HD120 MLC Research Articles

Comparison of 116 Radiosurgery Treatment Plans for Multi-Leaf and Cone Collimator on a Varian Edge Linac: Are Cones Superior in the Daily Routine?

Delivering focused radiation doses via linear accelerators is a crucial component of stereotactic radiosurgery (SRS) for brain metastases. The Varian Edge linear accelerator provides highly conformal radiation therapy through a high-definition multi-leaf collimator (HD120 MLC) and conical collimator (CC). HD120 MLC adapts to the shape of the target volume using movable tungsten leaves, while CC has a block of conical shape (cones). CC in SRS treatments of small brain metastases is preferred due to its mechanical stability and steeper dose fall-off, potentially sparing organs at risk (OARs) and the brain better than HD120 MLC. This study aims to determine if CC offers significant advantages over HD120 MLC for SRS treatments. For 116 metastatic lesions, CC and HD120 MLC treatment plans were created in Varian Eclipse TPS and compared based on various dose parameters, robustness tests, and QA measurements. The results indicate that CC provides no significant advantages over HD120 MLC, except for slight, clinically insignificant benefits in brain sparing and dose fall-off for the smallest lesions. HD120 MLC outperforms CC in almost every aspect, making it a better choice for irradiating brain metastases with 0.1 cm3 or higher volumes.

A feasibility study of "plan transformation": A novel workaround enabling patient transfer between different Varian machines.

The objective of this work is to explore a methodology to enable plan transfers from Standard MLC (STD) to High Definition MLC (HD) Varian units without replanning. Using "Plan Transformation", we aim to utilize full departmental capacity to disperse patients from STD Varian units to HD units, when the former goes down for repair. Our centre is equipped with four STD and two HD MLC Varian Truebeam units. Plan transformation is used to transfer patients from STD to HD units when needed. Static or manually segmented fields with Y jaws≤21.6cm are eligible for plan transformation. The MLC pattern for each field is exported out of Eclipse. An in-house program based on C# is used to convert those patterns from STD to HD. STD and HD MLCs are different in design but dosimetrically comparable for these type of plans. Still to avoid extra uncertainty, transformed plans are recalculated in Eclipse to enable the user to assess the quality of the process. On a given day, more than 25% of appointments on STD units were eligible for transformation. The majority of eligible plans were breast tangent and boost. The delivery and safety of plan transformation was examined directly on a Varian Truebeam linac. A gamma analysis comparison between measured and calculated transformed plans yields a pass-rate of 100% for 3%/3mm, identical to non-transformed plans. Plan transformation is feasible and saves time for planners as it takes less than a quarter the time required for a replan.

Impact of Breath-Hold MR-Guided Radiotherapy (MRgRTBH) vs. Free-Breathing CT Image-Guided Radiotherapy (CT-IGRTFB) on Gastrointestinal Sparing and Dose Conformality in Adrenal SBRT

<h3>Purpose/Objective(s)</h3> To quantitatively compare plan quality between mid-inspiration breath-hold (BH) MR-guided radiotherapy (MRgRT_BH) and free-breathing (FB) CT-based intensity modulated radiotherapy (CT-IGRT_FB) for the ablative treatment of adrenal malignancies. We hypothesized that MRgRT_BH would provide improved target coverage and lower organ-at-risk (OAR) doses due to smaller target volumes compared to CT-IGRT_FB. <h3>Materials/Methods</h3> Twenty adrenal metastasis patients were treated on a 0.35T MR-Linac with real-time sagittal tracking at four frames per second for gated BH delivery. All patients underwent IMRT planning to a median prescription dose of 50 Gy (range: 40-50 Gy) in 5 fractions. For MRgRT_BH, the CTV was delineated on the primary 3D TRUFI scan with no internal target volume (ITV). Each patient was retrospectively replanned using a 3-arc, VMAT CT-IGRT_FB treatment on a c-arm Linac (HD MLC). 4DCT respiratory evaluation was performed during MRgRT_BH simulation. The average intensity projection CT was used to determine the ITV for the CT-IGRT_FB plan. All plans used a 5 mm ITV to PTV margin. Target volume metrics used were: target coverage (TC) (PTV V100%/PTV vol), PTV D95%/Rx, PTV D90%/Rx, PTV D80%/Rx, homogeneity index (HI) (PTV D2%/ D98%), high dose conformity (PITV), low dose conformity (D_2cm), and gradient (R50%). Additional dose metrics were evaluated for mean ipsilateral kidney and 0.5 cc gastrointestinal (GI) OARs. <h3>Results</h3> The Wilcoxon signed-rank test showed a statistically significant increase (<i>P</i> < 0.05) in PTV volumes for CT-IGRT_FB over MRgRT_BH, with median (range) at 91.9 cc (32.0 – 481.4 cc) versus 53.6 cc (20.6 – 377.4 cc); a 71% relative increase for CT attributable to the need for an ITV. Target coverage metrics were only marginally worse for CT-IGRT_FB (<i>P</i> > 0.05). There were significant reductions (<i>P</i> < 0.05) in conformality metrics for CT-IGRT_FB in PITV (10%) and R50% (24%) relative to MRgRT_BH, due to the significant increase in target volumes for CT-IGRT_FB. Low dose conformality was equivalent for D_2cm. MRgRT_BH exhibited significant reductions (<i>P</i> < 0.05) in the median dose to 0.5 cc of the large bowel (15%), small bowel (28%) and duodenum (45%) over CT-IGRT_FB. <h3>Conclusion</h3> Intrafraction BH motion management of MRgRT_BH enabled significant reductions of target volumes (<i>P</i> < 0.05) compared to the ITV-approach for CT-IGRT_FB. Marginally improved target coverage (<i>P</i> > 0.05) and significant GI OAR sparing (<i>P</i> < 0.05) resulted from the smaller target volumes and reduced OAR-to-PTV proximity. MRgRT_BH was shown to be dosimetrically superior for OAR sparing to the small bowel, the large bowel and the duodenum (<i>P</i> < 0.05). Adaptive MRgRT_BH was used for all patients, but only initial treatment doses were considered in quantifying the impact of gated BH versus ITV for this study.

