Coplanar Arcs Research Articles

SU‐F‐T‐641: Comparative Study On Planning and Delivery Efficiency of Volumetric Modulated Arc Therapy Between Varian and Elekta Platforms for SBRT Lung Treatment

Purpose:To compare the plan quality between Eclipse (Varian) and Monaco (Elekta) TPS. To ascertain, if SBRT lung treatment could be delivered in a single coplanar arc (360 degrees) with both Elekta and Varian platforms. To assess if the smaller leaf width in Varian Millennium and Elekta Agility MLC heads have a dosimetric advantage over Elekta MLCi2 headMethods:Ten SBRT lung patients (PTV volumes ranging from 11 cc to 103cc) who were previously treated on Varian Linac with non‐coplanar arcs and received 50Gy in 5 fractions were chosen for this study. The patients were replanned in Eclipse TPS (AAA algorithm) using a 360 degree coplanar single arc (SA) delivery technique and 2 partial complimentary 180 degree arcs (PA). Treatment planning using single coplanar arc (360 degree arc) was also done on Monaco TPS (Montecarlo) for both Agility (160 leaf) and MLCi2 (80 leaf) Elekta MLC headsResults:The average monitor units to deliver 10 Gy across all delivery methods were 3000 ± 474 MU and did not vary with PTV size. Coplanar single arc and partial arc techniques did not compromise either the RTOG 0813 or 0915 low dose spillage criteria for R50% or the maximum dose to any point 2cm away from the PTV. OAR doses to spinal cord, heart, great vessels, esophagus, rib and lung were comparable on both Eclipse (Varian) and Monaco (Elekta) platforms regardless of the delivery method.Conclusion:SBRT lung tumors can be treated with a single coplanar 360 degree arc in both Varian and Elekta platforms. Non coplanar arcs and increasing arc degrees more than 360 degrees had no benefit in this study regardless of the volume of PTV. 0.5 cm leaf width used in Millennium and Agility MLC heads had no significant dosimetric improvement over 1 cm leaves in the MLCi2 head.

MO-FG-CAMPUS-TeP2-05: Optimizing Stereotactic Radiosurgery Treatment of Multiple Brain Metastasis Lesions with Individualized Rotational Arc Trajectories

Purpose: Radiosurgery of multiple (n>4) brain metastasis lesions requires 3–4 noncoplanar VMAT arcs with excessively high monitor units and long delivery time. We investigated whether an improved optimization technique would decrease the needed arc numbers and increase the delivery efficiency, while improving or maintaining the plan quality. Methods: The proposed 4pi arc space optimization algorithm consists of two steps: automatic couch angle selection followed by aperture generation for each arc with optimized control points distribution. We use a greedy algorithm to select the couch angles. Starting from a single coplanar arc plan we search through the candidate noncoplanar arcs to pick a single noncoplanar arc that will bring the best plan quality when added into the existing treatment plan. Each time, only one additional noncoplanar arc is considered making the calculation time tractable. This process repeats itself until desired number of arc is reached. The technique is first evaluated in coplanar arc delivery scheme with testing cases and then applied to noncoplanar treatments of a case with 12 brain metastasis lesions. Results: Clinically acceptable plans are created within minutes. For the coplanar testing cases the algorithm yields singlearc plans with better dose distributions than that of two-arc VMAT, simultaneously with a 12–17% reduction in the delivery time and a 14–21% reduction in MUs. For the treatment of 12 brain mets while Paddick conformity indexes of the two plans were comparable the SCG-optimization with 2 arcs (1 noncoplanar and 1 coplanar) significantly improved the conventional VMAT with 3 arcs (2 noncoplanar and 1 coplanar). Specifically V16 V10 and V5 of the brain were reduced by 11%, 11% and 12% respectively. The beam delivery time was shortened by approximately 30%. Conclusion: The proposed 4pi arc space optimization technique promises to significantly reduce the brain toxicity while greatly improving the treatment efficiency.

SU-G-BRC-06: Evaluation of a Novel Radiosurgery Software for Treating Multiple Brain Metastases Simultaneously in a Single Fraction

Purpose: To evaluate a new, automated brain metastases planning software designed to treat up to ten brain metastases simultaneously. Methods: We treated 61 patients with multiple brain metastases using the Elements software by BrainLab (Munich, Germany). Patients had between 2–10 metastases ranging from 0.01–8.64 cc. Dose prescription was 18–24 Gy. Plans use up to 5 non-coplanar arcs with a single isocenter at the metastases’ center of mass. The high degree of automation shortens the planning time to 15–20 minutes per patient.For comparison we planned 21 of the patients using Rapid Arc (Varian, Palo Alto CA) (RA). We used two coplanar arcs so as to keep planning times comparable to the Elements. We also planned 8 patients using iPlan software (BrainLab). We compared conformity index (CI), volume of brain receiving over 12 Gy (V12) and mean brain dose (MBD) for the three different planning systems (TPSs). Results: Plans from all TPSs were judged clinically acceptable. V12 and MBD were not statistically significantly different between TPSs.CI between RA and Elements was similar, however for iPlan CI was significantly worse compared to both RA and Elements (p<0.001). RA plans took approximately 40 minutes to plan (despite fusion and contouring being done in the Elements), and iPlan plans over an hour each. Delivery times were approximately 30 minutes for Elements, 10 minutes for RA, and up to 300 minutes for iPlan. Conclusion: Elements plans had good CI values and low brain doses. While treatment times for Elements were longer than for RA, 30 minutes is a significant improvement over conventional radiosurgery techniques where each metastasis is treated individually and delivery times to 10 metastases are close to 300 minutes.BrainLab Elements is a novel software allowing fast, automated planning and efficient irradiation of multiple brain metastases with minimal dose to healthy brain.

