MLC Leaf Research Articles

Optimizing LINAC-based Stereotactic Radiosurgery Dosimetric Outcomes Dictated by Beam Energy, MLC Leaf Width and Prescribed Isodose Level

LINAC-based stereotactic radiosurgery (SRS) has gained popularity in treating intracranial tumors. This study aims to compare the dosimetric outcomes of different SRS methods for typical brain tumors of various sizes, as dictated by beam energy, MLC leaf width, and prescribed isodose level (IDL). Hypothetical target lesions of 4 different sizes were contoured on brain MRI images of an index patient. The PTV were constructed to be within 4 respective categories (<0.5, 0.5-1.0, 1.0-4.0, 4.0-10.0 cc), in inverse proportion with prescription dose range of 16-22.5 Gy. For each lesion, SRS plans were analyzed independently between (1) two photon beam energies with different dose rates (6X-FFF with 1400 MU/min vs. 10X-FFF with 2400 MU/min), (2) two MLC leaf widths (0.5 cm vs. 0.25 cm), and (3) two different prescribed isodose lines (90% IDL vs. 80% IDL). The degree of normal tissue sparing was evaluated with the amount of brain volume receiving >12 Gy (V12) and the dose Gradient Index (GI) - defined as the ratio between volume receiving >50% of prescribed dose (V50%) and PTV volume. Tumor dose inhomogeneity was analyzed with respect to the maximum dose (Dmax) deposited, while conformity was assessed via Conformity Index (CI) - defined as the ratio between V100% and PTV volume. Treatment beam-on time was also compared. All 3 independent variables (beam energy, MLC leaf width, and prescribed IDL) were held constant except the one being analyzed, with mean relative differences of the dosimetric outcomes determined for all 4 lesions of respective volumes. The 4 PTV volumes contoured were: 0.35, 0.68, 1.76 and 5.60 cc, respectively. Dmax was affected only by the prescribed IDL, while the tumor CI did not exhibit any significant difference for all 3 variables analyzed. Compared to 10X-FFF, the 6X-FFF plans have lower GI by 11.51%±5.87%, lower V12 by 11.38%±6.98%, and longer beam-on time by 96.41%±4.44%. Compared to 0.25cm MLC leaf-width, the 0.5cm plans have higher GI by 22.12%±8.64%, higher V12 by 22.20%±11.12%, and shorter beam-on time by 2.63%±1.07%. Compared to 80%IDL, the 90%IDL plans have higher GI by 25.73%±9.09%, higher V12 by 25.27%±10.78%, lower Dmax by 11.02%±0.19%, and shorter beam-on time by 12.80%±0.78%. This study showed that (1) lower beam energy plans may provide better brain sparing but incur much longer beam-on time, (2) thinner MLC leaf width may provide better brain sparing at the expense of slightly longer treatment time, and (3) prescribing dose at lower IDL may provide better brain sparing but result in higher tumor dose inhomogeneity and longer beam-on time. It may thus provide useful information for LINAC-based SRS practitioners to better formulate customized treatment strategy based on how beam energy, MLC leaf width and prescribed IDL may each affect tumor dose coverage, normal brain tissue sparing or treatment duration.

Influence of maximum MLC leaf speed on the quality of volumetric modulated arc therapy plans.

PurposeMaximum leaf speed is a configurable parameter of MLC in a treatment‐planning system. This study investigated the influence of MLC on the quality of VMAT plans.MethodsSeven MLCs with different maximum leaf speeds (1.0, 1.5, 2.25, 3.5, 5.0, 7.5, and 10 cm/s) were configured for an accelerator in treatment‐planning system. Correspondingly, seven treatment plans, with the identical initial optimization parameter, were designed with the mdaccAutoPlan system. Six nasopharyngeal carcinoma (NPC) patients and nine rectal cancer patients were selected, representing complex and simple clinical circumstances. VMAT plan quality was evaluated with PlanIQTM software. The results were statistically analyzed with a one‐way analysis of variance (ANOVA) and pairwise comparison tests.ResultsThe relative changes of plan scores achieved by the seven configured accelerators, with specific maximum MLC leaf speed (MMLS) for each patient, were studied. Two apparent trends of MMLS influence on VMAT plan scores were observed: Plan scores increased with MMLS; Plan scores increased rapidly when MMLS increased from 1 to 3.5, thus the relative change of plan score decreased in this MMLS range. The stationary point of maximum MLC speed (MMSSP) is defined, for the specific MMLS when the relative changes of plan scores is first <5%, as MMLS increases from 1.0 to 10. For rectal plans, MMSSPs were 2.25 for six patients and 3.5 for the other three patients. For NPC plans, MMSSPs were 3.5 for five patients and 2.25 for one patient.ConclusionThis work indicates that MMLS directly influences VMAT plan quality in NPC cases and rectal cancer cases. VMAT plan quality improved conspicuously as MMLS increased from 1 to 3.5, VMAT plan quality with marginal improvement when MMLS is above 3.5.