Automated treatment planning of prostate stereotactic body radiotherapy with focal boosting on a fast-rotating O-ring linac: Plan quality comparison with C-arm linacs.

PurposeThe integration of auto‐segmentation and automated treatment planning methods on a fast‐rotating O‐ring linac may improve the time efficiency of online adaptive radiotherapy workflows. This study investigates whether automated treatment planning of prostate SBRT with focal boosting on the O‐ring linac could generate plans that are of similar quality as those obtained through manual planning on clinical C‐arm linacs.MethodsFor 20 men with prostate cancer, reference treatment plans were generated on a TrueBeam STx C‐arm linac with HD120 MLC and a TrueBeam C‐arm linac with Millennium 120 MLC using 6 MV flattened dual arc VMAT. Manual planning on the Halcyon fast‐rotating O‐ring linac was performed using 6 MV FFF dual arc VMAT (HA2‐DL10) and triple arc VMAT (HA3‐DL10) to investigate the performance of the dual‐layer MLC system. Automated planning was performed for triple arc VMAT on the Halcyon linac (ET3‐DL10) using the automated planning algorithms of Ethos Treatment Planning. The prescribed dose was 35 Gy to the prostate and 30 Gy to the seminal vesicles in five fractions. The iso‐toxic focal boost to the intraprostatic tumor nodule(s) was aimed to receive up to 50 Gy. Plan deliverability was verified using portal image dosimetry measurements.ResultsCompared to the C‐arm linacs, ET3‐DL10 shows increased seminal vesicles PTV coverage (D99%) and reduced high‐dose spillage to the bladder (V37Gy) and urethra (D0.035cc) but this came at the cost of increased high‐dose spillage to the rectum (V38Gy) and a higher intermediate dose spillage (D2cm). No statistically significant differences were found when benchmarking HA2‐DL10 and HA3‐DL10 with the C‐arm linacs. All plans passed the patient‐specific QA tolerance limit.ConclusionsAutomated planning of prostate SBRT with focal boosting on the fast‐rotating O‐ring linac is feasible and achieves similar plan quality as those obtained on clinical C‐arm linacs using manual planning.

MLC parameters from static fields to VMAT plans: an evaluation in a RT-dedicated MC environment (PRIMO)

BackgroundPRIMO is a graphical environment based on PENELOPE Monte Carlo (MC) simulation of radiotherapy beams able to compute dose distribution in patients, from plans with different techniques. The dosimetric characteristics of an HD-120 MLC (Varian), simulated using PRIMO, were here compared with measurements, and also with Acuros calculations (in the Eclipse treatment planning system, Varian).Materials and methodsA 10 MV FFF beam from a Varian EDGE linac equipped with the HD-120 MLC was used for this work. Initially, the linac head was simulated inside PRIMO, and validated against measurements in a water phantom. Then, a series of different MLC patterns were established to assess the MLC dosimetric characteristics. Those tests included: i) static fields: output factors from MLC shaped fields (2 × 2 to 10 × 10 cm2), alternate open and closed leaf pattern, MLC transmitted dose; ii) dynamic fields: dosimetric leaf gap (DLG) evaluated with sweeping gaps, tongue and groove (TG) effect assessed with profiles across alternate open and closed leaves moving across the field. The doses in the different tests were simulated in PRIMO and then compared with EBT3 film measurements in solid water phantom, as well as with Acuros calculations. Finally, MC in PRIMO and Acuros were compared in some clinical cases, summarizing the clinical complexity in view of a possible use of PRIMO as an independent dose calculation check.ResultsStatic output factor MLC tests showed an agreement between MC calculated and measured OF of 0.5%. The dynamic tests presented DLG values of 0.033 ± 0.003 cm and 0.032 ± 0.006 cm for MC and measurements, respectively. Regarding the TG tests, a general agreement between the dose distributions of 1–2% was achieved, except for the extreme patterns (very small gaps/field sizes and high TG effect) were the agreement was about 4–5%. The analysis of the clinical cases, the Gamma agreement between MC in PRIMO and Acuros dose calculation in Eclipse was of 99.5 ± 0.2% for 3%/2 mm criteria of dose difference/distance to agreement.ConclusionsMC simulations in the PRIMO environment were in agreement with measurements for the HD-120 MLC in a 10 MV FFF beam from a Varian EDGE linac. This result allowed to consistently compare clinical cases, showing the possible use of PRIMO as an independent dose calculation check tool.

