Leaf Gap Research Articles

Dosimetric Characterization of the Dual Layer MLC System for an O-Ring Linear Accelerator.

The Halcyon is Varian’s latest linear accelerator that offers a single 6X flattening-filter-free beam with a jawless design that features a new dual layer multileaf collimator system with faster speed and reduced transmission. Dosimetric characteristics of the dual layer multileaf collimator system including transmission, dosimetric leaf gap, and tongue and groove effects were measured. Ionization chambers, diode arrays, and an electronic portal imaging device were used to measure various multileaf collimator characteristics. Transmission through both multileaf collimator banks was found to be 0.008%, while the distal and proximal banks alone had transmission values of 0.4%. The penumbra was slightly sharper for fields using only the distal multileaf collimator bank but found to be largely independent of leaf position with values between 2.7 to 3.0 mm at dmax for the combined multileaf collimator banks. The dosimetric leaf gap was measured for the proximal and distal multileaf collimator banks both individually and together and found to have values of −0.216 mm, −0.225 mm, and 0.964 mm, respectively. Measurements of dosimetric leaf gap at the leaf edge and midline were also performed. Tongue and groove effects were investigated with both the electronic portal imaging device and a 2-dimensional array of diodes.

Validation and IMRT/VMAT delivery quality of a preconfigured fast-rotating O-ring linac system.

A fast-rotating O-ring dedicated intensity modulated radiotherapy (IMRT)/volumetric modulated arc therapy (VMAT) delivery system, the Halcyon, is delivered by default with a fully preconfigured photon beam model in the treatment planning system (TPS). This work reports on the validation and achieved IMRT/VMAT delivery quality on the system. Acceptance testing followed the vendor's installation product acceptance and was supplemented with mechanical QA. The dosimetric calibration was performed according to the IAEA TRS-398 code-of-practice, delivering 600 cGy/min at 10 cm depth, a 90 cm source-surface distance, and a 10 × 10 cm² field size. The output factors, multileaf collimator (MLC) transmission and dosimetric leaf gap (DLG) were validated by comparing measurements with the modeled values in the TPS. Validation of IMRT/VMAT was conducted following AAPM reports (MPPG 5.a, TG-119). Next, dose measurements were performed for end-to-end (E2E) checks in heterogeneous anthropomorphic phantoms using radiochromic film in multiple planes and using ionization chambers (IC) point measurements. E2E checks were performed for VMAT (cranial, rectum, spine, and head and neck) and IMRT (lung). Additionally, IROC Houston mailed dosimetry audits were performed for the beam calibration and E2E measurements using a thorax phantom (IMRT) and a head and neck phantom (VMAT). Lastly, extensive patient-specific QA was performed for the first patients of each new indication, 26 in total (nrectum = 2, nspine = 5, nlung = 5, nesophagus = 2, nhead and neck = 7, ncranial = 5), treated on the fast-rotating O-ring linac. The patient-specific QA followed the AAPM TG-218 guidelines and comprised of portal dosimetry, ArcCHECK diode array, radiochromic film dosimetry in a MultiCube phantom, and IC point measurements. The measured output factors showed an agreement <1% for fields ≥3 × 3 cm². Field sizes ≤2 × 2 cm² had a difference of <2%. The measured single-layer MLC transmission was 0.42 ± 0.01% and the measured DLG was 0.27 ± 0.22 mm. The AAPM MPPG 5.a measurements were fully compliant with the guideline criteria. Dose differences larger than 2% were found for the PDD at large depths (>25 cm). TG-119's confidence limits were achieved for the VMAT point dose measurements and for both the IMRT and VMAT radiochromic film measurements. The TG-119 confidence limits were not achieved for IMRT point dose measurements in both the target (5.9%) and the avoidance structure (6.4%). All E2E tests had point differences below 2.3% and gamma agreement scores above 90.6%. The IROC beam calibration audit showed agreement of <1%. The IROC lung IMRT audit and head and neck VMAT audit had results compliant with the IROC Houston's credentialing criteria. All IMRT and VMAT plans selected for patient-specific QA were within the action limits suggested by TG-218. The fast-rotating O-ring linac and its preconfigured TPS are compliant with the international commissioning criteria of AAPM MPPG 5.a and AAPM TG-119. E2E measurements on heterogeneous anthropomorphic phantoms were within clinically acceptable tolerances. IROC Houston's audits satisfied the credentialing criteria. This work comprises the first extensive dataset reporting on the preconfigured fast-rotating O-ring linac.