Comparison of VMAT complexity-reduction strategies for single-target cranial radiosurgery with the Eclipse treatment planning system.

Complexity in MLC‐based radiosurgery treatment delivery can be characterized by the efficiency of monitor unit (MU) utilization and the average MLC leaf separation distance for a treatment plan. A reduction in plan complexity may be desirable if plan quality is not impacted. In this study, a number of strategies are explored to determine how plan quality is affected by efforts to reduce plan complexity. Ten radiosurgery cases of varying complexity are retrospectively planned using six optimization strategies: an unconstrained volumetric modulated arc therapy (VMAT) technique, a MU‐constrained VMAT technique, three techniques using various strengths of the aperture shape controller (ASC), and a hybrid technique consisting of a final‐stage VMAT optimization applied to a dynamic conformal arc leaf sequence (ODCA). The plans are compared in terms of MU efficiency, MLC leaf‐separation, conformity index (CI), gradient index (GI), and QA measurement results. The five VMAT techniques exhibited only minor differences in CI and GI values, though the ASC and MU‐constrained techniques did require 6–20% fewer MU and had mean field apertures 5–19% larger. On average, the ODCA technique had CI values 3.5% lower and GI values 1.0–2.5% higher than the VMAT techniques, but also had a mean field aperture 24–47% larger and required 16–32% fewer MU. The QA measurement results showed a 0.61% variation in mean per‐field 2%/1 mm gamma passing rates across all techniques (range 96.81%–97.42%), with no observed correlation between passing rate and technique. For simple targets, the ODCA technique achieved CI results that were equivalent to the unconstrained VMAT technique with an average 30% reduction in required MU, an average 50% increase in mean leaf separation distance, and brain V12Gy values within 0.38 cc of the VMAT technique for targets up to approximately 2 cm diameter. For MLC‐based single‐target radiosurgery, plan complexity can often be significantly reduced without an equivalent reduction in plan quality.

Time and frequency to observe fiducial markers in MLC-modulated fields during prostate IMRT/VMAT beam delivery

ObjectiveThis work investigates the time and frequency to observe fiducial markers in MLC-modulated fields during intensity-modulated radiotherapy (IMRT) and volumetric-modulated arc therapy (VMAT) beam delivery for real-time prostate localization. MethodsThirty seven prostate patients treated with IMRT or VMAT were included in this retrospective study. DRR images were generated for all MLC segments/control points using the TPS. The MLC leaf pattern of each control point was overlaid on the DRR, and the number of fiducials within the MLC opening was analyzed. EPID images of fiducials in a pelvic phantom were obtained to demonstrate the fiducial visibility during modulated beam delivery. ResultsGold fiducials were visible on EPID images. The probability of seeing a number of fiducials within the MLC opening was analyzed. At least one fiducial was visible during 42 ± 2% and 52 ± 2% beam-on time for IMRT of the prostate with and without lymph nodes, and during 81 ± 4% and 80 ± 5% beam-on time for VMAT of the prostate with and without lymph nodes, respectively. The mean time interval to observe at least one fiducial was 8.4 ± 0.7 and 5.9 ± 0.5 s for IMRT of the prostate with and without the lymph nodes, respectively, and 1.6 ± 0.1 s for VMAT prostate patients. The estimated potential dosimetric uncertainty was 7% and 2% for IMRT and VMAT, respectively. ConclusionsOur results demonstrated that the time and frequency to observe fiducial markers in MLC-modulated fields during IMRT/VMAT beam delivery were adequate for real-time prostate localization. The beam’s eye view fiducial positions could be used for intrafractional target monitoring and motion correction in prostate radiotherapy.

Simulation of VMAT (Volumetric Modulated Arc Therapy) Delivery techniques on Cylinder Phantom Based on DICOM Data using Monte Carlo Method - EGSnrc

This research aims to study one of the radiotherapy techniques of VMAT and calculate the dose distribution on phantom cylinders using the Monte Carlo EGSnrc method. VMAT is one of radiotherapy technique where is all the fractions of the dose are given continuously when the gantry rotates around the patient with manages gantry rotation speed, MLC leaf position and the dose fraction is given. The simulation process begins with head linac modeling using BEAMnrc, and using the Monte Carlo VMAT simulation uses DOSXYZnrc software. The parameters for the BEAMnrc and DOSXYZnrc simulations using information from DICOM data set rtplan AAA are read using the pycom code. The results of this study provide information about the characteristics of 6 MV photon files such as fluence, fluence energy, spectral distribution, and fluence energy distribution, and dose distribution in phantom cylinders. The VMAT Monte Carlo simulation was carried out using 300 million particles which resulted in a VMAT dose distribution in the form of a dose profile curve and an isodose curve, the result shows that the curve of VMAT simulation using phantom cylinders produced the maximum dose distribution in the isocenter