Optimization of MLC parameters for TPS calculation and dosimetric verification: application to single isocenter radiosurgery of multiple brain lesions using VMAT

Linac and MLC-based stereotactic radiosurgery (SRS) using single-isocenter-multiple-target (SIMT) VMAT has become increasingly popular in the management of multi-focal cranial metastases. However, significant geometrical and dosimetric challenges exist due to the typically small target volumes and in most cases, non-isocentric locations. To the best of our knowledge, there hasn’t been a study in the optimization of MLC parameters, in the context of SIMT SRS, to ensure TPS calculation accuracy. In this work, we set out to optimize the dosimetric leaf gap (DLG) for the HD MLC installed on dedicated stereotactic Varian STx systems using a diverse group of 21 clinical SRS and SBRT plans. These plans featured a broad range of target sizes and target-to-isocenter distances that are typical of the stereotactic cases treated on these systems. Dose discrepancies between TPS calculations and verification measurements using a previously validated diode array Delta4 (ScandiDos) were minimized in a balanced manner to accommodate the variety of stereotactic plans. A DLG of 0.6 mm was found to be ‘optimal’ for the HD MLC and for the ‘typical’ plans treated on our STx systems. The finding was independently verified using commercially available 3D polymer gel dosimeter CrystalBallTM (MGS Research Inc.). 3D verification for 6 SIMT SRS plans, consisted of 5 to 15 targets, achieved an average gamma score of 97.3% (σ = 2.0%) on 3%/2 mm criteria with a cutoff isodose level of 20%. We further examined the practice of routine dosimetric verifications including the selection of appropriate detectors and optimal gamma parameters. We found that the commonly used standard 3%/3 mm criteria would have resulted in all but 4 (out of 2840) clinical plans achieving a gamma score of 95% or better, and therefore, losing sensitivity to detect potential dosimetric discrepancies. Based on the characteristics of stereotactic plans, a more stringent distance-to-agreement parameter is needed.

Development and Validation of End-to-End Test for Stereotactic Radiosurgery

The purpose of this study is to evaluate End to End test and dosimetric test of small field SRS MapCHECK dosimeter. The Fundamental properties of the SRS MapCHECK Diode Detector such as reproducibility, linearity, Energy dependence, Dose rate dependency and output factor are studied for 6MV FFF energy photon beams. All dosimetric test carried out on a medical accelerator consisting High definition 120 multileaf collimator (HD120 MLC), provide stereotactic treatments. End to End test of SRSMapCHECK carried out with StereoPHAN Cylindrical Phantom with CT and MRI inserts. The E2E test includes MRI scan, CT scan, Target contouring on CT/MRI scan, MRI/CT image fusion, Treatment planning and Dose delivery. SRS MapCHECK comprise of 1013 solid state diode detector, are equally spaced at 2.47mm. The active detector area is 0.48mm x 0.48mm and resolution is 0.007mm3. The maximum field size is 7 X 7 cm2. Before to start dosimetric measurement the calibration of SRS MapCHECK and StereoPHAN is necessary. The calibration process includes background measurement, array calibration and StereoPHAN factor calculation. The reproducibility of SRS MapCHECK checked by measuring output for 5 consecutive days for 6MV FFF beam with field size 5x5cm2. The Linearity of dose measurement also evaluated. The response of SRS MapCHECK to different energy (6, 10 & 15MV) also studied. Dose rate dependency also studied by varying dose rate from 400 to 1400 MU/min. Output Factor for different field sizes ranging from 1 x 1 cm2 to 7 x 7 cm2 was measured. Geometric accuracy of inserts volumes are find with CT and MRI fusion. Actual Insert volumes of Central Sphere, Right Sphere, Left Sphere are 4.17cc and 0.52 cc, 0.52cc respectively. The TPS measured volume on CT scan is 4.09cc, 0.52cc and 0.53 cc respectively similarly on MRI TPS measured volume is 4.22cc, 0.50cc and 0.52cc. The % variation on CT volume is less than 2%. For MRI the % difference for right sphere is -4.0%, central and left side are less than 2% The reproducibility of SRS MapCHECK output is well within less than 2% of the first day measurement. The SRS MapCHECK showed linear dose response for energy 6 MV FFF. Energy Dependency checked for 6MV, 10MV and 15MV with fixed dose rate 600MU/min it showed linear response to energy. We found SRS MapCHECK are independent on dose rate. Output factor was measured and compared with pinpoint chamber 0.015cc. SRS MapCHECK showed under response compared to pinpoint chamber for field size 1 x 1 cm2 to 4 x 4 cm2 and showed over response from the 5 x 5 cm2 to 7 x 7 cm2 The gamma analysis done with Dose difference 3% and Dose to agreement 3mm, we also checked with DAT 2mm, 1mm with threshold 10%. The result of gamma passing is 97.6%, 96.8% and 95.8 % respectively. All dosimetric test performed on SRS MapCHECK are comparable with ionization chamber measurement and clinically acceptable. It is an effective tool for E 2 E test and patient’s specific QA for SRS Patients.