168. Evaluation of a machine QA software tool for MLC performance checks

PurposeOne of the main requirement for a correct intensity modulated treatment (IMRT) delivery, is a periodic check of Multileaf Collimator (MLC) parameters. In some Treatment Planning System (TPS), the Dosimetric Leaf Gap (DLG) is one of the most important parameter for the correct calculation of dose distribution when using dynamic MLC. The purpose of this work is to evaluate the feasibility and time saving in using a dedicated machine quality assurance (QA) software tool to monitor over time the DLG parameter and leaves positions. MethodsTwo 6 MV Linac equipped with a 120 leaf MLC were compared to check the measured constancy of the DLG and leaves positions through use of a 3D diode array equipped with a dedicated machine QA software tool. The MLC checks were performed at 50° and 320° gantry angles due to phantom’s plate geometry. In this way it was possible also to verify gravity effects on leaves banks. The analysis was carried out with a version of picket fence test implemented in the software. ResultsThe Veroli mean DLG values was 2,0 ± 0.4 mm for 50° gantry angle and 2.1 ± 0.5 mm for 320° gantry angle. The Rieti mean DLG values was 1.9 ± 0.1 for 50° gantry angle and 2.0 ± 0.1 mm for 320° gantry angle. The check on the leaves movement has shown gravity effect on two different angles. In particular the Rieti DLG value carried out at gantry 50° was 1.6 ± 0.1 mm when there was a problem to leaf motor. ConclusionThe picket fence test, originally used with radiographic film to check the reproducibility of leaf gap, is extended in this machine QA software tool, including checks for gap width and leaf positions. This tool can be used as a routine MLC QA check for mean leaf deviation, DLG, and absolute positions of the leaves.

17 Patient Quality assurance of the new halcyon linear accelerator (Varian)

Introduction Recently commissioned in our centers, the latest generation HALCYON (Varian Medical Systems) 6xFFF (800UM/ Min) Linear Accelerator was installed with the TPS Eclipse V15.5, all the algorithms and dosimetric parameters of the multileaf collimator that have been preconfigured by the manufacturer: IMRT/ VMAT (PO V15.5.1.1), AAA, portal dosimetry, dosimetric Leaf Gap = 0.010 cm and slide transmittance = 0.005 (0.5%). The aim of this study is to validate the quality assurance of the generated plans to ensure the precision of the preconfigured algorithms with respect to the machine and to check if we need to adjust the beam modeling. Methods Twenty-one cases (Head & Neck, Prostate, SBRT lung/ liver/ spine, SRS, VMAT breast, etc.) from international dosimetry competitions, were downloaded from medicalaffairs.varian.com. 15 plans were imported with the RCMI technique and 14 VMAT plans. In addition, 10 breast cancer treatment IMRT plans have also been generated. The dosimetry tools used for validation are: portal dosimetry using the V15.5.1.1 Anisotropic Analytical Algorithm (AAA), the Mapcheck2 matrix (Sun Nuclear) and an Arccheck 3D detector (Sun Nuclear). Results For the portal dosimetry, we obtain on average for the IMRT plans a global gamma (3%, 3 mm) = 99.88 ± 0.19% and local gamma (3%, 3 mm, threshold 10%) = 99.06 ± 0.84%. For the VMAT plans, a mean global gamma (3%, 3 mm) = 99.68 ± 0.39% and a mean local gamma (3%, 3 mm, 10% threshold) = 98.08 ± 1.53% are found. For the 10 IMRT breast plans, the results between the portal dosimetry and the mapcheck2 are close and correspond respectively to a local mean gamma (3%, 3 mm, 10% threshold) = 99.9 ± 0.28% and 99.56 ± 0.65%. Other analyzes are currently being carried out and in particular with the Arccheck 3D detector for VMAT plans. Conclusions The first results show a very good agreement between measurements and calculations for the different controlled plans. The preconfigured algorithms show great robustness when used in clinical routine. Recently commissioned in our centers, the latest generation HALCYON (Varian Medical Systems) 6xFFF (800UM/ Min) Linear Accelerator was installed with the TPS Eclipse V15.5, all the algorithms and dosimetric parameters of the multileaf collimator that have been preconfigured by the manufacturer: IMRT/ VMAT (PO V15.5.1.1), AAA, portal dosimetry, dosimetric Leaf Gap = 0.010 cm and slide transmittance = 0.005 (0.5%). The aim of this study is to validate the quality assurance of the generated plans to ensure the precision of the preconfigured algorithms with respect to the machine and to check if we need to adjust the beam modeling. Twenty-one cases (Head & Neck, Prostate, SBRT lung/ liver/ spine, SRS, VMAT breast, etc.) from international dosimetry competitions, were downloaded from medicalaffairs.varian.com. 15 plans were imported with the RCMI technique and 14 VMAT plans. In addition, 10 breast cancer treatment IMRT plans have also been generated. The dosimetry tools used for validation are: portal dosimetry using the V15.5.1.1 Anisotropic Analytical Algorithm (AAA), the Mapcheck2 matrix (Sun Nuclear) and an Arccheck 3D detector (Sun Nuclear). For the portal dosimetry, we obtain on average for the IMRT plans a global gamma (3%, 3 mm) = 99.88 ± 0.19% and local gamma (3%, 3 mm, threshold 10%) = 99.06 ± 0.84%. For the VMAT plans, a mean global gamma (3%, 3 mm) = 99.68 ± 0.39% and a mean local gamma (3%, 3 mm, 10% threshold) = 98.08 ± 1.53% are found. For the 10 IMRT breast plans, the results between the portal dosimetry and the mapcheck2 are close and correspond respectively to a local mean gamma (3%, 3 mm, 10% threshold) = 99.9 ± 0.28% and 99.56 ± 0.65%. Other analyzes are currently being carried out and in particular with the Arccheck 3D detector for VMAT plans. The first results show a very good agreement between measurements and calculations for the different controlled plans. The preconfigured algorithms show great robustness when used in clinical routine.