Feasibility Study of a New Ring-Gantry Linac for Single Vertebral Segment SBRT

The novel, ring-gantry linear accelerator machine allows only translational and no rotational couch corrections. This study aims to compare treatment plan quality for single vertebral segment SBRT using the ring-gantry linear accelerator and the medical linear accelerator machines. In addition, this study investigates the effect of patient rotational setup errors on the delivered plan quality. We analyzed 20 patients with single-level spine metastasis, located between T7 and L5 near the spinal cord. The median tumor volume was 64.3 cm3 (17.9-139.1 cm3) and the median Z-axis tumor diameter was 3.1cm (2-5cm). The prescription doses were 12-16 Gy in 1 fraction or 18-24Gy in 3 fractions. All patients were originally treated in the TB machine with 2.5 mm leaf width, and replanned for the Halcyon machine with 5mm effective MLC leaf width. Offline CBCT fusion from 50 SBRT spine treatments showed an average rotation of 0.63, 1.19 and 0.79 degree in pitch, yaw and roll. Rotational error in roll was not considered in this study since the original TB plans used coplanar volumetric modulated arc therapy (VMAT) technique. Four CT image datasets were generated with different rotational combinations of pitch and yaw for 1 degree in Velocity to simulate different scenarios of patient setup errors. We recalculated the Halcyon plans on rotated images. Dosimetric plan quality was evaluated for tumor coverage, conformity index (CI), maximum dose to the cord, and volume of the cord receiving 8, 10, and 12 Gy. Paired student’s t-test was performed with a significance level of 0.01. The commercially available treatment planning system based VMAT plans for Halcyon achieved similar target coverage (92.3 ± 3.0% vs. 91.2 ± 5.5%, p=0.21), and CI (1.0 ± 0.1 vs. 1.1 ± 0.2, p=0.09) compared to TB plans. Maximum dose to the spinal cord was comparable (11.1 ± 2.8 Gy vs. 12.1 ± 4.2 Gy, p =0.05). So were the V8, V10 and V12 for the cord. The dosimetric influence of patient rotational setup error was statistically insignificant for rotations of up to 1 degree pitch/yaw (with similar target coverage, CI, max cord dose and V8, 10, 12 for cord). The total MUs for Halcyon (4739 ± 1522) were slightly less than TB (5412 ± 2057). VMAT plans for single spine metastasis were dosimetrically comparable to TB plans. As long as the patient rotation is within 1 degree in the pitch and yaw directions, the dosimetric effect was insignificant. Halcyon is an alternative to TB for the treatment of single spinal level metastases, but users need to be cautious about the patient rotational setup error.Abstract 3789; Table 1Comparison of Dosimetric indices on 1 degree rotated imageVariableTilt 01A1B1C1DConformity Index (CI)1.09±0.241.10±0.241.10±0.241.10±0.241.10±0.24PTV Coverage (%)91.2±5.4691.0±5.5591.0±5.5791.2±5.3791.1±5.64Max Cord (Gy)12.1±4.1712.0±4.2212.1±4.1812.1±4.2412.1±4.16Spinal Cord V8 (cc)2.57±2.102.52±2.082.53±2.072.52±2.072.51±2.06Pitch / Yaw: -1°/-1° (1A), -1°/+1° (1B), +1°/-1° (1C), +1°/+1° (1D); All P values>0.01 (n=20, paired t-test, 2 tails) Open table in a new tab

The Effect of MLC Leaf Motion Constraints on Plan Quality and Delivery Accuracy in VMAT

The Effect of MLC Leaf Motion Constraints on Plan Quality and Delivery Accuracy in VMAT

Single-arc VMAT optimization for dual-layer MLC

Dual-layer multi-leaf collimator (DLMLC) has recently attracted renewed interest due to its good balance among resolution, low leakage, and high fabricability. However, existing progressive sampling based volumetric modulated arc therapy (VMAT) algorithm is ineffective for DLMLC, requiring more arcs to achieve dosimetry comparable to VMAT plans with higher resolution single-layer MLC (SLMLC). In this study, we develop a novel single-arc VMAT optimization framework to take advantage of the unique DLMLC characteristics fully. Direct aperture optimization (DAO) for single-arc DLMLC VMAT was formulated as a least square dose fidelity objective, along with an anisotropic total variation term to regulate the fluence smoothness and a single segment term for forming simple apertures. The DAO was solved through alternating optimization approach. The DLMLC deliverability constraint and the MLC leaf speed constraint were formulated as the optimization constraints and solved using a graph optimization algorithm. Feasibility of the proposed framework was tested on a brain, a lung, and a prostate cancer patient. The framework was further adapted for a simultaneous integrated boost (SIB) case. The single-arc DLMLC-10 mm (leaf width) plan was compared against single-arc SLMLC VMAT plans including SLMLC-5mm, SLMLC-10mm, and SLMLC with 10 mm leaf width and 5 mm leaf step size (SLMLC-10mm-5mm). Compared with the SLMLC-10mm plan and the SLMLC-10mm-5mm plan, with the same target coverage, the DLMLC-10 mm plan reduced R50 by 30.7% and 10.0%, the average max OAR dose by 5.79% and 3.7% of the prescription dose, and the average mean OAR dose by 4.18% and 2.1% of the prescription dose, respectively. The plan quality is comparable to that of the SLMLC-5mm plan. The novel single-arc VMAT optimization framework for DLMLC utilizes two MLC layers to improve the effective modulation resolution and afford more sophisticated modulation. Consequently, DLMLC VMAT achieves superior dosimetry to SLMLC VMAT with the same leaf width.

