Leaf Transmission Research Articles

SU-E-T-560: A Method to Determine Optimal Dynamic MLC Parameters for Varian Truebeam with Millennium MLC and HDMLC

Purpose: Correct MLC modeling is essential to the accurate IMRT delivery. Most TPSs use simplified models with parameters of leaf transmission (LT) and dynamic leaf gap (DLG). The common way to determine them is through extrapolation from measurements. In this study, we propose a new technique to determine these parameters with EPID and ion chambers using specially designed fluence pattern. Methods: The fluence has symmetric twin peaks separated by 10cm and each has a width of 2cm. The DMLC files were generated based on initial values of LT and DLG from measurements. Plans were delivered to EPID and analyzed in portal dosimetry software. The FWHM of each peak was evaluated. The optimal DLG value was determined by iteratively adjusting its value and repeating calculation to match the FWHM between calculation and measurement. To determine LT, an ion chamber was placed at the central axis where dose is primarily from MLC leakage. Both Millennium MLC (MMLC) and HDMLC in Varian Truebeam were investigated for photon energies of 6X, 10X, 15X. QAs of realistic IMRT plans were performed and compared. Results: The MMLC has measured LT values from 1.3%–1.5%, and corresponding optimal values 1.6–1.9%, an increase of 20% on average. The DLGs extrapolated from measurement are 0.8–0.9 mm, and optimal at 1.2–1.6 mm, a 60% increase. For HDMLC, the LTs are similar. However, the DLGs are much smaller, with extrapolations at 0.15–0.21 mm, and optimal at 0.4–0.7 mm. The portal dosimetry QA for 6X plan with MMLC reduces pixels failing γ (criteria: 3%/1mm) from 9% to 3% with optimal parameters. Similarly, QA for 15X plan with HDMLC reduces from 21% to 6%. Conclusion: We have developed a method to determine optimal MLC parameters that minimize TPS modeling errors. This ensures that patient specific QA reflects the true discrepancies in treatment plan or machine delivery.

SU-E-P-11: Basic Characteristic Comparison of the the SIEMENS 160MLC and the VARIAN HD120MLC

Purpose: The objective of this work was to compare the basic physical characteristics of transmission, penumbra and Tongue & Groove of the Special Multi-leaf Collimator (MLC) 160 MLC, SIMENS Manufactured and HighDefinition120 (HD120), VARIAN Manufactured. Methods: Under the conditions of 6 MV, SCD 100 cm, D-Max (1.5 cm), transmission and penumbra were measured with 3D water phantom, RFA 300, dosimeter of ionization, CC13-S and Silicon photodiode detector, Type SFD. Leaf transmission was measured in the direction of the in-line and all leaves closed. End leaf transmission was measured in the direction of the cross-line and the part of all leaves closed was moved every 5cm in right and left. Penumbra was measured in the direction of the cross-line, and 10cm*10cm field was moved every 5cm in right and left. Tongue & Groove was measured with film, EDR2. Results: The average of Transmission of 160 MLC, SIEMENS was 0.42%, HD120, VARIAN was 1.6%. The average of End Leaf Transmission of 160 MLC, SIEMENS was 12.2%, HD 120, VARIAN was 26.0%. Penumbra at center of 160MLC, SIEMENS was 5.7mm, HD 120, SIEMENS was 5.5mm. Maximum of Penumbra of 160MLC, SIEMENS was 6.8mm, HD 120, VARIAN was 6.2mm. The effect of Tongue & Groove of 160 MLC, SIEMENS was about 18.0%, HD 120, VARIAN was about 20.0%. Conclusion: The difference was not found in Penumbra between 160MLC, SIEMENS and HD120, VARIAN. However, in Transmission and the effect of Tongue & Groove, 160MLC, SIEMENS was lower than HD120, VARIAN. These are depend on the effect of multiple-curved tip shape, high leaf and slant placement of leaves.