188. Optimization of the Pinnacle 9.10 Physics parameters for the volumetric modulated arc treatments with Autoplanning module

Purpose In order to optimize the agreement between the planned and the delivered dose distribution for VMAT treatments and thus obtain better pretreatment dosimetric evaluation, a set of Linac Elekta Synergy Agility specific machine parameters has been tuned in the TPS Pinnacle 9.10 with autoplanning module. Methods The introduction of the autoplanning module in the TPS resulted in a higher complexity of the treatment plans, worst affecting, as a consequence, the outcome of pretreatment dosimetric evaluation. This situation has been confirmed by a retrospective analysis of VMAT treatments of the Head&Neck district delivered in our center since 2015. The set of machine parameters considered in this study included dose rate, leaf motion, gantry speed and dynamic leaf gap. The association between these parameters and the outcome of pretreatment dosimetric evaluation by OCTAVIUS 4D (PTW) with 729 matrix has been performed. Results Our analysis suggests to use more stringent constraints for dose rate (by setting effective rather than nominal value as the maximum dose rate), leaf motion and gantry speed. As for the dynamic leaf gap, a commissioning of two machines was performed including a machine with the minimum achievable value to carry out stereotactic treatments in proximity of risk organs or multiple targets and a machine with a greater value for the remaining treatments. As a result, for the non SRT/SBRT VMAT treatment, these two machines enabled to obtain clinically equivalent plans in terms of PTV and OAR, but with less modulation. Conclusions The dynamic leaf gap resulted as the parameter impacting the most on the complexity of the plans [1] , also affecting the outcome of pretreatment dosimetric evaluation. To have a greater flexibility in the planning phase, it could be helpful to have the possibility of setting the value of this parameter outside the physics of the machine.

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.

A simple method for determining dosimetric leaf gap with cross-field dose width for rounded leaf-end multileaf collimator systems