Plan-Class Specific Reference Quality Assurance for Volumetric Modulated Arc Therapy

Background: There have been much efforts to develop the proper and realistic machine Quality Assurance (QA) reflecting on real Volumetric Modulated Arc Therapy (VMAT) plan. In this work we propose and test a special VMAT plan of plan-class specific (pcsr) QA, as a machine QA so that it might be a good solution to supplement weak point of present machine QA to make it more realistic for VMAT treatment. Materials and Methods: We divided human body into 5 treatment sites: brain, head and neck, chest, abdomen, and pelvis. One plan for each treatment site was selected from real VMAT cases and contours were mapped into the computational human phantom where the same plan as real VMAT plan was created and called plan-class specific reference (pcsr) QA plan. We delivered this pcsr QA plan on a daily basis over the full research period and tracked how much MLC movement and dosimetric error occurred in regular delivery. Several real patients under treatments were also tracked to test the usefulness of pcsr QA through comparisons between them. We used dynalog file viewer (DFV) and Dynalog file to analyze position and speed of individual MLC leaf. The gamma pass rate from portal dosimetry for different gamma criteria was analyzed to evaluate analyze dosimetric accuracy. Results and Discussion: The maxRMS of MLC position error for all plans were all within the tolerance limit of < 0.35 cm and the positional variation of maxPEs for both pcsr and real plans were observed very stable over the research session. Daily variations of maxRMS of MLC speed error and gamma pass rate for real VMAT plans were observed very comparable to those in their pcsr plans in good acceptable fluctuation. Conclusion: We believe that the newly proposed pcsr QA would be useful and helpful to predict the mid-term quality of real VMAT treatment delivery.

11. Sensitivity of patient specific quality assurance to simulated delivery errors for CyberKnife MLC treatments and effects on DVH

Purpose Accurate and reliable patient-specific QA is required for CyberKnife (CK) MLC treatment plans. Films are the current method of choice, but filmless solutions are starting to be introduced in the routine [1] . Aim of this study is to test the sensitivity of a filmless QA technique to simulated delivery errors for CK MLC plans, and to discuss which γ -index criteria are adequate; DVH implications were also analyzed. Methods Preliminarily, routine garden fence tests were employed to assess MLC leaf positioning accuracy. Test plans were then delivered to Octavius 1000SRS array (PTW) and EBT3 (Ashland), editing leaf positions of five clinical plans (prostate/pancreas) with three error types: MLC banks opened, both of 0.3 mm (03 mm OUT)/0.5 mm (05 mm OUT), one of 1 mm (1 mm X2)), closed (both of 0.5 mm (05 mm IN)) and shifted (both of 1 mm (1 mm X1X2)). Γ-index analysis was performed comparing the measured plans to the unedited dose distributions using three criteria: 2%2 mm, 3%1 mm, 2%1 mm. For patient plans, DVH implications were also considered. Results The garden fence leaf positioning distribution mean over 11578 values is 0.07 ± 0.16 mm. The 95% percentile is 0.34 mm, with 141 values (1.2%) outside the 0.5 mm tolerance. In Fig. 1 1000SRS passing rates for the edited and the unedited plans are reported for two cases. For all plans, the highest influence on pass-rate is found for increased segments size. With both banks opened of 0.3 mm a mean passing rate decrease from 94.0% to 80.2% was observed (3%1 mm). EBT3 show analogous results. In Table 1 the simulated errors effect on DVH parameters is reported for Plan2. A 0.3 mm opening produced a total of 7 OAR constraint violations over all plans. Conclusions High sensitivity in the detection of small delivery errors, slightly affecting DVHs, is achievable for CK MLC plans delivery QA even employing filmless solutions. A 2%2 mm criteria is not always tight enough.