199. Evaluation of beam’s attenuation using the Delta4 Discover transmission detector in various treatment application

Purpose Delta4 Discover (ScandiDos, Uppsala, Sweden) is a new device developed in order to monitor online complex radiation techniques such as volumetric-modulated arc therapy (VMAT). It can be inserted between the radiation source and the patient so it can potentially impact the beam quality that the patient receives. The aim of this work is to investigate, in terms of beam’s attenuation, the feasibility of using Discover transmission array [1] . Methods 9 VMAT treatment plans, generated using Pinnacle3’s SmartArc algorithm v.9.10 (Philips Fitchburg, USA), for various treatment sites, were delivered using Delta4 Phantom+ (ScandiDos, Uppsala, Sweden). The response, with and without the Discover mounted on the linac was evaluated. All plans were delivered on a Varian TrueBeam STx equipped with HD MLC. For phantom verification, plans were recalculated replacing the patient planning CT scan with a phantom CT scan. Dose distributions of each plan delivered with and without Discover were verified using the Delta4 Phantom + and compared to calculated dose. Results Analyzing dose difference profiles, the Relative Dose Difference, between the measured dose relative to both configurations, showed that the absorption of beam radiation observed was on average about 1%. About Median Dose Deviation between the calculated and measured dose for both configurations we found that the all Median Dose Deviation shift values are ranging from 0.2% to 1.3%. Conclusions The thin transmission detector Delta4 Discover shows good potential for real time quality assurance. The minimal attenuation observed potentially allows for a real time verification and independent monitoring of complex VMAT deliveries during patient treatment.

185. Characterization of TrueBeam STx for SRS/SBRT treatments with the new phantom StereoPHAN

PurposeTo apply StereoPHAN (Sun Nuclear), a phantom designed for end-to-end commissioning and QA on all parts of the SRS process, in detecting SRS/SBRT paths weaker parts. MethodsIrradiations were performed with TrueBeam STx (Varian) equipped with HD MLC (0.25 cm), while measurements with an Exradin A26 ionization chamber (IC, 0.015 cc, Standard Imaging) inserted in the phantom. Percentage dose difference (%DD) was calculated to evaluate: response linearity with dose, repeatability, dependence on the gantry rotation direction. Moreover, treatment plans with different plan parameters such as number and coplanarity of arcs, energy, dose rate, field dimensions and calculation grid sizes where simulated and measured. Finally, the same measurements where acquired also with the diode array ArcCheck (SunNuclear), the percentage gamma pass (%GP) calculated and isocenter doses reconstructed with 3DVH (Sun Nuclear). ResultsAverage %DD ± Standard Deviation was (2.6 ± 1)% for the linearity with dose and (3.0 ± 3.2)% for the simulated treatment plans; %DD correlations were found with the dose rate 1600 and 600 MU/min (%DD equal to (4.5 ± 1)% and (2.1 ± 0.8)% respectively, ANOVA p = 0.04) and number of monitor units (linear correlation, Pearson p = 0.01)). No correlation was found between %GP and %DD, while there was not a statistically significant difference between actual StereoPHAN measurements and 3DVH isocenter doses (paired sample t-test, p = 0.13). ConclusionsIn this work the new phantom StereoPHAN was applied to evaluate whether %DDs found in SRS and SBRT paths were due to limits of the instruments used to acquire measurements, TPS calculation or linac delivery. Correlation with planning parameters must be furtherly investigated. The lack of correlation between %DD and %GP reflects the intrinsic difference between point dose and dose maps measurements, but the lack of statistical difference between actual and simulated measurements validates StereoPHAN as in comparison with our gold standard measurements performed with ArcCheck.