PurposeThe dosimetric leaf gap (DLG) and multileaf collimator (MLC) transmission are two important systematic parameters used to model the rounded MLC leaf ends effect when commissioning an Eclipse treatment planning system (TPS). Determining the optimal DLG is a time consuming process. This study develops a simple and reliable method for determining the DLG using the cross-field dose width.Methods and materialsA Varian TrueBeam linac with 6 MV, 10 MV, 6 MV flattening filter free (FFF) and 10 MV FFF photon beams and equipped with the 120 Millennium MLC and the Eclipse™ TPS was used in this study. Integral sliding fields and static slit MLC field doses with different gap widths were measured with an ionization chamber and GAFCHROMIC EBT3 films, respectively. Measurements were performed for different beam energies and at depths of 5 and 10 cm. DLGs were derived from a linear extrapolation to zero dose and intercepting at the gap width axis. In the ion chamber measurements method, the average MLC leaf transmission to the gap reading for each gap (RgT) were calculated with nominal and cross-field dose widths, respectively. The cross-field dose widths were determined according to the dose profile measured with EBT3 films. Additionally, the optimal DLG values were determined using plan dose measurements, as the value that produced the closest agreement between the planned and measured doses. DLGs derived from the nominal and cross-field dose width, the film measurements, and the optimal process, were obtained and compared.ResultsThe DLG values are insensitive to the variations in depth (within 0.07 mm). DLGs derived from nominal gap widths showed a significantly lower values (with difference about 0.5 mm) than that from cross-field dose widths and from film measurements and from plan optimal values. The method in deriving DLGs by correcting the nominal gap widths to the cross-field dose widths has shown good agreements to the plan optimal values (with difference within 0.21 mm).ConclusionsThe DLG values derived from the cross-field dose width method were consistent with the values derived from film measurements and from the plan optimal process. A simple and reliable method to determine DLG for rounded leaf-end MLC systems was established. This method provides a referable DLG value required during TPS commissioning.

OA036] Clinical implementation and evaluation of the Mobius3D system for independent dose calculation

Purpose The complexity of dose planning and treatment delivery systems has increased and necessitates a more comprehensive physics quality assurance (QA). These QA checks can, however, be quite time consuming. Therefore, there is a need for automation of QA which usually rely on empirically established tolerance levels. Mobius3D is a commercial fully automated QA system that uses a collapsed cone convolution/superposition algorithm for verification of treatment plans. The purpose of this study was to investigate the level of deviation between this system and the clinical commissioned treatment planning system (Eclipse v.13.5) using the Anisotropic Analytical Algorithm when the MLC modeling was optimized. Methods The Mobius3D system comes with golden beam data and calculates the dose on the patient CT data set. For MLC optimization, 8 IMRT/VMAT plans from various anatomical regions were included and recalculated in a cylindrical plastic phantom containing two planar diode arrays (Delta4 phantom, ScandiDos). The plans were then verified experimentally at the accelerator and all passed a local 3%/2 mm gamma criteria (passing rate > 90%). The dosimetric leaf gap (DLG) used to model the MLC in Mobius3D was optimized to keep the PTV mean dose of the 8 plans within 2% in the Delta4 geometry resulting in a +0.6 mm adjustment of the DLG from the default value. To establish the level of dose deviation and establish acceptance criteria, 34 treatment plans in different geometries, such us lung (10), head and neck (HN, 10), and pelvis (14) were evaluated for both IMRT and VMAT. Results The absolute mean and max PTV dose deviation for (lung/HN/pelvis) were: 0.8/2.2/2.1% and 1.4/2.9/2.8%. All max deviations were positive, i.e. a higher dose estimate by Mobius3D. 5 HN IMRT plans before DLG adjustment all showed negative deviations of −3.12/−3.8% (PTV mean/max). Conclusions A random sample of treatment plans were within 2.5% in absolute mean dose of the PTV between Mobius3D and the TPS. MLC parameter optimization plays an important role in establishing an acceptance criterion.

Cyathealean Antarctic ferns from the Aptian Cerro Negro Formation: Rafaherbstia nishidai gen. et sp. nov. and associated fertile organs

New remains corresponding to Cyathealean tree ferns are described for the Aptian Cerro Negro Formation at Byers Peninsula, Livingston Island (South Shetland Islands, Antarctica). A new genus and species, Rafaherbstia nishidai, is proposed for several permineralized dictyostelic stems characterized by a heterogeneous pith with a central star-shaped sclerenchyma mass, U-shaped meristeles developing small hooks at the margins of the leaf gap, sclerenchyma sheaths surrounding the vascular strands, and leaf traces omega-shaped and multi parted. The new taxon shows a combination of features that preclude its placement in one of the eight recognized families of Cyatheales, supporting that previous diversity of tree ferns was higher than in modern times. Fragmentary isolated sori with preserved sporangia containing trilete spores, recovered from the sedimentary matrix surrounding the stems, are also described. These new tree ferns increase the diversity of Cyatheales in the Cerro Negro Formation, and support warm climatic conditions in this region during the Aptian.