SU-F-T-607: Study of Dose Falloff Slope in RapidArc Planning of Lung SBRT

Purpose: SBRT of lung tumor results in steep dose fall-off outside the PTV to limit the normal tissue dose. RTOG report 0915 recommended metrics for plan quality measure, CI, D2cm, R50% and HDloc were evaluated. This study was aimed at ascertaining moderate to low dose spillage using 5 mm ring structures outside the PTV. Methods: In this retrospective study, 41 clinical RapidArc lung SBRT plans (Eclipse TPS ver 8.9) previously treated at our institution in a TrueBeam STx linac were analyzed. The delivered plans were re-planed using either 2 or 3 complete or partial arcs in coplanar or non-coplanar arrangement with upto ±15 degree couch angle. Seven concentric 5 mm wide rings were created outside the PTV for evaluation of dose fall-off based on mean, maximum doses as a function of distance. Mean and maximum percent dose fall-off in the rings (% dose/mm) were compared between the delivered and research plans. The differences in the mean and maximum percent doses between coplanar and non-coplanar arc plans were studied for any significant differences. Results: Mean dose falloff in the rings outside the PTV followed an exponential curve. For all the patients, interestingly the slope of maximum dose falloff increased at the intersection of 1st and 2nd ring and monotonically fell at outer rings. Altering coplanar to non-coplanar arc arrangements had a minimal impact on the CI as well as D2cm, R50% values (p-value > 0.05). Conclusion: Slope of dose falloff from the PTV surface could serve as an additional metric. Coplanar arc plan had slightly steep dose falloff outside the PTV than non-coplanar arcs, but the differences are statistically insignificant.

SU-F-T-613: Multi-Lesion Cranial SRS VMAT Plan Quality

Purpose: Cranial SRS VMAT plans must have steep dose gradient around each target to reduce dose to normal brain. This study reports on the correlation between gradient index (GI=V50%/V100%), target size and target dose heterogeneity index (HI=PTV Dmax/prescription dose) for multi-lesion cranial SRS VMAT plans. Methods: VMAT plans for 10 cranial cases with 3 to 6 lesions (total 39 lesions) generated in Varian Eclipse V11.0.47 with a fine-tuned AAA beam model and 0.125 cm dose grid were analyzed. One or two iso centers were used depending on the spatial distribution of lesions. Two to nine coplanar and non-coplanar arcs were used per isocenter. Conformity index (CI= V100%/VPTV), HI, and GI were determined for each lesion. Dose to critical structures were recorded. Results: Lesion size ranged from 0.05–11.00 cm3. HI ranged from 1.2–1.4, CI ranged from 1.0–2.8 and GI from 3.1–8.4. Maximum dose to brainstem, chiasm, lenses, optic nerves and eyes ranged from 120–1946 cGy, 47–463 cGy, 9–121 cGy, 14–512 cGy, and 17–294 cGy, respectively. Brain minus PTV (Brain-PTV) V7Gy was in the range 1.1–6.5%, and Brain-PTV Dmean was in the range 94–324 cGy. Conclusion: This work shows that a GI 0.4cc. For smaller lesions, GI increases rapidly. GI is lower when HI is increased. Based on this study, recommend HI is 1.4, and recommended GI is for volumes 1.0cc GI<4. CI is < 1.3 for all lesions except for targets < 0.1cc. Cranial SRS VMAT plans must be optimized to lower the GI to reduce the dose to normal brain tissue.

Comparison of volumetric‐modulated arc therapy and dynamic conformal arc treatment planning for cranial stereotactic radiosurgery

