resulting in faster plan delivery. Depending on the gantry acceleration or deceleration requested by the LCS inhibits may still occur, although less frequent compared to delivery using discrete dose rates. For inhibit-free plan delivery a reduction of the maximum allowed gantry speed is also required. Clinics which do not have the CVDR option available may already improve linac behaviour by reducing the maximum allowed gantry speed or increase the ICD distance. Both options will result in slightly prolonged delivery times. Although much improvement has been made using CVDR and gantry speed limitations, more data is needed to clarify the influence of LCS parameters. This knowledge will ease and speed up the introduction of smooth and inhibit-free VMAT delivery for other disease sites.

RapidArc patient specific mechanical delivery accuracy under extreme mechanical limits using linac log files

To assess the accuracy of RapidArc (RA) delivery for treatment machine operation near allowable mechanical limits in dynamic multileaf collimator (DMLC) leaf velocities, gantry speeds, and dose rates. Thirty RA patient plans were created for treatment of lung, gastrointestinal, and head and neck cancers on a Trilogy unit. For each patient, three RA plans were generated; one with medium MLC velocities, highest gantry speeds, and dose rates (case A); one with maximal allowable MLC leaf velocities (case B); and one with lowest gantry speeds (case C). Combinations of dose rates (140-600 MU/min), gantry speeds (2-5.4°/s), and DMLC leaf velocities (1.3-2.4 cm/s) were utilized to test the RapidArc delivery accuracy. Linac delivery log files were acquired after delivery of each plan. In-house developed software was used to read in the original RapidArc DICOM plan and update the plan to reflect the delivered plan by using the leaf position (L), gantry position (G), and MU dose values (D) extracted from the linac log files. This modified DICOM RT plan was imported back to ECLIPSE and the delivered 3D dose map recomputed. Finally, the planned and delivered 3D isodose maps were compared under three criteria to evaluate the dosimetric differences: maximum percentage dose difference, 3D gamma analysis criteria for 3%/3mm DTA, number of dose voxels having a dose difference that is greater than 1%, 2%, or 3% of the maximum dose, and their respective percentages. For the three cases indicated above, MLC leaf position discrepancies between planned and delivered values are 0.8 ± 0.2, 1.2 ± 0.2, and 0.8 ± 0.2 mm; the maximum gantry position discrepancies are 0.9° ± 0.2°, 0.9° ± 0.2°, and 0.6° ± 0.1°, and the maximum differences in delivered MU per control point are 0.2 ± 0.1, 0.2 ± 0.1, and 0.04 ± 0.01, respectively. Maximum percentage dose difference observed is 6.7%, for a case where 1 cm MLC leaves were used with high MLC leaf velocity. Maximum number (percentage) of dose voxels having a dose difference that is greater than 1%, 2%, and 3% of the maximum dose were 4761 (0.35%), 897 (0.07%), and 188 (0.01%). This also corresponds to the plan utilizing the most number of 1 cm MLC leaves. The 3D Gamma factor acceptance rates are better than 99%. This work shows that the accuracy of RapidArc delivery holds across the full range of gantry speeds, leaf velocities, and dose rates with small dosimetric uncertainties for 0.5 cm MLC leaves. However, caution should be exercised when using large MLC leaves in RapidArc. A novel technique to obtain the delivered 3D dose distributions using machine log files is also presented.

Volumetric Modulation Arc Radiotherapy With Flattening Filter-Free Beams Compared With Static Gantry IMRT and 3D Conformal Radiotherapy for Advanced Esophageal Cancer: A Feasibility Study

A feasibility study was performed to evaluate RapidArc (RA), and the potential benefit of flattening filter-free beams, on advanced esophageal cancer against intensity-modulated radiotherapy (IMRT) and three-dimensional conformal radiotherapy (3D-CRT). The plans for 3D-CRT and IMRT with three to seven and five to seven fixed beams were compared against double-modulated arcs with avoidance sectors to spare the lungs for 10 patients. All plans were optimized for 6-MV photon beams. The RA plans were studied for conventional and flattening filter-free (FFF) beams. The objectives for the planning target volume were the volume receiving ≥ 95% or at most 107% of the prescribed dose of <1% with a dose prescription of 59.4 Gy. For the organs at risk, the lung volume (minus the planning target volume) receiving ≥ 5 Gy was <60%, that receiving 20 Gy was <20%-30%, and the mean lung dose was <15.0 Gy. The heart volume receiving 45 Gy was <20%, volume receiving 30 Gy was <50%. The spinal dose received by 1% was <45 Gy. The technical delivery parameters for RA were assessed to compare the normal and FFF beam characteristics. RA and IMRT provided equivalent coverage and homogeneity, slightly superior to 3D-CRT. The conformity index was 1.2 ± 0.1 for RA and IMRT and 1.5 ± 0.2 for 3D-CRT. The mean lung dose was 12.2 ± 4.5 for IMRT, 11.3 ± 4.6 for RA, and 10.8 ± 4.4 for RA with FFF beams, 18.2 ± 8.5 for 3D-CRT. The percentage of volume receiving ≥ 20 Gy ranged from 23.6% ± 9.1% to 21.1% ± 9.7% for IMRT and RA (FFF beams) and 39.2% ± 17.0% for 3D-CRT. The heart and spine objectives were met by all techniques. The monitor units for IMRT and RA were 457 ± 139, 322 ± 20, and 387 ± 40, respectively. RA with FFF beams showed, compared with RA with normal beams, a ∼20% increase in monitor units per Gray, a 90% increase in the average dose rate, and 20% reduction in beam on time (owing to different gantry speeds). RA demonstrated, compared with conventional IMRT, a similar target coverage and some better dose sparing to the organs at risk; the advantage against conventional 3D-CRT was more evident. RA with FFF beams resulted in minor improvements in plan quality but with the potential for additional useful reduction in the treatment time.

VMAT testing for an Elekta accelerator

Volumetric‐modulated arc therapy (VMAT) has been shown to be able to deliver plans equivalent to intensity‐modulated radiation therapy (IMRT) in a fraction of the treatment time. This improvement is important for patient immobilization/ localization compliance due to comfort and treatment duration, as well as patient throughput. Previous authors have suggested commissioning methods for this modality. Here, we extend the methods reported for the Varian RapidArc system (which tested individual system components) to the Elekta linear accelerator, using custom files built using the Elekta iComCAT software. We also extend the method reported for VMAT commissioning of the Elekta accelerator by verifying maximum values of parameters (gantry speed, multileaf collimator (MLC) speed, and backup jaw speed), investigating: 1) beam profiles as a function of dose rate during an arc, 2) over/under dosing due to MLC reversals, and 3) over/under dosing at changing dose rate junctions. Equations for construction of the iComCAT files are given. Results indicate that the beam profile for lower dose rates varies less than 3% from that of the maximum dose rate, with no difference during an arc. The gantry, MLC, and backup jaw maximum speed are internally consistent. The monitor unit chamber is stable over the MUs and gantry movement conditions expected. MLC movement and position during VMAT delivery are within IMRT tolerances. Dose rate, gantry speed, and MLC speed are accurately controlled. Over/under dosing at junctions of MLC reversals or dose rate changes are within clinical acceptability.PACS numbers: 87.55.de, 87.55.Qr, 87.56.bd