Analysis of activity driven by upstream regulatory modules (URM) of tapetum specific genes TA29 and A9 at ectopic locations in tobacco transgenics

TA29 and A9 are genes from Nicotiana tabacum and Arabidopsis thaliana respectively, which express in a tapetum specific manner. The upstream regulatory modules (URMs; i.e. the promoter and the 5′UTR) of these genes have been used in development of male sterile and restorer lines expressing the barnase and barstar genes for hybrid seed production. While initial studies show that these URMs drive the expression in a tapetum specific manner, there are no recordings of unintended (leaky) expression driven by these URMs at ectopic locations due to position effect in developed transgenic lines. The information on leaky expression driven by tissue specific URMs is important for their use in developing transgenic plants. The present study records the leaky activity of both these URMs in transgenic tobacco lines using β-glucuronidase as a reporter gene. Leaky activity was observed in about one-fourth of the lines developed with TA29. Most interestingly in these lines, the leaky expression of the reporter gene was observed to be restricted to the meristematic tip region of the roots and at the leaf gap from where leaf trace diverges from stem bundles. Such a restricted and unique pattern of leaky activity of a tissue specific promoter or a URM has never been reported before, including the URM of the A9 gene analyzed in the present study. This observation suggests the presence of cryptic cis-elements within the URM of TA29 gene that can possibly activate it in meristematic tissue when integrated at certain ectopic locations. The URM of the A9 gene was also observed to show leaky activity. However, there was no unique pattern as observed with that of TA29. Further, in the study we also show that while the smaller (290 bp) length of TA29 URM can be used to drive the expression of barnase gene to develop male sterile lines, it adversely affects the regeneration of transgenic tobacco lines due to leaky expression. This adverse effect is significantly reduced when the full length (1.5 kb) URM of the TA29 gene is used.

The virtual cone: A novel technique to generate spherical dose distributions using a multileaf collimator and standardized control-point sequence for small target radiation surgery

PurposeThe study aimed to develop and demonstrate a standardized linear accelerator multileaf collimator-based method of delivering small, spherical dose distributions suitable for radiosurgical treatment of small targets such as the trigeminal nerve. Methods and materialsThe virtual cone is composed of a multileaf collimator–defined field with the central 2 leaves set to a small gap. For 5 table positions, clockwise and counter-clockwise arcs were used with collimator angles of 45 and 135 degrees, respectively. The dose per degree was proportional to the sine of the gantry angle. The dose distribution was calculated by the treatment planning system and measured using radiochromic film in a skull phantom for leaf gaps of 1.6, 2.1, and 2.6 mm. Cones with a diameter of 4 mm and 5 mm were measured for comparison. Output factor constancy was investigated using a parallel-plate chamber. ResultsThe mean ratio of the measured-to-calculated dose was 0.99, 1.03, and 1.05 for 1.6, 2.1, and 2.6 mm leaf gaps, respectively. The diameter of the measured (calculated) 50% isodose line was 4.9 (4.6) mm, 5.2 (5.1) mm, and 5.5 (5.5) mm for the 1.6, 2.1, and 2.6 mm leaf gap, respectively. The measured diameter of the 50% isodose line was 4.5 and 5.7 mm for the 4 mm and 5 mm cones, respectively. The standard deviation of the parallel-plate chamber signal relative to a 10 cm × 10 cm field was less than 0.4%. The relative signal changed 32% per millimeter change in leaf gap, indicating that the parallel-plate chamber is sensitive to changes in gap width. ConclusionsThe virtual cone is an efficient technique for treatment of small spherical targets. Patient-specific quality assurance measurements will not be necessary in routine clinical use. Integration directly into the treatment planning system will make planning using this technique extremely efficient.

Institutional experience with SRS VMAT planning for multiple cranial metastases.