The aim was to analyze arc therapy techniques according to the number and position of the brain lesions reported by comparing dynamic noncoplanar conformal arcs (DCA), two coplanar full arcs (RAC) with volumetric‐modulated arc therapy (VMAT), multiple noncoplanar partial arcs with VMAT (RANC), and two full arcs with VMAT and 10° table rotation (RAT). Patients with a single lesion (n = 10), multiple lesions (n = 10) or a single lesion close to organs at risk (n = 5) and previously treated with DCA were selected. For each patient, the DCA treatment was replanned with all VMAT techniques. All DCA plans were compared with VMAT plans and evaluated in regard to the different quality indices and dosimetric parameters. For single lesion, homogeneity index (HI) better results were found for the RANC technique (0.17±0.05) compared with DCA procedure (0.27±0.05). Concerning conformity index (CI), the RAT technique gave higher and better values (0.85±0.04) compared with those obtained with the DCA technique (0.77±0.05). DCA improved healthy brain protection (8.35±5.61 cc vs. 10.52±6.40 cc for RANC) and reduced monitor unit numbers (3046±374 MU vs. 4651±736 for RANC), even if global room occupation was higher. For multiple lesions, VMAT techniques provided better HI (0.16) than DCA (0.24±0.07). The CI was improved with RAT (0.8±0.08 for RAT vs. 0.71±0.08 for DCA). The V10Gy healthy brain was better protected with DCA (9.27±4.57 cc). Regarding the MU numbers: RANC<RAT<RAC<DCA. For a single lesion close to OAR, RAT achieved high degrees of homogeneity (0.27±0.03 vs. 0.53±0.2 for DCA) and conformity (0.72±0.06 vs. 0.56±0.13 for DCA) while sparing organs at risk (Dmax=12.36±1.05 Gy vs. 14.12±0.59 Gy for DCA, and Dmean=3.96±3.57 Gy vs. 14.72±3.28 Gy for DCA). On the other hand, MU numbers were lower with DCA (2254±190 MU vs. 3438±457 MU for RANC) even if overall time was inferior with RAC. For a single lesion, DCA provide better plan considering low doses to healthy brain even if quality indexes are better for the others techniques. For multiple lesions, RANC seems to be the best compromise, due to the ability to deliver a good conformity and homogeneity plan while sparing healthy brain tissue. For a single lesion close to organs at risk, RAT is the most appropriate technique.PACS numbers: 87.55. dk, 87.56.bd

Treatment planning strategy for whole-brain radiotherapy with hippocampal sparing and simultaneous integrated boost for multiple brain metastases using intensity-modulated arc therapy

PurposeTo retrospectively evaluate the accuracy, plan quality and efficiency of intensity-modulated arc therapy (IMAT) for hippocampal sparing whole-brain radiotherapy (HS-WBRT) with simultaneous integrated boost (SIB) in patients with multiple brain metastases (m-BM). Materials and methodsA total of 5 patients with m-BM were retrospectively replanned for HS-WBRT with SIB using IMAT treatment planning. The hippocampus was contoured on diagnostic T1-weighted magnetic resonance imaging (MRI) which had been fused with the planning CT image set. The hippocampal avoidance zone (HAZ) was generated using a 5-mm uniform margin around the paired hippocampi. The m-BM planning target volumes (PTVs) were contoured on T1/T2-weighted MRI registered with the 3D planning computed tomography (CT). The whole-brain planning target volume (WB-PTV) was defined as the whole-brain tissue volume minus HAZ and m-BM PTVs. Highly conformal IMAT plans were generated in the Eclipse treatment planning system for Novalis-TX linear accelerator consisting of high-definition multileaf collimators (HD-MLCs: 2.5-mm leaf width at isocenter) and 6-MV beam. Prescription dose was 30Gy for WB-PTV and 45Gy for each m-BM in 10 fractions. Three full coplanar arcs with orbit avoidance sectors were used. Treatment plans were evaluated using homogeneity (HI) and conformity indices (CI) for target coverage and dose to organs at risk (OAR). Dose delivery efficiency and accuracy of each IMAT plan was assessed via quality assurance (QA) with a MapCHECK device. Actual beam-on time was recorded and a gamma index was used to compare dose agreement between the planned and measured doses. ResultsAll 5 HS-WBRT with SIB plans met WB-PTV D2%, D98%, and V30Gy NRG-CC001 requirements. The plans demonstrated highly conformal and homogenous coverage of the WB-PTV with mean HI and CI values of 0.33 ± 0.04 (range: 0.27 to 0.36), and 0.96 ± 0.01 (range: 0.95 to 0.97), respectively. All 5 hippocampal sparing patients met protocol guidelines with maximum dose and dose to 100% of hippocampus (D100%) less than 16 and 9Gy, respectively. The dose to the optic apparatus was kept below protocol guidelines for all 5 patients. Highly conformal and homogenous radiosurgical dose distributions were achieved for all 5 patients with a total of 33 brain metastases. The m-BM PTVs had a mean HI = 0.09 ± 0.02 (range: 0.07 to 0.19) and a mean CI = 1.02 ± 0.06 (range: 0.93 to 1.2). The total number of monitor units (MU) was, on average, 1677 ± 166. The average beam-on time was 4.1 ± 0.4 minute . The IMAT plans demonstrated accurate dose delivery of 95.2 ± 0.6%, on average, for clinical gamma passing rate with 2%/2-mm criteria and 98.5 ± 0.9%, on average, with 3%/3-mm criteria. ConclusionsAll hippocampal sparing plans were considered clinically acceptable per NRG-CC001 dosimetric compliance criteria. IMAT planning provided highly conformal and homogenous dose distributions for the WB-PTV and m-BM PTVs with lower doses to OAR such as the hippocampus. These results suggest that HS-WBRT with SIB is a clinically feasible, fast, and effective treatment option for patients with a relatively large numbers of m-BM lesions.

Potential for reduced radiation-induced toxicity using intensity-modulated arc therapy for whole-brain radiotherapy with hippocampal sparing.