SU‐E‐T‐631: A Fixed Jaw Method to Protect Critical Organs in Intensity‐Modulated Radiotherapy

Purpose: For some patients to be treated with IMRT, extremely low tolerances of critical organs(such as lens, ovaries and testicles) could not be met during treatment planning. The aim of this work is to introduce a new planning method to overcome this problem. Methods: In current planning practice, jaw positions are automatically set to cover all target volumes by planning system(eg, Pinnacle3 system). Because of such setting, critical organs may be fully blocked by MLC, but sit in the field shaped by collimator jaws. These critical organs receive doses from MLC leaf transmission and leakage. To reduce their doses further, we manually fixed jaw positions to block them. This method has been used for different treatment sites in our clinic. Here we take 8 patients with radical hysterectomy plus ovarian transposition as example, to demonstrate its performance. For each patient, two treatment plans were generated with the same optimization parameters: the original plan(called O‐plan), and the plan with fixed jaw positions(named F‐plan). In F‐plan, the jaws were manually fixed to block the ovaries. Results: For target coverage, the mean CI of F‐plan was remarkable lower than O‐plan(p<0.05). Meanwhile, for the critical organ sparing, the mean dose of left and of right ovary were much lower in F‐plan than O‐plan(p<0.05). V20, V30 and V40 of bladder were also lower with the F‐plan (p<0.05). The maximum dose to the spinal cord PRV of the O‐plan and the F‐plan were with significant differences(p<0.01). But there were 1.4 times MUs in F‐plan compared with the O‐plan, thus the treatment time could be longer by using the F‐plan. Conclusion: A fixed‐jaw method was developed to protect critical organs. Since it results in better plan quality but more MUs, it is recommended to be used only clinical requirements to critical organs cannot be met with regular method.

SU-E-T-128: A Comprehensive Dosimetric Characterization of the New 160 Leaf Elekta Agility Collimator System

Purpose: A new MLC has been released that includes 160 leaves of 5mm width at isocenter that allows 40×40 cm2, rapid leaf speed, interdigitation, and reduced leaf transmission. We evaluate the dosimetric characteristics of this new Agility Collimator and its impacts on clinical applications. Methods: A dosimetric evaluation of the Agility and the MLCi2 collimator system is performed using a Wellhofer water tank, diode and ion chamber detectors, edge detectors, and rotational delivery QA tools. Characterization includes a comprehensive verification of beam quality (6, 10, and 15 MV), MLC transmission, and delivery efficiency and accuracy of IMRT and VMAT. Results: The Agility system has one set of diaphragm and the MLC replaces the entire X diaphragm, while the older MLCi2 has two sets of diaphragms. Examination of percent depth dose of the photon beam quality is identical except in the buildup region. MLCi2 has a slightly higher dose in the buildup region; attributed to more collimation material in the beam path. For Agility, the averaged MLC leakage is 0.003, and for MLCi2 the averaged leakage is 0.013. Agility (MLC along) is equivalent to MLCi2 (with backup diaphragm) to achieve low out‐of‐field radiation. With equivalent quality Philips Pinnacle3 IMRT and SmartArc/VMAT the delivery time is shorter on AGILITY because of higher MLC speeds and improvements in linac control software (Integrity R3.0) which supports continuously variable dose rate. Conclusion: This work demonstrates that the new Agility collimator assembly has similar beam quality as the original MLCi2 collimator system, except in the buildup region. Agility has less MLC leakage and increased delivery efficiency compared to MLCi2. These findings allow for an expedited commissioning of upgrade to the Agility from the MLCi2. Further features of the leaf speed are under investigation, including their potential for higher modulation in VMAT.

Sci-Thur AM: Planning - 05: Lung SBRT: Dosimetric accuracy of the Analytical Anisotropic Algorithm (AAA) for 6MV FFF RapidArc planning.

