Gantry Speed Research Articles

MO-SAM-BRB-01: Principles and Clinical Application of Volumetric Modulated Arc Therapy

Intensity modulated arc therapy (IMAT), first proposed by C. Yu in 1995, is a multiple arc technique in which the aperture defined by the multileaf collimator(MLC) of a conventional accelerator changes dynamically while gantry rotation speed and dose rate remain constant. IMAT was shown to deliver highly conformal dose distributions in times comparable to other conformal techniques; however, widespread adoption of IMAT was limited by the lack of commercial systems. Recently, volumetric modulated arc therapy (VMAT), in which gantry speed and dose rate are modulated in addition to MLC aperture, has been demonstrated and become commercially available. Commercial VMAT delivery systems are available from Elekta AB (Stockholm, Sweden) and Varian Medical Systems, Inc. (Palo Alto, CA). Although the dose distributions produced by VMAT are similar to those produced by fixed gantry intensity modulated radiation therapy(IMRT), VMAT is a distinct delivery technique with unique clinical considerations. We will describe the clinical implementation of VMAT, including commissioning, treatment planning, and patient and machine specific quality assurance. We will also review the elements of starting a VMAT program. Learning Objectives: 1. Describe volumetric modulated arc therapy (VMAT). 2. Survey VMAT treatment planning systems and planning techniques. 3. Describe commissioning of a VMAT delivery system. 4. Review patient‐ and machine‐specific VMAT quality assurance. 5. Summarize the key elements of starting a VMAT program. Conflict of Interest (only if applicable): Richard Popple and Cedric Yu each have a sponsored research agreement with Varian. David Shepard has a sponsored research agreement with Elekta.

SU-GG-T-114: A Feasibility Study of a 4D Intensity-Modulated Arc Tracking Technique for Treatment of Moving Targets

Purpose: A major challenge of Intensity‐modulated arc therapy(IMAT) is in dealing with moving targets since the conventional gating technique may not be easily applied. The purpose of this work is to develop a 4D IMAT tracking technique for moving targets and investigate its feasibility for clinical use. Method and Materials: 4DCT images at 10 different breathing phases were acquired. The target was contoured for each phase and a 3D RapidArc plan was generated based on the 50%phase CTimages. Each control point in the RapidArc plan was then correlated to the patient's respiration phases with consideration given to changes in gantry speed and dose rate. A novel program was developed to read the RapidArc plan and target contours from all 10 phases. In order to generate a 4D RapidArc plan with the capability of DMLC tracking, the MLC modification algorithm originally developed for on‐line plan modification was used to modify the MLC leaf positions at each control point to correspond to the respiration phases. 4D RapidArc plans for phantom and lung cases were used to evaluate the proposed technique. Experimental measurements for both 4D and 3D plans in a moving phantom were performed and the results were compared with the calculated and measured results for the 3D RapidArc plans in a “static” phantom. Results: Analysis of isodose and gamma distributions indicated that the dose distribution in the moving target delivered using the proposed technique is comparable to that in a static target delivered using a 3D RapidArc plan. The results were much improved using the 4D IMAT delivery compared with the 3D plans for a moving target. Conclusion: A 4D RapidArc tracking technique can be implemented and our results indicated that it is feasible to incorporate the intra‐fraction organ motion into a 4D IMAT planning to track the moving targets.

SU-GG-T-246: Systematics Review of Patient-Specific Plan Dose Verification in IMRT and RapidArc Treatments by Ion-Chamber Measurements