190. Correlation between modulation degree and dosimetric accuracy of prostate VMAT treatment planning

Purpose To evaluate the impact of modulation degree (MD) on patient specific pre-treatment quality assurance for prostate VMAT treatment planning. Methods Ten patients with similar PTV and rectum size were included in the study. The prescribed fractional dose to PTV was 2,5 Gy daily. VMAT plans were calculated using a MonteCarlo-based TPS. For each patient we optimized 4 different plans using the same constraint cost-functions for PTV and organ-at-risk (OARs), the same plan parameters (arc number, maximum number of control points for arc) but changing the minimum distance between opposing MLC leafs (MDW = 0.5, 0.8, 1, 1.5 cm). Such parameter influences the MD and the plan quality. MD was calculated from the DICOM-RT files using the mean segment area value and total Monitor Unit. To evaluate the plan quality we took into account the PTV coverage and rectum dose. Gamma index analysis was performed to compare measured and calculated dose. Plans were accepted if 95% of the points satisfied the 2% dose-difference and distance-to-agreement 2 mm (γ2%,2 mm). All plans were delivered by a 6 MV Linac and the patient specific pre-treatment quality assurance verification was performed using a cylindrical diode array-phantom. Descriptive statistic was performed to evaluate the correlation between MD and gamma passing rate measured. Results We observed a negative linear correlation (Fig. 1) between MD values and γ2%,2 mm (correlation coefficient r = 0.7, p = 0.0004). The plans with MD 4 yielded a γ2%,2 mm inferior than 95% in 100% of cases (Fisher-exact-test p = 0.002). 93% of plans with MDW > 0.8 cm had a MD Conclusions The gamma index value was significantly related to MD 4. Rival plans with a MD 0.8 cm allows to obtain a desirable MD value. The calculation of MD for prostate VMAT treatment planning is important to predict the patient-QA in term of gamma index analysis.

16 Dosimetry improvement of an EPID-based pre-treatment verification software by the implementation of a dual photon beam model from linear accelerators

Introduction Point-kernel convolution-based dose calculation algorithms require an accurate modelling of the incident energy. The contribution of diffused photons by the primary collimator and the flattening filter (extra-focal source) is added to the primary source. Deviations up to 5% can be observed (Alaei, 2010) compared to the measurement for certain settings of collimation elements without an explicit modelling of the extra-focal source according to the position of the various collimation elements. EPID-based pre-treatment verification solutions generally uses fluence-based convolution models with the impulsional response of the detector. To be perfectly reliable, these methods must accurately predict the portal image to be compared with the acquired image. The aim of this work concerns the implementation and validation of an extra-focal source model for the computation of the predicted EPID image for standard (FF) and FFF (Flattening Filter Free) beams. Methods The linear accelerator used in this study is a TrueBeam STx® equipped with MLC HD120 and EPID as1200 with 6MV FF and FFF photons. Square fields with swapped openings of jaws and MLC leafs of 5 to 22 cm are designed with Eclipse® (Varian). Reference dose measurements in water are carried out on the central axis, with a microDiamond detector at SSD = 95 cm and 5 cm deep. The predicted and acquired portal images are processed with the EPIbeam software (Dosisoft). The prediction model calculates the expected dose map at SSD = 95 cm and 5 cm depth in water from the DICOM RT plan file, which is then compared to the acquired portal image converted to dose. The accuracy of the TPS calculations, dose conversion and prediction models in EPIbeam, without and with implementation of an extra-focal source model including only the effects of the flattening filter, are confronted with the reference measures in water. Results The maximum discrepancy between the TPS calculations and the measurements is less than 1%. The dose values on the converted EPID image accurately match to microDiamond measurements (less than 1% of error). With the primary source model alone, the EPIbeam prediction model for FF photons shows dose differences up to 4% with the microDiamond detector with small MLC openings and retracted jaw positions. The observed deviations after the implementation of the extra-focal source model remain less than 1% for all collimations. For the FFF photons, deviations are in all cases less than 1%. Conclusions For FFF photons, the implementation of an extra-focal source model is optional since the contribution of the primary collimator appears negligible. However, this study confirms the importance of modelling the extra-focal source for FF photons to enhance the accuracy of the dosimetry calculations for pre-treatment checking.

11 Evaluation of delivery analysis software (Accuray), comparison to Delta 4 Phantom

Introduction Delivery Analysis software uses the signal received from xenon detectors of Tomotherapy to calculate the MLC leafs open time (LOTs) and compares the measured sinogram with the planned one. We have used Delivery analysis for Tomotherapy pretreatments controls for more than two years. Each measure was also carried out using the Delta 4 phantom. In this study, we propose a comparison of these two measurement tools. Methods First, an evaluation of the repeatability and the reproducibility of both tools was realized.Then, a retrospective study of the results obtained for 82 patients, regardless the localization, was led. The percentages of points passing the gamma index criterion 3 %-3 mm with a 10 % threshold were analyzed according to the detector type as well as treatments plans parameters. Finally, a set of measures of deliberately modified plans was realized on the Delta 4 in order to test a potential dependence of the results with the gantry period. To achieve this, nine plans were created from the same MLC sequencing by modifying the period of rotation between 11 s and 30 s while keeping the same LOTs. Results The repeatability and the reproducibility of Delivery Analysis software were respectively 0.06% and 0.51% versus 0.43% and 0.92% for the Delta 4. The retrospective study did not make it possible to highlight a correlation between the results obtained with the Delta 4 and Delivery Analysis. However, concerning the Delivery Analysis and looking at the treatments plans parameters, we observed a strong correlation of the percentage between points passing the gamma criterion and the gantry period which was not visible with the Delta 4 (Fig. 1). Download : Download high-res image (211KB) Download : Download full-size image Measurements made for different gantry periods with the Delta 4 seemed to confirm that there was no correlation between the performances of the machine and the gantry period. Indeed, a decrease of only 2 % of the percentage of points crossing the criteria gamma was noticed for gantry periods ranging from 30 s to 11 s. Conclusions Using Delivery Analysis software for pretreatments quality controls could be very interesting to save time. Furthermore, they could be done with a better repeatability/ reproducibility of measurements than with the Delta 4. However, the results obtained with the software are difficult to correlate to those obtained with the Delta 4 phantom. In addition, they seem abnormally correlated to the gantry period. These results should be confirmed with further experiments.