Background and PurposeThis study summarizes the cranial stereotactic radiosurgery (SRS) volumetric modulated arc therapy (VMAT) procedure at our institution.Materials and MethodsVolumetric modulated arc therapy plans were generated for 40 patients with 188 lesions (range 2–8, median 5) in Eclipse and treated on a TrueBeam STx. Limitations of the custom beam model outside the central 2.5 mm leaves necessitated more than one isocenter pending the spatial distribution of lesions. Two to nine arcs were used per isocenter. Conformity index (CI), gradient index (GI) and target dose heterogeneity index (HI) were determined for each lesion. Dose to critical structures and treatment times are reported.ResultsLesion size ranged 0.05–17.74 cm3 (median 0.77 cm3), and total tumor volume per case ranged 1.09–26.95 cm3 (median 7.11 cm3). For each lesion, HI ranged 1.2–1.5 (median 1.3), CI ranged 1.0–2.9 (median 1.2), and GI ranged 2.5–8.4 (median 4.4). By correlating GI to PTV volume a predicted GI = 4/PTV0.2 was determined and implemented in a script in Eclipse and used for plan evaluation. Brain volume receiving 7 Gy (V 7 Gy) ranged 10–136 cm3 (median 42 cm3). Total treatment time ranged 24–138 min (median 61 min).ConclusionsVolumetric modulated arc therapy provide plans with steep dose gradients around the targets and low dose to critical structures, and VMAT treatment is delivered in a shorter time than conventional methods using one isocenter per lesion. To further improve VMAT planning for multiple cranial metastases, better tools to shorten planning time are needed. The most significant improvement would come from better dose modeling in Eclipse, possibly by allowing for customizing the dynamic leaf gap (DLG) for a special SRS model and not limit to one DLG per energy per treatment machine and thereby remove the limitation on the Y‐jaw and allow planning with a single isocenter.

Automated EPID-based measurement of MLC leaf offset as a quality control tool

The multi-leaf collimator (MLC) dosimetric leaf gap (DLG) offset and transmission are measured parameters which influence the dosimetric accuracy of radiation therapy treatment plans. An international consortium developed an efficient method of automatically measuring the two-dimensional MLC DLG and transmission using the electronic portal imaging device (EPID) on multiple Varian TrueBeam linacs incorporating Millennium and high definition (HD) MLCs. Quality control was implemented as part of each test. Results were compiled including comparisons to baseline measurements and between machines. Validations were accomplished using ion chambers (IC) and a 2D IC array. Sensitivity was investigated by introducing deliberate leaf position offsets, repeatability was assessed, and performance was analyzed using statistical process control methods. The EPID measured DLG (EDLG) and transmission were consistently smaller than the IC measured DLG and transmission for all machines tested. EDLG variations across two dimensions averaged more than 0.24 mm for both Millennium and HD MLCs, demonstrating that leaf-to-leaf DLG variations should be assessed for both MLC types. The two-dimensional correlation coefficient between the IC array and EPID measurements was at least 0.937. A nearly linear relationship between changes in the EPID measured leaf gap and actual leaf positions was measured, with R2 > 0.999. EDLG results were analyzed as a difference relative to a baseline EDLG measurement for each machine, a metric we labeled the leaf offset constancy (LOC). The maximum change in LOC out of the 20 repeatability tests was −0.094 mm. The LOC QC process was found to be capable (Cpk > 1) for 4 machines using a ± 0.15 mm specification limit. LOC results showed that leaves may deviate from their reference positions at a level that approaches dosimetric significance for small stereotactic and highly modulated treatment fields, with average excursions measuring up to 0.21 mm for a Millennium MLC and 0.16 mm for a HD MLC. The effects of initialization, gantry angle, and common repairs are also reported.

A Practical Method to Optimize Quality Assurance Results of Arc Therapy Plans in Beam Modeling.

Dosimetric accuracy of a volumetric modulated arc therapy (VMAT) plan is directly related to the beam model, particularly with multileaf collimator characterization. Inappropriate dosimetric leaf gap (DLG) value can lead to a suboptimal beam model, with significant failure in patient-specific quality assurance (PSQA) of VMAT plans. This study addressed the systematic issue of beam modeling and developed a practical method to determine the optimal DLG value for a beam model. Several complex VMAT plans were selected for the quality assurance analysis using the variable DLG values. The results of three-dimensional (3D) Gamma analysis as a function of the DLG at 3%/3 mm, 2%/2 mm, and 1%/1 mm criteria were fitted by a polynomial curve. The DLG value corresponding to the maximum Gamma passing rate for each polynomial fitting function was derived, and the average was calculated to be the optimal DLG value for each model. The 3D Gamma analysis was repeated with the optimal DLG value to verify the dosimetric accuracy of each VMAT case by PSQA. Gamma passing rates are seen to vary considerably with the DLG values and different analysis criteria (3%/3 mm, 2%/2 mm, and 1%/1 mm) for each case. The optimal DLG derived for each model was 1.16 mm and 1.10 mm, much larger than the measured value (about 0.3 mm). The beam models with the optimal DLG was able to produce an average Gamma passing rate of 97.1% (range, 94.6%– 99.1%) at 3%/3 mm and 93.5% (range, 89.0%– 96.5%) at 2%/2 mm for one beam model, and 97.1% (range, 94.8%– 99.1%) at 3%/3 mm, and 93.3% (range, 88.8%– 96.7%) at 2%/2 mm for another. The overall accuracy of dose calculation for VMAT plans should be optimized with a compromise of varied modulation complexities in a beam model. We have developed a practical method to derive the optimal DLG value for each beam model based on the Gamma passing criterion. This technique should be applicable in general for all beam energies and patient cases.