The purpose of this study was to retrospectively investigate the accuracy, plan quality, and efficiency of using intensity‐modulated arc therapy (IMAT) for whole brain radiotherapy (WBRT) patients with sparing not only the hippocampus (following RTOG 0933 compliance criteria) but also other organs at risk (OARs). A total of 10 patients previously treated with nonconformal opposed laterals whole‐brain radiotherapy (NC‐WBRT) were retrospectively replanned for hippocampal sparing using IMAT treatment planning. The hippocampus was volumetrically contoured on fused diagnostic T1‐weighted MRI with planning CT images and hippocampus avoidance zone (HAZ) was generated using a 5 mm uniform margin around the hippocampus. Both hippocampi were defined as one paired organ. Whole brain tissue minus HAZ was defined as the whole‐brain planning target volume (WB‐PTV). Highly conformal IMAT plans were generated in the Eclipse treatment planning system for Novalis TX linear accelerator consisting of high‐definition multileaf collimators (HD‐MLCs: 2.5 mm leaf width at isocenter) and 6 MV beam for a prescription dose of 30 Gy in 10 fractions following RTOG 0933 dosimetric criteria. Two full coplanar arcs with orbits avoidance sectors were used. In addition to RTOG criteria, doses to other organs at risk (OARs), such as parotid glands, cochlea, external/middle ear canals, skin, scalp, optic pathways, brainstem, and eyes/lens, were also evaluated. Subsequently, dose delivery efficiency and accuracy of each IMAT plan was assessed by delivering quality assurance (QA) plans with a MapCHECK device, recording actual beam‐on time and measuring planed vs. measured dose agreement using a gamma index. On IMAT plans, following RTOG 0933 dosimetric criteria, the maximum dose to WB‐PTV, mean WB‐PTV D2%, and mean WB‐PTV D98% were 34.9±0.3 Gy,33.2±0.4 Gy, and 26.0±0.4 Gy, respectively. Accordingly, WB‐PTV received the prescription dose of 30 Gy and mean V30 was 90.5%±0.5%. The D100%, and mean and maximum doses to hippocampus were 8.4±0.3 Gy,11.2±0.3 Gy, and 15.6±0.4 Gy, on average, respectively. The mean values of homogeneity index (HI) and conformity index (CI) were 0.23×0.02 and 0.96×0.02, respectively. The maximum point dose to WB‐PTV was 35.3 Gy, well below the optic pathway tolerance of 37.5 Gy. In addition, compared to NC‐WBRT, dose reduction of mean and maximum of parotid glands from IMAT were 65% and 50%, respectively. Ear canals mean and maximum doses were reduced by 26% and 12%, and mean and maximum scalp doses were reduced by 9 Gy (32%) and 2 Gy (6%), on average, respectively. The mean dose to skin was 9.7 Gy with IMAT plans compared to 16 Gy with conventional NC‐WBRT, demonstrating that absolute reduction of skin dose by a factor of 2. The mean values of the total number of monitor units (MUs) and actual beam on time were 719×44 and 2.34×0.14 min, respectively. The accuracy of IMAT QA plan delivery was (98.1±0.8) %, on average, with a 3%/3 mm gamma index passing rate criteria. All of these plans were considered clinically acceptable per RTOG 0933 criteria. IMAT planning provided highly conformal and homogenous plan with a fast and effective treatment option for WBRT patients, sparing not only hippocampi but also other OARs, which could potentially result in an additional improvement of the quality life (QoL). In the future, we plan to evaluate the clinical potential of IMAT planning and treatment option with hippocampal and other OARs avoidance in our patient's cohort and asses the QoL of the WBRT patients, as well as simultaneous integrated boost (SIB) for the brain metastases diseases.PACS number: 87

A modular approach to intensity-modulated arc therapy optimization with noncoplanar trajectories

Utilizing noncoplanar beam angles in volumetric modulated arc therapy (VMAT) has the potential to combine the benefits of arc therapy, such as short treatment times, with the benefits of noncoplanar intensity modulated radiotherapy (IMRT) plans, such as improved organ sparing. Recently, vendors introduced treatment machines that allow for simultaneous couch and gantry motion during beam delivery to make noncoplanar VMAT treatments possible. Our aim is to provide a reliable optimization method for noncoplanar isocentric arc therapy plan optimization. The proposed solution is modular in the sense that it can incorporate different existing beam angle selection and coplanar arc therapy optimization methods.Treatment planning is performed in three steps. First, a number of promising noncoplanar beam directions are selected using an iterative beam selection heuristic; these beams serve as anchor points of the arc therapy trajectory. In the second step, continuous gantry/couch angle trajectories are optimized using a simple combinatorial optimization model to define a beam trajectory that efficiently visits each of the anchor points. Treatment time is controlled by limiting the time the beam needs to trace the prescribed trajectory. In the third and final step, an optimal arc therapy plan is found along the prescribed beam trajectory. In principle any existing arc therapy optimization method could be incorporated into this step; for this work we use a sliding window VMAT algorithm.The approach is demonstrated using two particularly challenging cases. The first one is a lung SBRT patient whose planning goals could not be satisfied with fewer than nine noncoplanar IMRT fields when the patient was treated in the clinic. The second one is a brain tumor patient, where the target volume overlaps with the optic nerves and the chiasm and it is directly adjacent to the brainstem.Both cases illustrate that the large number of angles utilized by isocentric noncoplanar VMAT plans can help improve dose conformity, homogeneity, and organ sparing simultaneously using the same beam trajectory length and delivery time as a coplanar VMAT plan.