Purpose: To review the accuracy and systematic trend of patient‐specific plan dose verification by ion chamber measurements for 272 IMRT and RapidArc clinical cases of brain, spine, head‐and‐neck, prostate, lung,liver and pelvis‐non‐prostate treatments. Methods & Materials: A BrainLAB IMRT QA Phantom and a 0.125 cc ion‐chamber (PTW 31010) was used to measure composite dose at a high‐dose and low‐gradient point near iso‐center. Measured doses are compared to calculated doses in treatment planning systems for a total of 272 clinical cases, of which 203 are IMRT treatments planned in BrainSCAN and 69 are RapidArc treatments planned in Eclipse. Distributions of percentage differences between measured dose and planned dose are reviewed to extract the mean, standard deviation and range of the differences, from which confidence levels of |mean deviation|+1.96×SD are determined and compared to the action levels recommended by AAPM TG‐119. Results: For the 203 IMRT cases, the mean percentage difference was −1.89%, the standard deviation was 1.28%, the range of the differences was −4.67% to 4.17%, the confidence level was determined to be 4.40%. For the 69 RapidArc cases, the mean percentage difference was −0.17%, the standard deviation was 2.38%, range of the differences was −4.76% to 5.15%, the confidence level was determined to be 4.83%. Conclusion: The results of ion‐chamber dose verification for 272 patient‐specific IMRT and RapidArc plans were reviewed. The confidence levels determined from the IMRT cases (4.40%) and the RapidArc cases (4.83%) are better or close to the confidence levels of 4.5%∼4.7% established in AAPM TG‐119 by planning and delivering multiple IMRT plans on a suite of 7 phantom cases among 9 institutions. Significantly more clinical cases will be included for extracting disease‐site specific confidence levels, particularly for RapicArc treatments where DMLC motion speed, gantry speed and dose‐rate are all utilized in plan optimization and delivery.

SU‐GG‐J‐41: Treatment Beam's‐Eye‐View Speed‐Modulated CBCT Imaging Used in a Triple Imaging Modality to Track Moving Tumors during Treatment Delivery

Purpose: To track the target trajectory during the treatment based on the pre‐treatment CBCT projection images when the gantry is selectively slowed down at the treatment angles, the synchronized 3D surface imaging and the EPID in movie mode. Methods and Materials: A gantry‐speed device slows down the CBCT gantry speed (2min/360degrees) by half for a 6 degrees arc centered on the MV treatment beam, allowing us acquire twice as many frames within the treatment arc (22frames/5.5fps). The treatment arc is chosen to accommodate a typical full breathing period (4s). A CBCT kV beam‐on trigger is used to acquire 3D surface images using the AlignRT cameras. Phantom experiments were performed using a programmable respiratory motion platform. During the treatment delivery, MV imaging was performed in movie mode (3fps) while the surface cameras tracked the chest‐wall component. A mutual information algorithm was employed to compare the kV an MV images. Results: CBCT reconstruction was performed using every other frame when in the slow treatment arc for correct reconstruction. The longer scanning times were translated in ∼10–15% additional CBCT dose when 4 beams were used. For simple trajectory, the kV and MV images were phase correlated ∼98% of the time for the high contrast tumor and ∼92% for the low contrast tumor. Phase knowledge provided by the surface cameras increased the later kV‐MV correlation to ∼97%. Patient‐like trajectories were tested where the most irregular trace (3mm baseline‐shift) showed a moderate kV‐MV correlation, enabling one to interrupt the treatment for a second on‐line baseline correction. Conclusion: The selective beams‐eye‐view slow‐arc CBCT technique in conjunction with surface cameras and MV imaging has been demonstrated to be an effective tool for tumor tracking due to its treatment time immediacy and minimal impact on treatment flow.

SU‐DD‐A3‐01: Comparison of Respiration‐Correlated and Uncorrelated Cone‐Beam CT for Correcting Target‐Positioning Errors in Radiotherapy of Thoracic and Abdominal Cancer

Purpose: This study compares respiration‐correlated kV cone beam CT (RC‐kVCBCT) image guidance to standard (uncorrelated) kVCBCT for target localization in radiation treatment of thoracic and abdominal tumors. Method and Materials: In an IRB‐approved study, 9 stage II–IV non‐small cell lung cancer (NSCLC) patients and 1 stage III stomach tumor patient with fiducials received a respiration‐correlated planning CT (RCCT) at simulation and five RC‐CBCTs during the first week of treatment. The GTV volumes ranged from 2.8 cm3 – 286 cm3. Kilovoltage RC‐CBCT scans are acquired (Varian Trilogy) while recording respiration and with reduced gantry speed (0.2 rpm), yielding 3D images at six respiratory phases. The tumor in each phase of the RC‐CBCT is registered to the end‐expiration (EE) phase image to obtain the respiration‐averaged tumor displacement relative to its EE position. This process is repeated in the RCCT image set. The RCCT and RC‐CBCT images at EE are registered to obtain the tumor displacement at the EE position. A vector sum of the three displacements yields the correction to the respiration‐averaged tumor position. For comparison, the correction is also computed by matching an uncorrelated CBCT reconstructed with all projections and representing a respiration‐averaged image, to the average intensity projection image computed from the RCCT. Results: The mean difference between the two correction methods in 10 patients was 1.4mm left‐right (L/R), 1.6mm anterior‐posterior (A/P), and 1.7mm superior‐inferior (S/I). The maximum difference was 5.0mm (L/R), 6.0mm (A/P), 7.0mm (S/I). The GTV range‐of‐motion at simulation ranged from 2–14mm. Conclusion: Our preliminary results suggest that for many thoracic and abdominal tumors, position corrections using uncorrelated, respiration‐averaged 3D imaging are consistent with those from respiration‐correlated imaging; however, respiration‐correlated imaging may be indicated for cases of large tumor motion. Research supported by Varian and by NIH grant T32 CA61801.