Relationship between dosimetric leaf gap and dose calculation errors for high definition multi-leaf collimators in radiotherapy.

Background and purposeDosimetric leaf gap (DLG) is a parameter to model the round-leaf-end effect of multi-leaf collimators (MLC) that is important for treatment planning dose calculations in radiotherapy. In this study we investigated on the relationship between the DLG values and the dose calculation errors for a high-definition MLC.Materials and methodsThree sets of experiments were conducted: (1) physical DLG measurements using sweeping-gap technique, (2) DLG adjustment based on spine radiosurgery plan measurements, and (3) DLG verification using films and ion-chambers (IC). All experiments were conducted on a Varian Edge machine equipped with HD120 MLC for 6X, 6XFFF, and 10XFFF (FFF: flattening filter free). The Analytical Anisotropic Algorithm was used for all dose calculations.ResultsThe measured physical DLGs were 0.39 mm, 0.27 mm, and 0.42 mm for 6X, 6XFFF, and 10XFFF respectively. The calculated doses were lower by 4.2% (6X), 3.7% (6XFFF), and 6.8% (10XFFF) than the measured, while the adjusted DLG values with minimum errors were 1.1 mm, 0.9 mm, and 1.5 mm. The IC measurement errors were < 1%, and the film gamma pass rates (3%/3 mm) were greater than 97% for the spine plans.ConclusionsThe calculated doses were systematically lower than measured doses with the physical DLG values. It was necessary to increase the DLG values to minimize the dose calculation uncertainty. The optimal DLG values may be specific to individual MLCs and beams and, thus, careful evaluation and verification are warranted.

Verification of Mechanical Leaf Gap Error and VMAT Dose Distribution on Varian VitalBeam™ Linear Accelerator

Verification of Mechanical Leaf Gap Error and VMAT Dose Distribution on Varian VitalBeam™ Linear Accelerator

Validation of Dosimetric Leaf Gap (DLG) prior to its implementation in Treatment Planning System (TPS): TrueBeam™ millennium 120 leaf MLC.

Objective of present study is to determine optimum value of DLG and its validation prior to being incorporated in TPS for Varian TrueBeam™ millennium 120 leaves MLC. Partial transmission through the rounded leaf ends of the Multi Leaf Collimator (MLC) causes a conflict between the edges of the light field and radiation field. Parameter account for this partial transmission is called Dosimetric Leaf Gap (DLG). The complex high precession technique, such as Intensity Modulated Radiation Therapy (IMRT), entails the modeling of optimum value of DLG inside Eclipse Treatment Planning System (TPS) for precise dose calculation. Distinct synchronized uniformed extension of sweeping dynamic MLC leaf gap fields created by Varian MLC shaper software were use to determine DLG. DLG measurements performed with both 0.13cc semi-flex ionization chamber and 2D-Array I-Matrix were used to validate the DLG; similarly, values of DLG from TPS were estimated from predicted dose. Similar mathematical approaches were employed to determine DLG from delivered and TPS predicted dose. DLG determined from delivered dose measured with both ionization chamber (DLGIon) and I-Matrix (DLGI-Matrix) compared with DLG estimate from TPS predicted dose (DLGTPS). Measurements were carried out for all available 6MV, 10MV, 15MV, 6MVFFF and 10MVFFF beam energies. Maximum and minimum DLG deviation between measured and TPS calculated DLG was found to be 0.2mm and 0.1mm, respectively. Both of the measured DLGs (DLGIon and DLGI-Matrix) were found to be in a very good agreement with estimated DLG from TPS (DLGTPS). Proposed method proved to be helpful in verifying and validating the DLG value prior to its clinical implementation in TPS.