SU‐E‐T‐153: Burst‐Mode Modulated Arc Therapy with Flattening‐Filter‐Free Beams Versus Flattening‐Filtered Beams

Purpose:To compare the dosimetry and delivery of burst‐mode modulated arc radiotherapy using flattening‐filter‐free (FFF) and flattening‐filtered (FF) beams.Methods:Burst‐mode modulated arc therapy (mARC, Siemens) plans were generated for six prostate cases with FFF and FF beam models, using the Elekta Monaco v. 5.00 planning system. One 360‐degree arc was used for five cases, and for one case two 360‐degree coplanar arcs were used. The maximum number of optimization points (OPs) per arc was set to 91, and OPs with less than 4 MU were disregarded. All plans were delivered on the Siemens Artiste linear accelerator with 6MV FF (300 MU/min) and comparable‐energy FFF (2000 MU/min, labeled as 7UF) beams.Results:For all cases studied, the plans with FFF beams exhibited DVHs for the PTV, rectum, and bladder that were nearly identical to those for the plans with FF beams. The FFF plan yielded reduced dose to the right femoral head for 5 cases, and lower mean dose to the left femoral head for 4 cases. For all but the two‐arc case, the FFF and FF plans resulted in an identical number of segments. The total number of MUs was slightly lower for the FF plans for five cases. The total delivery time per fraction was substantially lower for the FFF plans, ranging from 25 to 50 percent among all cases, as compared to the FF plans.Conclusion:For mARC plans, FFF and FF beams provided comparable PTV coverage and rectum and bladder sparing. For the femoral heads, the mean dose was slightly lower in most cases when using the FFF beam. Although the flat beam plans typically required slightly fewer MUs, FFF beams required substantially less time to deliver a plan of similar quality.This work was supported by Siemens Medical Solutions and the MCW Cancer Center Fotsch Foundation.

SU‐E‐T‐536: LINAC‐Based Single Isocenter Frameless SRT for Brain Metastases

Purpose:Single‐isocenter Stereotactic Radiotherapy of multiple brain metastases with Varian 21 IX LINAC, using Aktina Pinpoint system for patient setup.Methods:In 2014, five single‐isocenter RapidArc SRT plans were delivered to five patients with 2 to 8 brain metastases using Varian 21 IX. Aktina Pinpoint system was used for setup and 2mm PTV margin were used. CBCT was acquired before and after the beam delivery. The prescription is 2100 cGy in 3 fractions. Eclipse planning system was used for treatment planning. Depending on the number of metastases and their locations, 1 to 5 coplanar or non coplanar arcs were used. Typically, 2 or 3 arcs are used. IMRT QAs were performed by comparing an A1SL ion chamber point dose measurement in solid water phantom to point dose of the plan; also, based on EPID measurement, 3D spatial dose was calculated using DosimetryCheck software package from MathResolutions Inc. The EPID system has an active area of 40cm by 30cm with 1024 by 768 photodiodes, which corresponds to a resolution of 0.4mm by 0.4mm pixel dimension.Results:for all the plans, at least 95% PTV coverage was achieved for full prescription dose, with plan normalization &gt; 75%. RTOG conformity indices are less than 1.1 and Paddick gradient indices are less than 4.5. The distance from prescription IDL to 50% IDL increases as the number of metastases increases, and it ranges from 0.6mm to 0.8mm. Treatment time varies from 10mins to 30mins, depending on the number of arcs and if the arcs are coplanar. IMRT QA shows that the ion chamber measurement agree with the eclipse calculation within 3%, and 95% of the points passed Gamma, using 3% dose difference and 3mm DTAConclusion:High quality single isocenter RapidArc SRT plan can be optimized and accurately delivered using Eclipse and Varian 21IX.

SU‐E‐T‐514: Investigating the Dose Distributions of Equiangular Spaced Noncoplanar Beams

PurposeIt has been demonstrated that the use of noncoplanar beams in radiation therapy may Result in dose distributions that are comparable or better than standard coplanar beams [Pugachev, 2001]. A radiation therapy system designed with a noncoplanar beam geometry could allow for a full ring diagnostic quality imaging system to be placed around the patient. Additionally, if the noncoplanar beams were fixed in number and in their angle with respect to the patient's axial plane, then both treatment and imaging could be achieved concurrently without the need for moving parts, which could greatly reduce treatment times. For such a system to be designed, it is necessary to determine the appropriate number of beams and the beam angles to achieve optimal dose distributions. For simplicity, the beam angles are assumed to be equiangular in the patient's axial plane, and only the beam angle with respect to the axial plane are varied. This study aims to investigate the dose distributions produced by equiangular noncoplanar beams for multiple beam numbers and beam angles, and to compare these dose distributions with distributions achieved in coplanar volumetric arc therapy (VMAT).MethodsDose distributions produced by noncoplanar beams were calculated using the Varian Eclipse treatment planning system by varying the gantry, collimator, and couch angles to simulate the noncoplanar delivery method. Noncoplanar intensity‐modulated (NC‐IMRT) beams using 8, 12, and 16 beams with angles varying from 45 degrees to 54 with respect to the patient's axial plane were studied.ResultsThe NC‐IMRT beams produced dose distributions comparable to VMAT plans for a number of treatment sites, and were capable of meeting similar dose‐volume histogram constraints.ConclusionThis study has demonstrated that a noncoplanar beam delivery method with fixed beam numbers and beam angles is capable of delivering dose distributions comparable to VMAT plans currently in use.