167. Evaluation of a radiotherapy-dedicated Monte Carlo environment (PRIMO) for verification of clinical VMAT plans for brain and lung metastases

PurposePRIMO is a graphical environment for MonteCarlo (MC) simulations. The Dose Planning Method (DPM) is a fast MC algorithm for the simulation of the deposited dose under radiotherapy conditions. The objective of this work was to validate the DPM-calculated beams against our LINAC EDGE and compare a clinical algorithm (Acuros, Varian) with PRIMO DPM MC in two different clinical scenarios. MethodsThe beams were evaluated in terms of profiles, PDDs, Output Factors (OF), Dosimetric Leaf Gap (DLG) and transmitted dose of the 120 HD MLC. PDDs were also evaluated in a slab-phantom with different materials. A set of 20 patients was selected for this study: 10 patients with brain metastases and 10 with lung metastases. For all patients VMAT plans with 10 MV FFF beam were optimized in Eclipse and calculated with Acuros. The DICOM files (plan, structure and images) were imported in PRIMO. DPM was used to calculate the dose distribution in the patients. The plans were compared in terms of DVHs through Homogeneity (HI) and Gradient (GI) Indexes. ResultsThe agreement between simulated and real beam was good with differences ⩽1.5% for all parameters considered. Some differences were observed in the MLC modeling. PRIMO showed a larger DLG with values ∼2 mm greater than both Acuros and the related experimental measurements (Fig. 1).The analysis of PDDs in the slab-phantom materials showed discrepancies of ∼1.2% between Acuros and PRIMO in cartilage and lung (Fig. 2). The MC dose distributions of the VMAT plans always had mean uncertainties within the PTV < 1.5%. Comparing the VMAT plans an overall good agreement was observed. ConclusionsPRIMO is an interesting tool for the clinical environment, useful to verify and support the commercially available TPS. The agreement with Acuros is good though further analysis should be done.

OA150] Influence of extrafocal dose in the out-of-field dose distribution in a paediatric anthropomorphic phantom, irradiated with a FFF beam and a 120 HD MLC – Monte Carlo simulations and gafchromic EBT3 dose distribution

Purpose Since paediatric patients have higher life expectancy than adults and tend to receive higher secondary organ doses due to geometrical factors, secondary induced cancer due to out-of-field dose among paediatric patients is thus an increased concern. This work focusses on the influence of extrafocal dose in the out-of-field dose distribution using a paediatric anthropomorphic phantom. Monte Carlo simulations (Penelope) and Gafchromic EBT3 2D dose distribution measurement will be assessed. Methods A 5-year pediatric phantom (hypothetical brain lesion) was irradiated by Varian TrueBeam 120HD MLC linac, using four 6MV Flattening Filter Free (FFF) static fields. A cranio-caudal beam was employed. The plan was simulated by Monte-Carlo (MC) simulation using PRIMO software. The 2D dose distributions at several distances from the isocenter were measured using Gafchromic EBT3 films. The results were evaluated using 2D Gamma index on PTW-Verisoft software. MC simulations were previously validated by gamma index method (98% of points 1; 2mm, 2%). Results Dose distributions were evaluated at 10, 15 and 17.5 cm away from the isocenter. The maximum local measured doses were 3.5, 1.6 and 1cGy per Gy at the isocenter respectively. A well-defined rectangular shaped dose distribution was noticed in the films located at 15 and 17.5 cm from the isocenter. The same shape was also detected in all the MC simulations of the same planes. The rectangular shape was not observed in the TPS (Acuros nor AAA). The center of this rectangular shape is aligned with the isocenter, and the width is consistent with the X-jaws opening direction. The dose values inside this rectangle are approximately 150% of the dose values in the surrounding regions. Conclusions The rectangular shaped dose distribution was linked to the extrafocal dose due to the cranio-caudal beam, which propagates significantly in the out-of-field regions (lungs, heart, etc.), even after applying known methodologies to lower it such as using FFF modes; choosing lower-energy beams; and limiting the Y-field size. The authors acknowledge support from the project NORTE-01-0145-FEDER-000027, supported by Norte Portugal Regional Operational Programme (NORTE 2020), under the PORTUGAL 2020 Partnership Agreement, through the European Regional Development Fund (ERDF).

Modeling the target dose fall-off in IMRT and VMAT planning techniques for cervical SBRT

There has been growing interest in the use of stereotactic body radiotherapy (SBRT) technique for the treatment of cervical cancer. The purpose of this study was to characterize dose distributions as well as model the target dose fall-off for intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) delivery techniques using 6 and 10 MV photon beam energies. Fifteen (n = 15) patients with non-bulky cervical tumors were planned in Pinnacle3 with a Varian Novalis Tx (HD120 MLC) using 6 and 10 MV photons with the following techniques: (1) IMRT with 10 non-coplanar beams (2) dual, coplanar 358° VMAT arcs (4° spacing), and (3) triple, non-coplanar VMAT arcs. Treatment volumes and dose prescriptions were segmented according to University of Texas Southwestern (UTSW) Phase II study. All plans were normalized such that 98% of the planning target volume (PTV) received 28 Gy (4 fractions). For the PTV, the following metrics were evaluated: homogeneity index, conformity index, D2cc, Dmean, Dmax, and dose fall-off parameters. For the organs at risk (OARs), D2cc, D15cc, D0.01cc, V20, V40, V50, V60, and V80 were evaluated for the bladder, bowel, femoral heads, rectum, and sigmoid. Statistical differences were evaluated using a Friedman test with a significance level of 0.05. To model dose fall-off, expanding 2-mm-thick concentric rings were created around the PTV, and doses were recorded. Statistically significant differences (p < 0.05) were noted in the dose fall-off when using 10 MV and VMAT3-arc, as compared with IMRT. VMAT3-arc improved the bladder V40, V50, and V60, and the bowel V20 and V50. All fitted regressions had an R2 ≥ 0.98. For cervical SBRT plans, a VMAT3-arc approach offers a steeper dose fall-off outside of the target volume. Faster dose fall-off was observed in smaller targets as opposed to medium and large targets, denoting that OAR sparing is dependent on target size. These improvements are further pronounced with the use of 10-MV photons.