Stereotactic Body Radiation Therapy (SBRT) requires the delivery of a high biologically effective dose in only a few fractions. These large doses per fraction can necessitate long treatment times. The Varian Truebeam is capable of RapidArc delivery and also has the optional Flattening Filter Free (FFF) modes which greatly increase the dose rate. We have commissioned the 6MV FFF beam (1400 MU/min) for RapidArc lung SBRT, and verified heterogeneous dose calculations with Monte Carlo (MC). The standard commissioning data was acquired for Varian's Analytical Anisotropic Algorithm (AAA) beam model. Measurements were acquired with the IBA Blue Phantom, using the CC13 and CC01 ion chambers and PTW diode. MLC-defined fields were also acquired for model verification. The Dosimetric Leaf Gap (DLG) was measured and then optimized using RapidArc lung SBRT plans, matching Eclipse with ion chamber measurements. Heterogeneous dose calculations were independently verified using MC. There were some discrepancies regarding leaf transmission and penumbra, but the AAA model was generally well within 2% and 2 mm. A nominal DLG value of 1.6 mm was chosen. A representative lung SBRT case utilizing FFF RapidArc was calculated with MC. For the high dose region, 99% matched Eclipse within 3% and 3 mm. The mean dose difference of the PTV was 0.7%. Although we have observed some minor infield discrepancies between the AAA and Monte Carlo calculations in heterogeneous media, the Eclipse AAA is reasonably accurate for complex FFF, RapidArc, SBRT lung planning.

Nitrogen Fertilization for Optimizing the Quality and Yield of Shade Grown Cuban Cigar Tobacco: Required Nitrogen Amounts, Application Schedules, Adequate Leaf Nitrogen Levels, and Early Season Diagnostic Tests

Abstract Nitrogen (N) fertilizers have a decisive influence on the yield and quality of tobacco. Yield, percentage of plant N, wrapper leaf quality, and nicotine content are all important quality characteristics in tobacco growing. This work is an attempt to provide a tool for optimizing mineral N nutrition for Cuban cigar tobacco, using a strategy that links N supply with leaf N concentration and wrapper yield. Similar approaches developed worldwide have mainly involved Virginia and Burley tobacco types but not Cuban cigar tobacco. The objective of the current work is to identify the effects of fertilizer N levels and timing of application on each of the mentioned quality factors for shade grown Cuban cigar tobacco. Another purpose is to explore the usefulness of a quick method of assessing the N status of plants based on measuring leaf transmission at two different wavelengths (650 and 940 nm). The experiments were done in the main tobacco growing area of Cuba (Vueltabajo). In each experiment, nine separate treatments were used covering different levels and times of fertilizer N application. The same experiment was carried out in three different years (2005-2006, 2006-2007, 2007-2008) but as the results were similar only one set of data is described (2006-2007). The patterns of response to N fertilizer of all four quality measurements, including yield and wrapper leaf quality, were similar in the different replications of the experiments. The optimal fertilizer level was 140-190 kg N/ha (40% applied on days 8-10 after transplanting and 60% on days 18-20 after transplanting). The optimal N concentration of leaves taken at the central foliar level of the middle stalk position was 4.3-4.7% at harvest time. Leaf transmission measurements by means of the SPAD-502 Chlorophyll Meter in the early stages of growth were correlated with leaf chlorophyll and N concentration and provide an excellent guide for predicting Cuban cigar tobacco wrapper leaf yield.

前立腺3DCRTおよびIMRTにおける High-definition Multileaf Collimatorの影響

The focus of this work is to evaluate the dosimetric impact of treatment planning for three-dimensional conformal radiotherapy (3DCRT) and intensity-modulated radiotherapy (IMRT) of prostate cancer using Varian/BrainLAB 120-leaf high-definition multileaf collimator (HD120 MLC) with 2.5 mm leaf width and Varian 120-leaf millennium multileaf collimator (M120 MLC) with 5 mm leaf width. We measured the leaf transmission and dosimetric leaf gap (DLG) of two multileaf collimator (MLC) systems using Farmer ionization chamber. The dosimetric impact of treatment planning for 3DCRT and IMRT of prostate cancer for ten clinical cases using two MLC systems was evaluated quantitatively. 3DCRT was divided to 3DCRT(middle) as fitting at middle of leaf tip and 3DCRT(outside) as fitting at outside of leaf tip. The leaf transmission factor and DLG of HD120 MLC for 6 and 10 MV X-ray decreased by 0.2% and 1 mm, respectively, compared to M120 MLC. The mean conformity index of PTV of treatment planning for prostate 3DCRT(middle), 3DCRT(outside) , and IMRT decreased by 0.9%, 6.6%, and 0.9% and the mean homogeneity index increased 2.3%, 13.0%, and 4.2%, respectively. The mean V20, V40, and V65 decreased by 2.4%, 6.6%, and 4.5% for bladder and 3.3%, 6.1%, and 5.9% for rectum, respectively. The results of this work demonstrated that the dose conformity of PTV improved and the dose of bladder and rectum decreased for 3DCRT and IMRT of prostate cancer using HD120 MLC compared to M120 MLC, because of reduction of leaf width, leaf transmission, and rounded leaf end transmission.