SU‐E‐T‐331: Dosimetric Impact of Multileaf Collimator Leaf Width On Stereotactic Radiosurgery (SRS) RapidArc Treatment Plans for Single and Multiple Brain Metastases

Purpose:To determine the effects of multileaf collimator (MLC) leaf width on normal‐brain‐tissue doses and dose conformity of SRS RapidArc treatment plans for brain tumors.Methods:Ten patients with 24 intracranial tumors (seven with 1–2 and three with 4–6 lesions) were planned using RapidArc for both Varian Millennium 120 MLC (5 mm leaf width) and high definition (HD) MLC (2.5 mm leaf width). Between 2 and 8 arcs were used with two full coplanar arcs and the rest non‐coplanar half arcs. 6 MV beams were used and plans were optimized with a high priority to the Normal Tissue Objective (to achieve dose conformity and sharp dose fall‐off) and normal brain tissue. Calculation was done using AAA on a 1 mm grid size. The prescription dose ranged from 14–22 Gy. Plans were normalized such that 99% of the target received the prescription dose. Identical beam geometries, optimizations, calculations, and normalizations were used for both plans. Paddick Conformity Index (PCI), V4, V8 and V12 Gy for normal brain tissue and Integral Dose were used for analysis.Results:In all cases, HD MLC plans performed better in sparing normal brain tissue, achieving a higher PCI with a lower Integral Dose. The average PCI for all 24 targets was 0.75±0.23 and 0.70±0.23 (p ≤0.0015) for HD MLC and Millennium MLC plans, respectively. The average ratio of normal brain doses for Millennium MLC to HD MLC plans was 1.30±0.16, 1.27±0.15, and 1.31±0.18 for the V4, V8, and V12, respectively. The differences in normal brain dose for all criteria were statistically significant with p‐value &lt; 0.02. On average Millennium MLC plans had a 16% higher integral dose than HD MLC plans.Conclusion:Significantly better dose conformity with reduced volume of normal brain tissue and integral dose was achieved with HD MLC plans compared to Millennium MLC plans.

SU-D-BRB-04: Plan Quality Comparison of Intracranial Stereotactic Radiosurgery (SRS) for Gamma Knife and VMAT Treatments

Purpose: To compare treatment plan quality of intracranial stereotactic radiosurgery (SRS) for VMAT (RapidArc) and Gamma Knife (GK) systems. Methods: Ten patients with 24 tumors (seven with 1–2 and three with 4–6 lesions), previously treated with GK 4C (prescription doses ranging from 14–23 Gy) were re-planned for RapidArc. Identical contour sets were kept on MRI images for both plans with tissues assigned a CT number of zero. RapidArc plans were performed using 6 MV flattening-filter-free (FFF) beams with dose rate of 1400 MU/minute using two to eight arcs with the following combinations: 2 full coplanar arcs and the rest non-coplanar half arcs. Beam selection was based on target depth. Areas that penetrated more than 10 cm of tissue were avoided by creating smaller arcs or using avoidance sectors in optimization. Plans were optimized with jaw tracking and a high weighting to the normal-brain-tissue and Normal-Tissue-Objective without compromising PTV coverage. Plans were calculated on a 1 mm grid size using AAA algorithm and then normalized so that 99% of each target volume received the prescription dose. Plan quality was assessed by target coverage using Paddick Conformity Index (PCI), sparing of normal-brain-tissue through analysis of V4, V8, and V12 Gy, and integralmore » dose. Results: In all cases critical structure dose criteria were met. RapidArc had a higher PCI than GK plans for 23 out of 24 lesions. The average PCI was 0.76±0.21 for RapidArc and 0.46±0.20 for GK plans (p≤0.001), respectively. Integral dose and normal-brain-tissue doses for all criteria were lower for RapidArc in nearly all patients. The average ratio of GK to RapidArc plans was 1.28±0.27 (p=0.018), 1.31±0.25 (p=0.017), 1.81±0.43 (p=0.005), and 1.50±0.61 (p=0.006) for V4, V8, and V12 Gy, and integral dose, respectively. Conclusion: VMAT was capable of producing higher quality treatment plans than GK when using optimal beam geometries and proper optimization techniques.« less

SU‐E‐T‐326: Dosimetric Impact of Beam Energies and Jaw Tracking On Intracranial Tumors Using RapidArc