Adapting VMAT plans optimized for an HD120 MLC for delivery with a Millennium MLC.

Linac downtime invariably impacts delivery of patients' scheduled treatments. Transferring a patient's treatment to an available linac is a common practice. Transferring a Volumetric Modulated Arc Therapy (VMAT) plan from a linac equipped with a standard‐definition MLC to one equipped with a higher definition MLC is practical and routine in clinics with multiple MLC‐equipped linacs. However, the reverse transfer presents a challenge because the high‐definition MLC aperture shapes must be adapted for delivery with the lower definition device. We have developed an efficient method to adapt VMAT plans originally designed for a high‐definition MLC to a standard‐definition MLC. We present the dosimetric results of our adaptation method for head‐and‐neck, brain, lung, and prostate VMAT plans. The delivery of the adapted plans was verified using standard phantom measurements.

Patient daily treatment verification using MLC log files

Purpose To investigate the viability of patient specific IMRT QA method which uses MLC log files to recreate a delivered fluence and compare to the planned fluence. Materials and methods A Varian Novalis Tx equipped with a HD120 MLC was used in this study. A total of 40 patients of both VMAT and step and shoot of various sites were used. Error containing plans were generated for 5 of the 40 patients. The errors introduced were MLC shifts, gantry angle errors and collimator errors. The treatment plans were delivered in air and the MLC log files recorded. An in-house MatLab program was used to recreate the planned fluence from the dicomRT file for reference and recreate the delivered fluence using data from the MLC log files. The resulting beam fluence maps and composite fluence maps were then compared using the gamma index. Gamma passing percentages were then compared to the results of the original IMRT QA (performed using the Delta 4 to establish a correlation and compare error detection capabilities. Results The overall mean gamma values for the Delta 4 and the fluence comparisons were similar. Gamma passing percentages for the Delta 4 were 96.9% while the fluence comparisons resulted in averages of 99.7% and 98.8% for 3%/3 mm and 2%/2 mm respectively. The error detection capabilities of the fluence comparison method was on par with that of the Delta 4 . Conclusion Using fluences created from MLC log files for patient specific IMRT QA is a valid alternative to traditional methods. The ease of use and efficiency of the fluence method makes it an attractive option for routine patient specific IMRT QA.

SU-F-T-590: Modeling PTV Dose Fall-Off for Cervical Cancer SBRT Treatment Planning Using VMAT and Step-And-Shoot IMRT

Purpose: Due to the high dose per fraction in SBRT, dose conformity and dose fall-off are critical. In patients with cervical cancer, rapid dose fall-off is particularly important to limit dose to the nearby rectum, small bowel, and bladder. This study compares the target volume dose fall-off for two radiation delivery techniques, fixed-field IMRT & VMAT, using non-coplanar beam geometries. Further comparisons are made between 6 and 10MV photon beam energies. Methods: Eleven (n=11) patients were planned in Pinnacle3 v9.10 with a NovalisTx (HD120 MLC) machine model using 6 and 10 MV photons. The following three techniques were used: (1) IMRT (10 non-coplanar beams) (2) Dual, coplanar 360° VMAT arcs (4° spacing), and (3) Triple, non-coplanar VMAT arcs (1 full arc and dual partial arcs). All plans were normalized such that 98% of the PTV received at least 28Gy/4Fx. Dose was calculated using a 2.0mm isotropic dose grid. To assess dose fall-off, twenty concentric 2mm thick rings were created around the PTV. The maximum dose in each ring was recorded and the data was fitted to model dose fall-off. A separate analysis was performed by separating target volumes into small (0–50cc), medium (51–80cc), and large (81–110cc). Results: Triple, non-coplanar VMAT arcs showed the best dose fall-off for all patients evaluated. All fitted regressions had an R2≥0.99. At 10mm from the PTV edge, 10 MV VMAT3-arc had an absolute improvement in dose fall-off of 3.8% and 6.9% over IMRT and VMAT2-arc, respectively. At 30mm, 10 MV VMAT3-arc had an absolute improvement of 12.0% and 7.0% over IMRT and VMAT2-arc, respectively. Faster dose fall-off was observed for small volumes as opposed to medium and large ones—9.6% at 20mm. Conclusion: Triple, non-coplanar VMAT arcs offer the sharpest dose fall-off for cervical SBRT plans. This improvement is most pronounced when treating smaller target volumes.