Monte Carlo and analytic modeling of an Elekta Infinity linac with Agility MLC: Investigating the significance of accurate model parameters for small radiation fields.

PurposeTo explain the deviation observed between measured and Monaco calculated dose profiles for a small field (i.e., alternating open‐closed MLC pattern). A Monte Carlo (MC) model of an Elekta Infinity linac with Agility MLC was created and validated against measurements. In addition, an analytic model which predicts the fluence at the isocenter plane was used to study the impact of multiple beam parameters on the accuracy of dose calculations for small fields.MethodsA detailed MC model of a 6 MV Elekta Infinity linac with Agility MLC was created in EGSnrc/BEAMnrc and validated against measurements. An analytic model using primary and secondary virtual photon sources was created and benchmarked against the MC simulations and the impact of multiple beam parameters on the accuracy of the model for a small field was investigated. Both models were used to explain discrepancies observed between measured/EGSnrc simulated and Monaco calculated dose profiles for alternating open‐closed MLC leaves.Results MC‐simulated dose profiles (PDDs, cross‐ and in‐line profiles, etc.) were found to be in very good agreements with measurements. The best fit for the leaf bank rotation was found to be 9 mrad to model the defocusing of Agility MLC. Moreover, a very good agreement was observed between results from the analytic model and MC simulations for a small field. Modifying the radial size of the incident electron beam in the BEAMnrc model improved the agreement between Monaco and EGSnrc calculated dose profiles by approximately 16% and 30% in the position of maxima and minima, respectively.ConclusionAccurate modeling of the full‐width‐half‐maximum (FWHM) of the primary photon source as well as the MLC leaf design (leaf bank rotation, etc.) is essential for accurate calculations of dose delivered by small radiation fields when using virtual source or MC models of the beam.

Treating Benign Cranial Lesions - Treatment Planning System Comparison

To compare three treatment planning systems for treating benign cranial lesions using stereotactic radiosurgery (SRS). Three treatment planning software systems were compared for treating benign cranial lesions. iPlan (Brainlab, Germany) is a dedicated software for SRS planning using non-coplanar dynamic conformal arcs. The dose prescription, arc setup and MLC positions are planner controlled. Cranial SRS (Brainlab, Germany) (CSRS) is a new software for treating benign cranial lesions using up to 5 intensity-modulated arcs. The user can select the degree of modulation, (from minimal modulation to full VMAT). The couch angles, number of arcs and their lengths are optimization parameters and the treatment planning is highly automated. Eclipse RapidArc (Varian, Palo Alto) (RA) uses an inverse planning approach where the planner determines the number of arcs, their lengths and couch angles, as well as the optimization objectives for the PTV and organs at risk (OARs). The optimization parameters can be adjusted "on the fly" as the optimization progresses. The comparison metrics were conformity index (CI), gradient index (GI), volume of brain receiving over 12 Gy (V12) and mean brain dose between the three TPSs. We compared 10 plans for acoustic neuromas and meningiomas to plans generated and treated by iPlan, our current clinical standard. We also compared 4 cases that were treated with RA due to unusual shape, for which iPlan did not yield acceptable plans. All CSRS and RA plans for all indications were judged clinically acceptable. The average CI was 1.38 and 1.30 for CSRS and RA, respectively, and the average GI was 3.76 and 4.11 for CSRS and RA, respectively. No metrics were significantly different compared to iPlan. However, for those lesions for which iPlan could not generate acceptable plans, treatment plans with CSRS were more homogenous than those generated by RA. Planning times were significantly shorter for CSRS compared to the other TPSs. This was especially noticeable for iPlan, where MLC leaf shaping is a manual, labor-intensive process. Eclipse took much longer to perform the actual calculation using a 1 mm grid than either Brainlab TPS. iPlan used less MUs compared to either RA or CSRS: an average of 1910 MUs for iPlan, 3883 MUs for RA and 2033 MUs for CSRS. Planning times for CSRS were much shorter than for either iPlan or RA and planning was more efficient due to the high level of automation. However, the need to pre-define settings in a template outside the CSRS workspace can be cumbersome. Higher modulation in CSRS led to lower CI and GI values and lower OAR doses without affecting planning times. For complex lesions for which iPlan was not adequate, both VMAT TPSs yielded highly conformal treatment plans that satisfied OAR constraints, however CSRS was faster to plan, gave more homogeneous dose distributions and used less MUs.