Purpose:To determine the dosimetric impact of jaw tracking and beam energies on dose conformity and normal‐brain‐tissue doses for intracranial tumors using VMAT (RapidArc).Methods:Seven patients with 1–2 and three patients with 4–6 intracranial tumors were planned using RapidArc for Varian TrueBeam STx machine with beam energies 6MV‐FFF (Flattening‐Filter‐Free), 8MV, 10MV, and 10MV‐FFF. The prescription dose ranged from 14–23Gy. Between 2 and 8 arcs were used with the following geometries: 2 full coplanar arcs and the non‐coplanar half arcs. Plans were optimized (jaw tracking ON) with a high priority to Normal‐Tissue‐Objective and normal‐brain‐tissue. Plans were calculated on 1mm grid size using AAA algorithm and then normalized so that 99% of each target volume received the prescription dose. Plans for the 6MV‐FFF were also optimized without jaw tracking (No‐JT) for comparison. Plan quality was assessed by target coverage using Paddick Conformity Index (PCI), sparing of normal‐brain‐tissue through analysis of V4Gy, V8Gy and V12Gy, and integral dose.Results:The average PCI ± standard deviation was 0.76±0.21 and 0.76±0.22 for 6MV‐FFF and 10 MV‐FFF, respectively. The average ratio in normal brain tissue volume (reported as follows V4,V8,V12) were (1.12±0.07,1.12±0.07,1.14±0.04), (1.12±0.08,1.12±0.09,1.13±0.06), (1.19±0.10,1.18±0.10,1.20±0.04), and (1.04±0.03,1.03±0.03,1.03±0.04) for 8MV/6MV‐FFF, 10MV‐FFF/6MV‐FFF, 10MV/6MV‐FFF, 6MV‐FFF No‐JT/6MV‐FFF, respectively. Statistically significant differences in normal‐brain‐tissue for V4, V8, and V12 were observed in all cases for the different energies (p‐values &lt;0.05). V4 data shows significant differences in JT vs. No‐JT (p=0.04), however no difference was found for V8 and V12. Brain tissue sparing from best to worst occurred in this order 6MV‐FFF, 6MV‐FFF no‐JT, 10MV‐FFF, 8MV, and 10MV. The average ratio of integral brain dose was 1.05±0.04 (p=0.21), 1.04±0.05 (p=0.33), 1.09±0.06 (p=0.04), and 1.02±0.06 (p=0.61) for 8MV/6MV‐FFF, 10MV‐FFF/6MV‐FFF, 10MV/6MV‐FFF, and 6MV‐FFF No‐JT/6MV‐FFF, respectively.Conclusion:Normal brain tissue and integral dose improved using the lower energy and FFF beams, though plan conformity showed energy independence.

EP-1463: VMAT for intracranial SRS: coplanar or non-coplanar arcs ñ high dose conformity versus accurate delivery

EP-1463: VMAT for intracranial SRS: coplanar or non-coplanar arcs ñ high dose conformity versus accurate delivery

Is high–dose rate RapidArc-based radiosurgery dosimetrically advantageous for the treatment of intracranial tumors?

In linac-based stereotactic radiosurgery (SRS) and radiotherapy (SRT), circular cone(s) or conformal arc(s) are conventionally used to treat intracranial lesions. However, when the target is in close proximity to critical structures, it is frequently quite challenging to generate a quality plan using these techniques. In this study, we investigated the dosimetric characteristics of using high–dose rate RapidArc (RA) technique for radiosurgical treatment of intracranial lesions. A total of 10 intracranial SRS/SRT cases previously planned using dynamic conformal arc (DCA) or cone-based techniques have been included in this study. For each case, 3 treatment plans were generated: (1) a DCA plan with multiple noncoplanar arcs, (2) a high–dose rate RA plan with arcs oriented the same as DCA (multiple-arc RA), and 3) a high–dose rate RA plan with a single coplanar arc (single-arc RA). All treatment plans were generated under the same prescription and similar critical structure dose limits. Plan quality for different plans was evaluated by comparing various dosimetric parameters such as target coverage, conformity index (CI), homogeneity index (HI), critical structures, and normal brain tissue doses as well as beam delivery time. With similar critical structure sparing, high–dose rate RA plans can achieve much better target coverage, dose conformity, and dose homogeneity than the DCA plans can. Plan quality indices CI and HI, for the DCA, multiple-arc RA, and single-arc RA techniques, were measured as 1.67 ± 0.39, 1.32 ± 0.28, and 1.38 ± 0.30 and 1.24 ± 0.11, 1.10 ± 0.04, and 1.12 ± 0.07, respectively. Normal brain tissue dose (V12Gy) was found to be similar for DCA and multiple-arc RA plans but much larger for the single-arc RA plans. Beam delivery was similar for DCA and multiple-arc RA plans but shorter with single-arc RA plans. Multiple-arc RA SRS/SRT can provide better treatment plans than conventional DCA plans, especially for complex cases.

Radiotherapy for unresectable sinonasal cancers: Dosimetric comparison of intensity modulated radiation therapy with coplanar and non-coplanar volumetric modulated arc therapy

Background and purposeTo compare volumetric modulated arc therapy (VMAT) and intensity modulated radiation therapy (IMRT) plans for treatment of unresectable paranasal sinuses cancers (PNSCs) with different clinical presentations. Material and methodsFour patients treated for primary target volume only (group 1), four requiring elective nodal irradiation (group 2) and four with positive nodes in macroscopic disease (group 3) were selected. For each patient were generated 7 fields IMRT, coplanar VMAT (c-VMAT) and non-coplanar VMAT (nc-VMAT) treatment plans. Total doses were 70Gy and 54Gy to high dose planning target volume (HD-PTV) and low-dose-PTV, respectively. Dose–volume histogram, conformity and homogeneity index (CI and HI), and monitor units (MUs) per Gy were evaluated. ResultsVMAT provided significantly better target coverage, in terms of V100% (Volume encompassed by the isodose 100%), than IMRT, in particular when nc-VMAT was used. In general, organ at risk sparing is similar with the three approaches, although nc-VMAT can allow a statistically significant reduction of dose to contralateral parotid gland and cochlea for all three groups. ConclusionsVMAT can offer significant improvement of treatment for all unresectable PNSCs over existing IMRT techniques. In particular, nc-VMAT may be a further advantage for those patients with sinonasal cancers and involvement of the nodes in whom large volumes and complex/irregular shape have to be irradiated, even if clinical benefits should be established in the future.