MO‐EE‐A3‐05: Assessment of Volumetric Arc Therapy Plans for Constant and Variable Dose Rates

Introduction: Recently, volumetric arc therapy (VMAT) that combines dynamic gantry motion with beam intensity modulation has become clinically available for planning and delivery of dynamic arc IMRT treatments. During VMAT optimization process, several parameters can be changed, including gantry speed and dose rate. The goal of this work was to assess the differences between the VMAT plans generated with constant and variable dose rates. Methods and Materials: Patients with H&N cancer who were treated with step‐and‐shoot IMRT (DMPO) were selected for this study. Plans were generated using Pinnacle's Smart Arc software which supports generation of both constant and variable dose rate VMAT plans. VMAT plans for 100 – 500 constant dose rates (DR) and variable dose rate were generated. Plans consisted of two 360 ° arcs with 4° arc sampling. Plans were compared using DVH parameters for target(s) and OARs and MUs. VMAT plans were also verified in a cylindrical phantom using film and ion chamber. Results: The MUs for DR=100 were 22% lower as compared to the DMPO MUs and increased with the increasing DR. The MUs for DR=400, however, is 21% higher than the DMPO MUs. The MUs for variable dose rate is the lowest (581MUs) as compared to 100 dose rate plan (832MUs). PTV coverages were almost the same up to DR=300 and degraded significantly for 400, 500 and 600. Phantom verification of all plans showed that the calculated and measured ion chamber doses at isocenter were within 4%. Conclusion: This study showed that the constant dose rate VMAT plans up to 300 produced similar plans to the ones obtained with variable dose rate and DMPO plans, in terms of target coverage and critical structure sparing. However, variable dose rate plans still the choice in terms of MU efficiency and plan quality.

Pre-clinical evaluation of respiratory-gated delivery of volumetric modulated arc therapy with RapidArc

A study was carried out to evaluate the possibility of delivering volumetric modulated arc therapy with the RapidArc technology under respiratory-gated conditions. The experiments were performed in the framework of a non-clinically released environment. Plans of six patients, all realized for a single arc, were used for the experiments. The Real-time Position Management™ (RPM) respiratory gating system from Varian was used to generate gate-open signals of different durations. Arcs were delivered applying the different gates creating sequences of beam-hold/beam-on during the dose delivery: the average number of interruptions for a single arc ranged from 0 to 45. Dose prescription was set to 2 Gy and different gate-open periods of 30, 15 and 5 s to keep gantry speed constant at maximum. 5 Gy and 15 Gy doses were then applied to gate-open signals of 5 and 8 s, respectively, to mimic the most challenging conditions of slow gantry rotation and high-frequency interruptions. The 5 and 15 Gy experiments represent dose conditions of clinical interest for stereotactic treatments. For each patient and gating condition, pre-treatment 2D verification measurements were performed using the PTW-729 array in conjunction with the Octavius phantom (PTW, Freiburg); measurements were performed on different days (one per patient, with the complete setup of phantom and detectors every time), while each gating experiment was repeated seven consecutive times for reproducibility (without a new setup of the measurement equipment). Measurements were compared with dose calculations in the treatment planning system (performed without any gating) to appraise the dosimetric impact of the presence of gating and the eventual dependence from the number of interruptions during a single arc. Analysis of machine-registered log files in terms of average deviations between actual and expected positions (from automatic measurements every 50 ms) resulted in mean ΔMU (monitor units) <0.02% for all gating conditions. Δ(Gantry angle) = 0.38 ± 0.01° for 2 Gy (all gate periods), 0.24 ± 0.01° for 5 Gy, and 0.10 ± 0.01° for 15 Gy deliveries. Average deviations for multileaf collimator (MLC) positions (root mean square over all 120 leaves) were 0.45 ± 0.01 mm for 2 Gy (all gate periods), 0.32 ± 0.01 mm for 5 Gy and 0.14 ± 0.01 mm for 15 Gy. Results in terms of dose measurements confirmed that the application of gating to RapidArc delivery does not affect the quality of the dose delivery. With criteria of ΔD = 3%, DTA = 3 mm, the gamma test was passing in a range of 99 to 100% of the measured points for most of the cases (with maximum number of interruptions of about 20 per arc) and from 97 to 98% for the extreme case of 15 Gy and 8 s gate-open signal (corresponding to almost 50 interruptions per arc). In conclusion, RapidArc delivery proved, in a pre-clinical phase and non-clinically released framework, to be reliable and dosimetrically accurate also when applied in conjunction with gating procedures.