Margin expansion for treatment field of volumetric modulated arc therapy for breast cancer

Objective We developed a method to expand margin of beam aperture for VMAT treatment planning of breast cancer and evaluated its effectiveness in clinic. Methods An expanded VMAT (E-VMAT) plan for breast cancer using Pinnacle3 9.10 planning system was made. A small tissue-equivalent bolus was added at the middle of the breast surface as part of the target and the VMAT plan was generated with beam aperture automatically expanded. Then the tissue-equivalent bolus was removed and the final dose distribution was calculated. Ten patients with breast cancer undergoing breast conserving surgery were tested. The dosimetrical metrics and delivery efficiency were compared between the E-VMAT plans and the routine VMAT plans. Results On the BEV view, it was observed that in E-VMAT plans the collimator and MLC leaf positions was extended outside the skin along the anterior-posterior direction of the thorax. There were no significant differences in conformity index and homogeneity index of target volume between the two types of plans and the planned delivery efficiency was consistent (P>0.05). Conclusions The method established in this study can automatically generate expansion of VMAT beam apertures and prevent missing of target volume due to respiratory motion and/or setup error of the patient. The method does not require programming skill and available for different treatment planning systems. Key words: Breast neoplasms/volumetric modulated arc therapy; Beam aperture

Evaluation of delivered dose to a moving target by 4D dose reconstruction in gated volumetric modulated arc therapy.

PurposeTo develop a 4D dose reconstruction method and to evaluate the delivered dose in respiratory-gated volumetric modulated arc therapy (VMAT).Materials and methodsA total 112 treatment sessions of gated VMAT for 30 stereotactic body radiotherapy (SBRT) patients (10 lung, 10 liver, and 10 pancreas) were evaluated. For respiratory-gated SBRT, 4DCT was acquired, and the CT data at the end-exhale phase was used for a VMAT plan. The delivered dose was reconstructed using a patient’s respiratory motion and machine motion acquired during the beam delivery. The machine motion was obtained from the treatment log file, while the target position was estimated from an external respiratory marker position. The target position was divided into 1-mm position bins, and sub-beams with beam isocenters corresponding to each position bin were created in a motion mimicking plan, reflecting motion data including MLC leaf positions and gantry angle and target position data during beam treatment. The reconstructed 4D dose was compared with the dose of the original plan using these dosimetric parameters; the maximum dose (Dmax) and mean dose (Dmean) of gross target volume (GTV) or organs at risk (spinal cord, esophagus, heart, duodenum, kidney, spinal cord, and stomach). The minimum dose (Dmin) to GTV was also calculated to verify cold spots in tumors.ResultsThere was no significant difference of dose parameters regard to the GTV in all tumors. For the liver cases, there were significant differences in the Dmax of duodenum (-4.2 ± 1.4%), stomach (-3.5 ± 4.2%), left kidney (-4.1 ± 2.8%), and right kidney (-3.2 ± 1.3%), and in the Dmean of duodenum (-3.8 ± 1.4%), stomach (-3.9 ± 2.2%), left kidney (-3.1 ± 2.8%), and right kidney (-4.1 ± 2.6%). For the pancreas cases, there were significant differences in the Dmax of stomach (2.1 ± 3.0%), and in the Dmean of liver (1.5 ± 0.6%), duodenum (-1.0 ± 1.4%), stomach (2.1 ± 1.6%), and right kidney (-1.3 ± 0.9%). The average gamma pass rates were 97.6 ± 4.8% for lung cases, 99.6 ± 0.5% for liver cases, and 99.5 ± 0.5% for pancreas cases. Most cases showed insignificant dose variation, with gamma pass rates higher than 98%, except for two lung cases with gamma pass rates of 86.9% and 90.6%. The low gamma pass rates showed larger global motion ranges resulting from the baseline shift during beam delivery.ConclusionThe actual delivered dose in thoracic and abdominal VMAT under breathing motion was verified by 4D dose reconstruction using typical treatment equipment and software. The proposed method provides a verification method for the actual delivered dose and could be a dosimetric verification QA tool for radiation treatment under various respiratory management techniques.