Optimization of Multi-leaf Collimator Leaf Offset Table for a Treatment Planning System

For accurate dose delivery using a rounded leaf-end multi-leaf collimator (MLC), a consistent understanding of MLC leaf offset is required between a linear accelerator (linac) controller and a treatment planning system (TPS). We have found that the leaf offset tables were inconsistent between an Elekta linac controller and a Pinnacle TPS. An optimum leaf offset table for Pinnacle was determined by operating the TPS and the linac controller. It was suggested that the MLC leaf transmission value and the MLC leaf offset table stored in Pinnacle may need to be revised to improve dose verification results for volumetric modulated arc therapy.

Accuracy of Acuros XB and AAA dose calculation for small fields with reference to RapidArc® stereotactic treatments

To assess the accuracy against measurements of two photon dose calculation algorithms (Acuros XB and the Anisotropic Analytical algorithm AAA) for small fields usable in stereotactic treatments with particular focus on RapidArc(®). Acuros XB and AAA were configured for stereotactic use. Baseline accuracy was assessed on small jaw-collimated open fields for different values for the spot sizes parameter in the beam data: 0.0, 0.5, 1, and 2 mm. Data were calculated with a grid of 1 × 1 mm(2). Investigated fields were: 3 × 3, 2 × 2, 1 × 1, and 0.8 × 0.8 cm(2) with a 6 MV photon beam generated from a Clinac2100iX (Varian, Palo Alto, CA). Profiles, PDD, and output factors were measured in water with a PTW diamond detector (detector size: 4 mm(2), thickness 0.4 mm) and compared to calculations. Four RapidArc test plans were optimized, calculated and delivered with jaw settings J3 × 3, J2 × 2, and J1 × 1 cm(2), the last was optimized twice to generate high (H) and low (L) modulation patterns. Each plan consisted of one partial arc (gantry 110° to 250°), and collimator 45°. Dose to isocenter was measured in a PTW Octavius phantom and compared to calculations. 2D measurements were performed by means of portal dosimetry with the GLAaS method developed at authors' institute. Analysis was performed with gamma pass-fail test with 3% dose difference and 2 mm distance to agreement thresholds. Open square fields: penumbrae from open field profiles were in good agreement with diamond measurements for 1 mm spot size setting for Acuros XB, and between 0.5 and 1 mm for AAA. Maximum MU difference between calculations and measurements was 1.7% for Acuros XB (0.2% for fields greater than 1 × 1 cm(2)) with 0.5 or 1 mm spot size. Agreement for AAA was within 0.7% (2.8%) for 0.5 (1 mm) spot size. RapidArc plans: doses were evaluated in a 4 mm diameter structure at isocenter and computed values differed from measurements by 0.0, -0.2, 5.5, and -3.4% for Acuros XB calculations (1 mm spot size), and of -0.1, 0.3, 6.7, and -1.2% for AAA, respectively for J3 × 3, J2 × 2, J1 × 1H, J1 × 1L RapidArc plans. Gamma Agreement Index from 2D dose analysis was higher than 95% for J3 × 3 and J2 × 2 plans, being around 80% for J1 × 1 maps. Sensitivity with respect to the dosimetric leaf gap and transmission factor MLC parameters was evaluated in the four RapidArc plans, showing the need to properly set the dosimetric leaf gap for accurate calculations. Acuros XB and AAA showed acceptable characteristics for stereotactic small fields if adequate tuning of configuration parameters is performed. Dose calculated for RapidArc stereotactic plans showed an acceptable agreement against point and 2D measurements. Both algorithms can therefore be considered safely applicable to stereotactic treatments.