Volumetric modulated arc therapy for delivery of hypofractionated stereotactic lung radiotherapy: A dosimetric and treatment efficiency analysis

Purpose/objective(s) Volumetric modulated arc therapy (VMAT) allows for intensity-modulated radiation delivery during gantry rotation with dynamic MLC motion, variable dose rates and gantry speed modulation. We compared VMAT plans with 3D-CRT for hypofractionated lung radiotherapy. Materials/methods Twenty-one 3D-CRT plans for Stage IA lung cancer previously treated stereotactically were selected. VMAT plans were generated by optimizing machine aperture shape and radiation intensity at 10° intervals. A partial arc range of 180° was manually selected to coincide with tumor location. The arc was resampled down to 5° intervals to ensure dose calculation accuracy. Identical planning objectives were used for VMAT/3D-CRT. Parameters assessed included dose to PTV and organs-at-risk (OAR), monitor units, and multiple conformity and homogeneity indices. Plans were delivered to a phantom for time comparison. Results Lung V 20/12.5/10/5 were less with VMAT (relative reduction 4.5%, p = .02; 3.2%, p = .01; 2.6%, p = .01; 4.2%, p = .03, respectively). Mean/maximum-doses to PTV, dose to additional OARs, 95% isodose line conformity, and target volume homogeneity were equivalent. VMAT improved conformity at both the 80% (1.87 vs. 1.93, p = .08) and 50% isodose lines (5.19 vs. 5.65, p = .01). Treatment times were reduced significantly with VMAT (mean 6.1 vs. 11.9 min, p < .01). Conclusions Single arc VMAT planning achieves highly conformal dose distributions while controlling dose to critical structures, including significant reduction in lung dose volume parameters. Employing a VMAT technique decreases treatment times by 37–63%, reducing the chance of error introduced by intrafraction variation. The quality and efficiency of VMAT is ideally suited for stereotactic lung radiotherapy delivery.

ペースメーカ植込み患者における被ばく低減を考慮した冠動脈ＭＤＣＴの撮影法

We proposed a new acquisition method of coronary MDCT achieved by pacing rate resetting and/or propranolol or verapamil injection in patients with a pacemaker. Coronary MDCT was undertaken in 57 patients with a pacemaker (DDD: 51, VVI: 6) and in 2975 patients with sinus rhythm as control using Aquilion 64 (Toshiba). Pacing rate was reset to 60 beats per minute (bpm) in DDD, and spontaneous beats were suppressed by propranolol injection. Pacing rate was reset to 70 bpm in atrial fibrillation with VVI, and spontaneous beats were suppressed by verapamil injection. Coronary MDCT was undertaken using as high a beam pitch (BP) as possible. When spontaneous beats were not suppressed, we selected the optimal gantry speed and BP to get the highest temporal resolution. Image quality makes no significant difference between pacemaker and sinus rhythm. When spontaneous beats were completely suppressed (all pacing), mean radiation dose and acquisition time, respectively, decreased by 33.0% and 35.2% in DDD compared with the method recommended by Heart Navi (by Toshiba), and they decreased by 38.1% and 25.9%, respectively, in VVI compared with the method recommended by Heart Navi. We could not estimate coronary stenosis in the proximal right coronary artery by lead artifacts in 30% of DDD pacemakers. In conclusion, the new method is useful for not only reducing radiation dose and acquisition time, but also for maintaining image quality in patients with a pacemaker.