SU-G-BRC-05: Conundrum for VMAT Cranial Multiple Lesions Treated with HD120 MLC

Purpose: To commission a custom 6MV-SRS-AAA Eclipse beam model for VMAT multiple lesions cranial SRS treatment on a Varian TrueBeam STx. Methods: Six clinical plans were created using a customized beam model with dosimetric-leaf-gap(DLG) optimized for clinical treatments. Each plan had 4–6 non-isocentric targets with size from 0.2 to 7.1cc. All fields were measured with EBT3 film in the coronal plane in a solid water phantom and with an AS1000 EPID using gantry rotation. In addition, an end-to-end test was performed with coronal and sagittal films in an anthropomorphic phantom verifying dosimetry and localization accuracy. Portal dose distributions were generated with a custom portal dosimetry algorithm(PDIP). Measured dose distributions were compared with calculations using average dose difference (DD), and gamma function, γ. Using a 1.25mm grid, the γ criteria, local DD ≤ 3% and 2mm distance-to-agreement, were applied in regions with dose 50% of maximum. Results: The respective DD and γ for all films were 94.2%. The portal dose γ scores for all the plans were >94.9%. However, local regions with underdose >10%, were observed when targets were treated with the 5mm leaves. The same plans re-optimized with two isocenters such that all lesions were under the 2.5mm leaves did not show this effect. The DD and localization error of the end-to-end test were within 3.4% and 1.0mm respectively. Conclusion: The custom AAA beam model is capable of calculating acceptable dosimetry for targets using only the 2.5 mm leaves. This restricts lesions to within ±4cm of isocenter. The observed underdose beneath the 5mm leaves is attributed to a limitation in Eclipse that uses a single DLG representing the DLG's of both 2.5mm and 5mm leaves. If lesions are >4cm from isocenter, a multiple isocenter technique should be considered to allow the use of only the 2.5mm leaves.

SU‐G‐TeP4‐13: Interfraction Treatment Monitoring Using Integrated Invivo EPID Images

Purpose:To investigate inter‐fraction differences of dose delivery by analyzing portal images acquired during treatment and implement an automated system to generate a report for each fraction. Large differences in images between fractions can alert the physicist of possible machine performance issues or patient set‐up errors.Methods:A Varian Novalis Tx equipped with a HD120 MLC and aS1000 electronic portal imaging device (EPID) was used in our study. EPID images are acquired in continuous acquisition mode for 32 volumetric arc therapy (VMAT) patients. The images are summed to create an image for each arc and a single image for each fraction. The first fraction is designated as the reference unless a machine error prevented acquisition of all images. The images for each beam as well as the fraction image are compared using gamma analysis at 1%/1mm, 2%/2mm and 3%/3mm. A report is then generated using an in house MatLab program containing the comparison for the current fraction as well as a history of previous fractions. The reports are automatically sent via email to the physicist for review. Fractions in which the total number of images was not within 5% of the reference number of images were not included in the results.Results:91 of the 182 fractions recorded an image count within 5% of the reference. Gamma averages over all fractions and patients were 96.2% ±0.8% at 3%/3mm, 92.9% ±1% at 2%/2mm and 80.6% ±1.8% at 1%/1mm. The SD between fractions for each patient ranged from .004% to 10.4%. Of the 91 fractions 3 flagged due to low gamma values. After further investigation no significant errors were found.Conclusion:This toolkit can be used for in‐vivo monitoring of treatment plan delivery an alert the physics staff of any inter‐fraction discrepancies that may require further investigation.

SU-F-T-594: Dosimetric Impact of Multileaf Collimator Leaf Width On Single and Multiple Isocenter Stereotactactic IMRT Treatment Plans for Four Or More Intracranial Tumors

Purpose: To compare the impact of MLC width on the dose conformity, doses received by the normal brain tissue, and other critical structures for single isocenter (SI) and multiple isocenter (MI) stereotactic IMRT (SRT) treatments for patients with at least four brain tumors. Methods: Six patients (three with seven and three with four lesions) planned in BrainLab iPlan were evaluated using Varian HD MLC (2.5mm leaf-width) and Millennium-120 MLC (5mm leaf-width). Prescribed doses in all plans were 25 Gy delivered in five fractions normalized to 98% dose covering 98% of the target volume. Results: All plans were judged clinically acceptable. The average Paddick conformity index for 2.5mm vs 5mm SI and MI plans were (0.48±0.22 vs. 0.38±0.21, p<.01) and (0.49±0.18 vs. 0.39±0.16, p<.01), respectively. The average percent volumes of normal brain receiving, V12, V10, and V8 Gy for (2.5mm vs 5mm) SI plans were (15.3±11.5 vs. 17.8±13.2, p=0.04), (20.8±14.8 vs. 23.5±15.7, p=0.01), (28.1±18.0 vs. 30.7±18.1, p=0.05), respectively. For MI plans these values were respectively (11.8±8.3 vs. 13.2±8.9, p<0.01), (16.3±11.5 vs. 17.7±11.9, p=0.01), and (22.4±15.5 vs. 23.8±15.4, p=0.01). The average mean and maximum brainstem doses for (SI and MI) plans were (7% and 5%) and (6% and 2%) lower in 2.5mm plans than 5mm plans, respectively. For (SI and MI) plans the average mean and maximum hippocampus doses were (11% and 11%) and (7% and 5%) lower in 2.5mm plans than 5mm plans, respectively. The average mean and maximum chiasm doses for (SI and MI) plans were (5% and 30%) and (7% and 28%) lower in 2.5mm plans than 5mm plans, respectively. Conclusion: MI stereotactic IMRT treatment plans, especially with HD MLC, were capable of achieving significantly higher dose conformity, lower normal brain tissue doses, and lower doses to critical structures than SI plans for four or more intracranial tumors.