Dose uncertainty from calculation grid resolution and its alignment with MLC

The aims are (1) to present a clinical brain case where the dose distribution calculated at 2.5-mm grid resolution was significantly different from that of 1.0 mm with Varian HD120 multileaf collimator (MLC; 2.5-mm central leaf width), and (2) to present our analysis results to better understand the influence of dose calculation grid resolution and its spatial alignment with MLC. A simulation using a static MLC pattern with 10 open strips was conducted to demonstrate the possibility that the characteristics of MLC dose profiles can be completely hidden. The effects of the spatial MLC alignment with the dose grid were investigated for grid resolutions from 1.0 mm to 5.0 mm. In the end, 20 recent stereotactic radiotherapy plans (5 brain, 5 lung, 5 spine, and 5 other) were retrospectively selected, and their dose distributions were recalculated at 1.0 mm, 1.5 mm, and 2.5 mm resolutions for comparisons. Film dosimetry results were used as references. At 2.5-mm grid resolution, the MLC open-strip patterns were not visible in the dose distribution when the MLC leaf interspaces were at exact alignment with the dose grid. The effects were smaller at 1.0-mm grid resolution. The dose error at the centers of the open strips varied significantly (7% to 39%) depending on the alignments. Overall, the planning target volume mean dose of 1.5-mm plans was higher than 1.0-mm plans by 1.3 ± 0.3% (mean ± 1SD) with a maximum of 2.6%, while the mean dose of 2.5-mm plans was lower by −0.5 ± 0.5% with a maximum mean dose of −3.7%. In conclusion, the choice of dose calculation grid resolution can affect the plan dose distribution significantly. The dose grid resolution must be smaller than the width of MLC leaf at a minimum. A finder resolution is expected to produce less uncertainty in the calculated dose and influenced less by the MLC-grid spatial alignment.

P215] Quality assurance of MLC using a software for automated quality control and an electronic portal device

Purpose Nowadays, there are two methods of MLC quality control. First method the patient specific quality assurance assumed that before every treatment start, plans are verified using for example electronic portal dosimetry. Second one the machine specific quality assurance assumed that MLC parameters like for example leaf position accuracy for static tests or picket fence for dynamic tests should be verified every established period of time. Both methods have specific advantages and disadvantages. The aim of this study was to introduce ARTISCAN software from AQUILAB for automated quality control for MLC machine specific quality assurance. Methods The Artiscan software for automated quality control is using mathematical formulas to provide quantitative information about MLC parameters. So far in our department after MLC commissioning we only did patient specific quality assurance. To improve quality of treatment we have implemented this software. We made proper plans and realized them on our three Clinac 2300CD, one Unique and two TrueBeam equipped with MLC 120 Millennium and one TrueBeam equipped with MLC HD all from Varian. We analyzed all available parameters for static and dynamic tests. We manly focused on MLC leaf position static test and all picket fence dynamic tests. Plans have been realized for three months. Almost 3000 DICOM images from linacs were analyzed in ARTISCAN software. Results For four linacs the software for automated quality control showed almost 1.0 mm error for leaf position. We reported that to our service and service engineer calibrated MLC using standard procedure. After that calibration leaf position error decreased to maximum 0.5 mm. No tolerance deviations were observed for the rest of analyzed parameters. Conclusions Both methods are complimentary but machine specific quality assurance approach can detect smaller difference and can improve accuracy of treatment. For patient specific quality assurance using electronic portal and gamma analysis we routinely use criteria 3% dose difference and 3 mm distance to agreement. These criteria will not detect such small differences in MLC working like 1 mm. However we still need more measurements to observe variability of parameters to adopt tolerance and to create levels of respond.

Improving accuracy for stereotactic body radiotherapy treatments of spinal metastases.

PurposeUse of SBRT techniques is now a relatively common recourse for spinal metastases due to good local control rates and durable pain control. However, the technique has not yet reached maturity for gantry‐based systems, so work is still required in finding planning approaches that produce optimum conformity as well as delivery for the slew of treatment planning systems and treatment machines.MethodsA set of 32 SBRT spine treatment plans based on four vertebral sites, varying in modality and number of control points, were created in Pinnacle. These plans were assessed according to complexity metrics and planning objectives as well as undergoing treatment delivery QA on an Elekta VersaHD through ion chamber measurement, ArcCheck, film‐dose map comparison and MLC log‐file reconstruction via PerFraction.ResultsAll methods of QA demonstrated statistically significant agreement with each other (r = 0.63, P < 0.001). Plan complexity and delivery accuracy were found to be independent of MUs (r = 0.22, P > 0.05) but improved with the number of control points (r = 0.46, P < 0.03); with use of 90 control points producing the most complex and least accurate plans. The fraction of small apertures used in treatment had no impact on plan quality or accuracy (r = 0.29, P > 0.05) but rather more complexly modulated plans showed poorer results due to MLC leaf position inaccuracies. Plans utilizing 180 and 240 control points produced optimal plan coverage with similar complexity metrics to each other. However, plans with 240 control points demonstrated slightly better delivery accuracy, with fewer MLC leaf position discrepancies.ConclusionIn contrast to other studies, MU had no effect on delivery accuracy, with the most impactful parameter at the disposal of the planner being the number of control points utilized.