Treatment planning for volumetric modulated arc therapy

Volumetric modulated arc therapy (VMAT) is a specific type of intensity-modulated radiation therapy (IMRT) in which the gantry speed, multileaf collimator (MLC) leaf position, and dose rate vary continuously during delivery. A treatment planning system for VMAT is presented. Arc control points are created uniformly throughout one or more arcs. An iterative least-squares algorithm is used to generate a fluence profile at every control point. The control points are then grouped and all of the control points in a given group are used to approximate the fluence profiles. A direct-aperture optimization is then used to improve the solution, taking into account the allowed range of leaf motion of the MLC. Dose is calculated using a fast convolution algorithm and the motion between control points is approximated by 100 interpolated dose calculation points. The method has been applied to five cases, consisting of lung, rectum, prostate and seminal vesicles, prostate and pelvic lymph nodes, and head and neck. The resulting plans have been compared with segmental (step-and-shoot) IMRT and delivered and verified on an Elekta Synergy to ensure practicality. For the lung, prostate and seminal vesicles, and rectum cases, VMAT provides a plan of similar quality to segmental IMRT but with faster delivery by up to a factor of 4. For the prostate and pelvic nodes and head-and-neck cases, the critical structure doses are reduced with VMAT, both of these cases having a longer delivery time than IMRT. The plans in general verify successfully, although the agreement between planned and measured doses is not very close for the more complex cases, particularly the head-and-neck case. Depending upon the emphasis in the treatment planning, VMAT provides treatment plans which are higher in quality and/or faster to deliver than IMRT. The scheme described has been successfully introduced into clinical use.

Evaluation of the Change in Respiration Motion using a Standard Elekta Cone Beam CT

Using 4D-CT the respiration induced tumor motion can be measured and used in the treatment planning. However, the measured tumor motion might not be representative for the motion during the entire fractionated treatment course. If a standard Cone Beam CT (CBCT) scan, acquired before the treatment, could be time resolved, this could be used to verify the size of the intra-fractional motion during the treatment course. Sonke et al (IJROBP 2008-70,590) has demonstrated that it is possible to track a tumor without mediastinal infiltration using a slow gantry speed 4D-CBCT.

Patient‐specific quality assurance method for VMAT treatment delivery

Volumetric modulated arc therapy (VMAT) is a system for intensity-modulated radiotherapy treatment delivery that achieves high dose conformality by optimizing the dose rate, gantry speed, and the leaf positions of the dynamic multileaf collimator (DMLC). The aim of this work is to present a practical approach for patient-specific volumetric reconstruction of the dose delivered of a VMAT treatment using the DMLC and treatment controller log (Dynalog) files. The accuracy of VMAT delivery was analyzed for five prostate patients. For each patient, a clinical treatment was delivered and values recorded in the log files for the gantry angle, dose rate, and leaf positions were converted to a new DICOM-compliant plan using a custom-developed software system. The plan was imported in a treatment planning system and the dose distribution was recreated on the original CT by simply recomputing the dose. Using the standard evaluation tools, it is straightforward to assess if reconstructed dose meets clinical endpoints, as well as to compare side-by-side reconstructed and original plans. The study showed that log files can be directly used for dose reconstruction without resorting to phantom measurements or setups. In all cases, analysis of the leaf positions showed a maximum error of -0.26 mm (mean of 0.15 mm). Gantry angle deviation was less than 1degree and the total MU was within 0.5 from the planned value. Differences between the reconstructed and the intended dose matrices were less than 1.46% for all cases. Measurements using the MATRIXX system in a phantom were used to validate the dosimetric accuracy of the proposed method, with an agreement of at least 96% in all pixels as measured using the gamma index. The methodology provides a volumetric evaluation of the dose reconstructed by VMAT plans which is easily achieved by automated analysis of Dynalog files without additional measurements or phantom setups. This process provides a valuable platform for adaptive therapy in the future.

Poster — Wed Eve—29: Validation of VMAT Delivery Using a Commercial 2D Diode Array