SU‐E‐T‐480: IMRT/VMAT QA Employing a High Resolution Measurement Array: Initial Experience and Limitations

Purpose:Patient specific QA for IMRT/VMAT is time‐consuming. A high resolution ionization chamber array was employed which rotates synchronously with the gantry. Commissioning, initial experience and limitations of this measurement technique will be discussed.Methods:A high resolution ionization chamber array, Octavius1500 (PTW‐Freiburg, Germany), was employed with the Octavius4D rotating cylindrical phantom. Measured plans are reconstructed into a 3D dose distribution using measured depth dose curves (Verisoft 6.0 PTW‐Freiburg). Plans of varying complexity were selected from our patient collective treated at a TrueBeam 1.5 linac (Varian Medical Systems) with HD120 MLC (VMS). Cross‐calibration was performed using a thimble chamber (30016, PTW‐Freiburg) placed within the cylindrical phantom (SSD 840mm, SCD 1000mm). All dose distributions were recalculated using a 1.5mm dose grid (Eclipse, AAA10.0.28/Acuros10.0.28, VMS) using a virtual homogeneous cylindrical phantom provided by the array manufacturer (density 1.016 gcm‐3). All analyses were performed using Verisoft and compared to routine QA analysis (Octavius729 with octagonal phantom) results.Results:Depth dose curves from 1cm × 1cm to 26cm × 26cm were measured for 3D dose calculation commissioning in Verisoft 6.0. Excellent agreement was observed between TPS PDD and PDD derived from the Octavius1500 array measurement. For highly modulated plans the overall 3D gamma (2.0mm, 3.0%) ranged between 91.3% to 96.9%. The Octavius729 (2D) array results ranged from 97.9 to 100.0% for PTVs.Conclusion:The Octavius1500 and 4D cylindrical phantom provides an integrated QA solution. Complex VMAT plans, comprising multiple PTVs, can be verified with a single (rotating) measurement of all arcs. Hitherto two measurements (horizontal and vertical) had to be performed per PTV. Overall the results are good. However, some refinement may need to be made to the reconstruction. Further work needs to be done to determine appropriate action levels for routine analysis.

WE-D-BRA-06: IMRT QA with ArcCHECK: The MD Anderson Experience

Purpose: The objective of this project was to report our initial IMRT QA results and experience with the SunNuclear ArcCHECK. Methods: Three thousand one-hundred and sixteen cases were treated with IMRT or VMAT at our institution between October 2013 and September 2014. All IMRT/VMAT treatment plans underwent Quality Assurance (QA) using ArcCHECK prior to therapy. For clinical evaluation, a Gamma analysis is performed following QA delivery using the SNC Patient software (Sun Nuclear Corp) at the 3%/3mm level. QA Gamma pass rates were analyzed based on categories of treatment site, technique, and type of MLCs. Our current clinical threshold for passing a QA (Tclin) is set at a Gamma pass rate greater than 90%. We recorded the percent of failures for each category, as well as the Gamma pass rate threshold that would Result in 95% of QAs to pass (T95). Results: Using Tclin a failure rate of 5.9% over all QAs was observed. The highest failure rate was observed for gynecological (22%) and the lowest for CNS (0.9%) treatments. T95 was 91% over all QAs and ranged from 73% (gynecological) to 96.5% (CNS) for individual treatments sites. T95 was lower for IMRT and non-HD (high definition) MLCs at 88.5% and 94.5%, respectively, compared to 92.4% and 97.1% for VMAT and HD MLC treatments, respectively. There was a statistically significant difference between the passing rates for IMRT vs. VMAT and for HD MLCs vs. non-HD MLCs (p-values << 0.01). Gynecological, IMRT, and HD MLC treatments typically include more plans with larger field sizes. Conclusion: On average, Tclin with ArcCHECK was consistent with T95, as well as the 90% action level reported in TG-119. However, significant variations between the examined categories suggest a link between field size and QA passing rates and may warrant field size-specific passing rate thresholds.