Dosimetric Evaluation of Volumetric-Modulated Arc Therapy (RapidArc) for Primary Leiomyosarcoma in the Spine

This study aims to investigate the suitability of volumetric-modulated arc therapy (VMAT) with RapidArc for primary leiomyosarcoma (LMS) in the spine, and present a new method to improve the target coverage and organs at risk (OAR) sparing. Five patients with LMS were retrospectively reviewed. The intensity-modulated radiotherapy (IMRT) with five coplanar beams (5b-IMRT) or seven coplanar beams (7b-IMRT), and VMAT using four quasi-quarter coplanar arcs (4q-VMAT) or two full coplanar arcs (2f-VMAT) were generated. Planning target volume (PTV) dose coverage, OAR dose sparing, conformity index (CI), and homogeneity index (HI) were evaluated. A hollow-cylinder model (HCM) was also used for feasible optimal beam arrangements. The mean doses to PTV were 95.2% ± 1.0%, 93.0% ± 1.0%, 97.9% ± 1.0% and 96.2% ± 1.5% for 4q-VMAT, 2f-VMAT, 5b-IMRT and 7b-IMRT respectively, while the mean maximum doses to spinal cord (SC) were 43.7 ± 0.9 Gy, 42.0 ± 0.8 Gy, 41.4 ± 1.2 Gy and 40.6 ± 1.4 Gy. Compared to 5b-IMRT, the mean doses delivered to kidneys decreased by about 35.1% (8.5 Gy), 2.5% (0.6 Gy) and 35.5% (8.6 Gy) for 4q-VMAT, 2f-VMAT, and 7b-IMRT, respectively. The CI proposed by Baltas et al. was twice as good with IMRT than with 4q-VMAT, and the numbers of monitor units were increased five- and threefold with 7b-IMRT and with 5b-IMRT compared to VMAT. The unexpected results we presented here show that VMAT technique can't achieve highly conformal treatment plans while maintaining SC sparing for LMS in the spine. An approach is proposed based on a hollow-cylinder model, but it is difficult to apply to clinical practice. In this case, VMAT is not superior to IMRT except for significant reduction in delivery time.

P13.04 * A STUDY ON DOSIMETRIC PERFORMANCE OF 5 MM MLCS FOR VMAT BASED STEREOTACTIC RADIOSURGERY

OBJECTIVE: In stereotactic radiosurgery (SRS), the high resolution multileaf collimator (HR-MLC) (i.e. 1 to 3mm leaf width) has the potential to provide improved dose conformity to target volume and lower doses to critical structures compared to the more prevalent standard MLCs (5 mm), when used in a classic conformal manner. In this study we investigate the dosimetric performance of standard 5 mm MLC for SRS targets of various sizes, when using a VMAT technique, to determine the limitations and/or need to increase planning complexity. METHODS AND MATERIALS: Spherical planning target volumes (PTV) were delineated in a test patient with diameters ranging from 1 to 3 cm in increments of 0.4 cm in an Eclipse treatment planning system. RapidArc plans using 6 MV photons were produced for each PTV using two coplanar arcs. Plan optimization and plan normalization was to obtain better PTV dose coverage (min V90 = 100% and medium PTV dose >100%) and dose fall-off controlled by the normal tissue objective and a ring structure 1 cm clear of PTV with an aim for dose of 40% of prescribed. PTV coverage was prioritized over dose fall off. All plans were calculated using Anisotropic Analytical Algorithm (AAA) with 1mm grid size. The plans were evaluated using dose conformity defined by the ratio of the percentage dose volume to volume of PTV for 100, 90, 80, 60 and 40 percent of prescribed (R100-40). Dosimetric machine accuracy is assessed using high resolution (0.1mm) GafChromic film analysis, an added limitation was to provide increased dose homogeneity compared to classical radiosurgery. RESULTS: Conformity index R100-40 was lower in plans with 1.8 cm diameter (volume <2.9 cc) or lower (R100 = 2.81, 1.84, 1.54, 1.39, 1.29, and 1.36 for target volumes with increasing diameter as defined above). Plotting the plan conformity and the calculated conformity given a defined fixed distance to dose fall off against PTV volume shows the two to be divergent at small volumes indicating the point at which conformity is compromised with 5 mm MLC. Gafchromic film analysis based on direct dose calibration showed good agreement for all volume sizes, showing that the planning system correctly predicts the dose variations even at this high resolution. CONCLUSIONS: Single plane RA-SRS using 5mm MLC is feasible down to target sizes of 2.9cc, while a clinical planning system allows accurate prediction of the dose. In future work the impact of multiple plane RA-SRS on feasibility will be presented.