Volumetric modulated arc therapy (VMAT) is a new modality that combines IMRT with rotational delivery. RapidArc (Varian Medical Systems) incorporates the capability to vary dose rate, gantry speed and dynamic multileaf collimators to optimize dose and delivery efficiency. While new systems are being developed for VMAT verification, diode arrays are efficient and convenient tools for beam‐wise verification of IMRT delivery, and also have been employed in composite rotational delivery verification. Mapcheck2 (Sun Nuclear Corporation) is a new diode array with 1527 detectors. In this study, we investigate the use of Mapchekc2 in the MapPhan solid water phantom to validate VMAT delivery. 5 VMAT plans were generated in Eclipse using the RapidArc optimizer. All plans were delivered several times to verify consistency of delivery and measurement. Gamma analysis was used to verify the results compared to calculated dose distributions. The system was found to be sensitive to small changes in position or orientation. RapidArc beam delivery has been verified to correspond well with calculated dose distributions. Gamma values were below 1 in greater than 90% of measured points for all treatment plans. Delivery was found to be insensitive to initial dose rate. The MapPhan, with Mapcheck2 2D array in solidwater is a convenient way to verify VMAT plans. MapPhan results showed that RapidArc delivery is stable and reproducible.

Some considerations concerning volume-modulated arc therapy: a stepping stone towards a general theory

In this paper it is formally shown that the dynamic multileaf collimator (MLC) IMRT delivery technique remains valid if the MLC is supported on a 1D moving platform. It is also shown that, in such circumstances, it is always time preferable to deliver overlapping modulating fields as a single swept field rather than as separate fields. The most general formulism is presented and then related to simpler equations in limiting cases. The paper explains in detail how a ‘small-arc approximation’ can be invoked to relate the 1D linear theory to the MLC-on-moving-platform-(gantry) delivery technique involving rotation therapy and known as volume-modulated arc therapy (VMAT). It is explained how volume-modulated arc therapy delivered with open unmodulated fields and which can deliver conformal dose distributions can be interpreted as an IMRT delivery. The (Elekta adopted) term VMAT will be used in a generic sense to include a similar (Varian) method known as RapidArc. Approximate expressions are derived for the ‘amount of modulation’ possible in a VMAT delivery. This paper does not discuss the actual VMAT planning but gives an insight at a deep level into VMAT delivery. No universal theory of VMAT is known in the sense that there is no theory that can predict precisely the performance of a VMAT delivery in terms of the free parameters available (variable gantry speed, variable fluence-delivery rate, set of MLC shapes, MLC orientation, number of arcs, coplanarity versus non-coplanarity, etc). This is in stark contrast to the situation with several other IMRT delivery techniques where such theoretical analyses are known. In this paper we do not provide such a theory; the material presented is a stepping stone on the path towards this.

SU-FF-T-233: Commissioning and Quality Assurance of VMAT with An Ion Chamber Profiler

Purpose: To develop efficient and quantitative commissioning and quality assurance (QA) procedures for volumetric modulated arc therapy (VMAT) with an ion chamber profiler. Method and Materials: Rotational treatment techniques such as VMAT utilize variable dose rate, gantry speed and dynamic MLC to achieve optimal dose distribution. We designed QA test procedures to check machine output and beam characteristics variation with variable dose rate and gantry speed. MLC leaf positioning accuracy and leaf speed control were also checked. A linear ion chamber profiler was rigidly attached to the gantry head for all measurements. The profiler measured 4 profiles simultaneously: x‐axis, y‐axis and two diagonal lines. Dose rate was changed with fixed gantry or during arc rotation. Gantry speed was changed by varying the dose rate for the same arc beam delivery. To test MLC movement, MLC openings decreased from 32×32 cm2 to 4×4 cm2 during 90° arc rotation. The arc beam was repeated with same dose rate but different total MU, thereby changing the leaf speed. In each test, machine output and beam characteristics (flatness, symmetry and field width) were evaluated. Results: Beam remained stable except at very low dose rate (32 MU/min). Output varied from −1.6% to 2.4% at different gantry angles and dose rates. In gantry rotation speed test, gantry speed varies from 0.26 to 8.32 degree/sec. Beam was stable with maximum output change of 0.43%. In MLC leaf test, the difference between measured and expected leaf positions were within 1 mm for all the leaf speeds tested. Conclusion: The ion chamber profiler is an efficient and convenient tool for quantifying mechanical and dosimetric accuracy for rotational therapy. VMAT machine beam and output are generally stable with variable dose rate and gantry speed. Variations were observed at very low dose rate. MLC leaf positioning accuracy was within tolerance.