Dosimetric Impact of Leaf Gap Error on Target Dose Using Volumetric Modulated Arc Therapy and Intensity-Modulated Radiation Therapy with Flattening Filter-Free Beam

Dosimetric Impact of Leaf Gap Error on Target Dose Using Volumetric Modulated Arc Therapy and Intensity-Modulated Radiation Therapy with Flattening Filter-Free Beam

A novel method for sub-arc VMAT dose delivery verification based on portal dosimetry with an EPID.

The EPID-based sub-arc verification of VMAT dose delivery requires synchronization of the acquired electronic portal images (EPIs) with the VMAT delivery, that is, establishment of the start- and stop-MU of the acquired images. To realize this, published synchronization methods propose the use of logging features of the linac or dedicated hardware solutions. In this study, we developed a novel, software-based synchronization method that only uses information inherently available in the acquired images. The EPIs are continuously acquired during pretreatment VMAT delivery and converted into Portal Dose Images (PDIs). Sub-arcs of approximately 10 MU are then defined by combining groups of sequentially acquired PDIs. The start- and stop-MUs of measured sub-arcs are established in a synchronization procedure, using only dosimetric information in measured and predicted PDIs. Sub-arc verification of a VMAT dose delivery is based on comparison of measured sub-arc PDIs with synchronized, predicted sub-arc PDIs, using γ-analyses. To assess the accuracy of this new method, measured and predicted PDIs were compared for 20 clinically applied VMAT prostate cancer plans. The sensitivity of the method for detection of delivery errors was investigated using VMAT deliveries with intentionally inserted, small perturbations (25 error scenarios; leaf gap deviations ≤ 1.5 mm, leaf motion stops during ≤ 15 MU, linac output error ≤ 2%). For the 20 plans, the average failed pixel rates (FPR) for full-arc and sub-arc dose QA were 0.36% ± 0.26% (1 SD) and 0.64% ± 0.88%, based on 2%/2 mm and 3%/3 mm γ-analyses, respectively. Small systematic perturbations of up to 1% output error and 1 mm leaf offset were detected using full-arc QA. Sub-arc QA was able to detect positioning errors in three leaves only during approximately 20 MU and small dose delivery errors during approximately 40 MU. In an ROC analysis, the area under the curve (AUC) for the combined full-arc/sub-arc approach was 0.90. A novel method for sub-arc VMAT dose delivery verification with EPIDs is proposed, using only dosimetric information in acquired EPIs for synchronization. Especially in combination with full-arc QA, the established sensitivity for detection of very small errors is high, with also a high specificity.

The Huperzia selago Shoot Tip Transcriptome Sheds New Light on the Evolution of Leaves.

Lycopodiophyta—consisting of three orders, Lycopodiales, Isoetales and Selaginellales, with different types of shoot apical meristems (SAMs)—form the earliest branch among the extant vascular plants. They represent a sister group to all other vascular plants, from which they differ in that their leaves are microphylls—that is, leaves with a single, unbranched vein, emerging from the protostele without a leaf gap—not megaphylls. All leaves represent determinate organs originating on the flanks of indeterminate SAMs. Thus, leaf formation requires the suppression of indeterminacy, that is, of KNOX transcription factors. In seed plants, this is mediated by different groups of transcription factors including ARP and YABBY.We generated a shoot tip transcriptome of Huperzia selago (Lycopodiales) to examine the genes involved in leaf formation. Our H. selago transcriptome does not contain any ARP homolog, although transcriptomes of Selaginella spp. do. Surprisingly, we discovered a YABBY homolog, although these transcription factors were assumed to have evolved only in seed plants.The existence of a YABBY homolog in H. selago suggests that YABBY evolved already in the common ancestor of the vascular plants, and subsequently was lost in some lineages like Selaginellales, whereas ARP may have been lost in Lycopodiales. The presence of YABBY in the common ancestor of vascular plants would also support the hypothesis that this common ancestor had a simplex SAM. Furthermore, a comparison of the expression patterns of ARP in shoot tips of Selaginella kraussiana (Harrison CJ, etal. 2005. Independent recruitment of a conserved developmental mechanism during leaf evolution. Nature 434(7032):509–514.) and YABBY in shoot tips of H. selago implies that the development of microphylls, unlike megaphylls, does not seem to depend on the combined activities of ARP and YABBY. Altogether, our data show that Lycopodiophyta are a diverse group; so, in order to understand the role of Lycopodiophyta in evolution, representatives of Lycopodiales, Selaginellales, as well as of Isoetales, have to be examined.