SU-FF-T-174: Evaluation of a New Dynamic Arc IMRT Optimization

Purpose: The quality and accuracy of delivery of both prostate and esophagus plans generated using of a new dynamic arc optimization algorithm (SmartArc) within the Pinnacle TPS have been evaluated and compared with standard step-and-shoot (SS) techniques. Method and Materials: Two types of single-arc plans have been evaluated, one that is delivered using a constant dose-rate (CDR) and gantry speed, and one that is delivered using both a variable dose-rate and gantry speed (VMAT). The former can be delivered on most linear accelerators; the latter requires an accelerator that has been licensed for VMAT treatments. Prostate plans were all optimized following the UK CHHIP trial protocol, which has three PTV dose levels. Esophagus plans were optimized following a local protocol. In both cases plans were created for Varian accelerators equipped with a 120-leaf MLC. The delivery of plans was evaluated using Delta-4 (ScandiDos). Results: In all prostate cases plans met the target coverage criteria. The CDR arc plans tended to give the highest hot-spot. For the prostate plans the CDR arc plans and SS plans gave approximately equivalent rectal and bladder DVHs, with the arc-plan femoral head DVHs being better. The VMAT plans gave improved DVHs for all outlined organs-at-risk for prostate plans. For esophagus plans, the CDR arc plans struggled to meet the target coverage criteria, whereas the VMAT plans gave acceptable coverage and improved organ-at-risk sparing. In all cases the measured quality assurance results were good with no significant differences between the different delivery methods. For the VMAT plans delivery time was 75–90 seconds and 2.5–3.0 minutes for the CDR-arc plans. Conclusion: The Pinnacle SmartArc optimizer provides a very useful tool for generating accurately deliverable IMRT plans on a wide range of accelerators. Conflict of Interest: This work was performed in collaboration with Philips Radiation Oncology Systems

SU‐FF‐T‐98: Evaluation of RapidArc for Locoregional Radiotherapy of Left‐Sided Breast Cancer: Comparison with Conventional Intensity Modulated Radiotherapy(cIMRT)

Purpose: RapidArc is a novel volumetric modulated arc technique with dynamically variable MLC positions, dose rate and gantry speed. RapidArc is highly efficient and has the potential to reduce monitor units and treatment times compared to cIMRT. This report presents our first experience in performing treatment planning for breast cancer using Eclipse RapidArc optimization software 8.4.10 that supports multiple arcs. RapidArc plans were compared with cIMRT for locoregional radiotherapy of left‐sided breast cancer, including the internal mammary nodes (IMN). Method and Materials: 4 patients previously treated with cIMRT to the left breast/chest wall and regional nodes, including internal mammary chain, were planned (50Gy/25fractions) using the same contours and CT dataset with the RapidArc technique. Different RapidArc planning strategies were investigated. The endpoints of the comparison between RapidArc and cIMRT plans were breast PTV homogeneity, conformity of the plans, heart V30, left lung V20, mean doses to surrounding structures, number of monitor units (MUs) and treatment time. Results: 2 arcs were used due to the large treatment volumes to be optimized and the finite travel range of the multileaf collimator (MLC). Treatment plans generated using RapidArc optimization resulted in similar PTV homogeneity as compared with cIMRT. The best RapidArc plans (2 short arcs of 190 degrees each) reduced the mean dose to heart from 11.2±1.0Gy to 9.7±0.7Gy and ipsilateral lung from 14.5±0.8 to 13.6±0.6Gy. The number of MUs was reduced by 50% and the mean beam on time was reduced from 5.85±1.04 minutes to 1.4±0.05 minutes for RapidArc as compared with cIMRT. Conclusions: RapidArc optimization employing 2 arcs is feasible and yielded similar dosimetric plan quality as compared to cIMRT. RapidArc reduced the MUs and significantly reduced the beam on time as compared to cIMRT.

SU-FF-T-649: A Comparison of Constant Dose Rate SmartArc and Helical Tomotherapy

Purpose: The purpose of this study was to compare constant dose rate and gantry rotation speed intensity modulated arc delivery planned with the new Pinnacle SmartArc planning system and delivered on a Varian 21EX with helical tomotherapy (HT). Materials and Methods: A series of thirty patients that were treated with HT were randomly selected and re‐planned for SmartArc treatment delivery on a Varian 21EX equipped with a 120‐leaf MLC. All SmartArc plans were optimized for delivery using a constant dose rate and gantry speed, which eliminated the need for special VMAT licensees. The same contours, objectives, and prescriptions were used for the HT and SmartArc plans. SmartArc dosimetric verification was performed using a cylindrical solid water phantom with radiographic film and with an ionization chamber array. The MLC log files for the SmartArc treatment deliveries were also downloaded and analyzed. Results: 25 of the test cases were planned with a single arc, while 5 cases were planned with double arcs. The mean delivery time was 2.5 minutes, and the mean number of monitor units was 547. For the prostate test cases, the SmartArc plans were remarkably similar to the tomotherapy plans. The dose uniformity of the PTV for the SmartArc cases was actually superior to the tomotherapy dose uniformity for most of the prostate cases. For both the lung cases and H&N cases, the PTV uniformity and normal tissue DVHs were slight worse in the SmarArc plans as compared to the HT plans. Conclusion: SmartArc treatment plans can be created for delivery using a constant dose rate and gantry rotation speed that are very similar to helical tomotherapy plans for prostate, lung, and simple H&N patients. These plans can be delivered in 2–3 minutes per arc without the need to purchase special VMAT licenses on most accelerators.

SU-FF-T-218: Volumetric Patient-Specific Quality Assurance for VMAT Treatment Delivery Using the Dynalog Files

Purpose: The Volumetric Modulated Arc Therapy (VMAT) technology is a novel delivery method that is capable of producing highly conformal dose distributions though concomitant optimization of MLC shapes, dose rate and gantry speed. The aim of this work is to present a practical approach for patient—specific volumetric reconstruction of the dose delivered of a VMAT treatment using the DMLC and treatment controller log (Dynalog) files. Method: The accuracy of VMAT delivery was analyzed for prostate patients. For each patient, a clinical treatment was reconstructed and values recorded in the log files for the gantry angle, dose rate and leaf positions were converted to a new DICOM-compliant plan using a custom-developed software system. The plan was imported in a treatment planning system and the reconstructed dose was recreated on the original CT by simply recomputing the dose. Reconstructed and planned doses together with original CT scan and delineated structures are in the same coordinate systems and can be combined directly or verified through dose-volume histograms to asses treatment objective degradation due to machine imprecision. Results: In all cases, analysis of the leaf positions showed a maximum error of −0.26 mm (mean 0.15 mm). Gantry speed deviation was less than 1° and the delivered MU was within 0.001 % from the planned value. Measurements using the Matrixx system in a phantom were used to validate the dosimetric accuracy of the proposed method, with an agreement of at least 96% using the gamma index. Conclusions: The methodology provides an volumetric evaluation of the dose delivered by VMAT plans, easily achieved by automated analysis of Dynalog files without additional measurements or phantom setups. It provides a valuable platform for adaptive therapy in future.

TU‐E‐304A‐06: A 3D Solution for Advanced Photon Arc Therapy Quality Assurance

Purpose: Recently, radiation treatment technology has out‐paced the advances in QA technology, and a critical gap has been created within the arc motion delivery systems such as Rapid Arc®, High Art®, and VMAT®. Gantry position, speed and MU rate create extra degrees of complexity unaccounted for by several traditional 2D QA verifications due to directional dependence and/or inadequate spatial resolution. We present the 1st 3D dosimetry verification of a Rapid Arc treatment and general dosimetry technique based on a radio‐chromic plastic (PRESAGE™)/optical‐CT (OCTOPUS™) combination suitable for today's complex treatment technology. Method and Materials: A radio‐chromic plastic cylinder 17cm diameter × 11cm height was used to verify a simplistic Rapid Arc 6MV prostate plan. The radiation induced OD change (proportional to dose delivered) was acquired by an optical‐CT scanner with a voxel size of 1×1×2.5mm3. The measured distribution was then compared with the corresponding dose distribution calculated by the treatment planning system (Eclipse® AAA, voxel size 2.5×2.5×2.5mm3). Comparisons between the two dose distributions were made using dose profiles and stacked 2D gamma maps (with criteria 3% dose difference and 3mm distance to agreement) for a quasi 3D gamma volume. Results: The 3D dose distribution measured profiles in the dosimeter showed agreement amongst the calculated treatment plan down to lower dose regions (down to ∼35%). Gamma map comparisons show the dosimeter measurements generally agree with the calculated treatment plan with a few low dose bath problem areas superior and inferior. More traditional QA techniques showed the same discrepancy areas of the plan. This aided in the verification of the system commissioning process. Conclusion: Radio‐chromic plastic/optical‐CT dosimetry techniques are capable of providing 3D, high spatial resolution verification for patient QA with advanced treatment techniques or commissioning, and the potential for its use at other treatment sites where the fluence is more complex.