Commonly applied smoothing of IR spectra showed unappropriate for the identification of plant leaf samples

The use of infrared spectroscopy has spread from single compound research to the investigation of complex biological samples. In infrared spectroscopy, spectral pre-treatment techniques have been assumed to be equally applicable and effective in the analysis of biological samples with complex chemical composition and structure. In this research, the most commonly used pre-treatment techniques were investigated based on the identification of species from whole leaf samples of pharmaceutically important Epilobium and Hypericum genera. Two spectral collection modes were used; whole leaf transmission and KBr-tablet transmission mode. The results of this study do not support the current standard in pre-treatment methods. After the frequency decomposition of a spectral signal by Fourier transform and wavelet decomposition, it was revealed that the important information of whole plant leaf as an example of biological samples was contained in the spectral details. Therefore, smoothing techniques were not appropriate because high frequency information is lost. A vast majority of published work used a Savitzky–Golay smoothing method on infrared spectra of complex biological samples. This method was shown to be less effective. In contrast, taking the derivative of the spectra showed significantly better results; with this pre-treatment method, the overlapping bands become more evident.

SU‐E‐T‐867: Improving the IMRT Plan Quality by Minimizing the Discrepancy Between the Optimized and Segmented Intensity Maps

Purpose: In step‐and‐shoot IMRT, beam intensity maps are first optimized, followed by a leaf‐sequencing to translate them into deliverable segments. The differences between the optimized and segmented intensity maps may degrade the deliverable dose distribution. Aiming to decrease this degradation, we present a method to minimize the discrepancy between the optimized and segmented intensity maps. Methods: The general idea of our algorithm is to minimize the sum of absolute differences (SOAD) between the optimized and sequenced intensity maps, similar to the work of Daliang Cao et al (Med. Phys. 2006, 33(4): 859–867). The improvement of our method includes: (1) the initial segment shapes and weights are produced based on the optimized intensity maps with a fast Xia and Verheyˈs leaf sequencing algorithm; (2) the segment weights and shapes are iteratively optimized with a modified gradient conjugate algorithm and genetic algorithm; and (3) the MLC‐dependent leaf transmission and penumbra are both accounted for. Results: For a simulated case involving the left and right lungs and an organ at risk (OAR) surrounded by a C‐shaped planning target volume (PTV), five beams were used. For the five optimized beam intensity maps, the segmented maps were computed using the proposed algorithm. The average computation time was about 17 sec per beam. Both of the dose distributions and dose volume histograms (DVHs) showed the improvements given by our method: more uniform dose distributions for PTV of the both cases, and lower doses for OARs of the clinical case, than that when the MLC‐dependent leaf transmission and penumbra were ignored. Also, the needed numbers of segment and monitor unit were reduced. Conclusions: The proposed method could produce a better segmentation result by improving the dose distributions and reducing the segment number.Funding support: This work was supported by the Program for New Century Excellent Talents in University of China (NCET‐07‐0151) and the Natural Science Foundations of China (61075109, 30730036).

SU‐E‐T‐428: 1D Direct Dosimetric Leaf Separation Measurements as An M Quality Control Tool

Purpose: To demonstrate the use of a diode array for the direct measure of the dosimetric leaf separation (DLS) associated with a multi collimator (MLC). Methods: Using the Profiler(TM) diode array, the has been directly measured at multiple points (diode positions). These measurements were obtained by collecting the dose rate spectra as a gap‐sliding window is delivered over the diode array. The DLS is de from the full width at half maximum (FWHM) determined by fitting sigmoid functions to the dose rate spectra and using the MLC spee calculate an effective gap. Taking the difference between the effective nominal gap provides a directly measured DLS for every diode position. In the present study, 49 measurements were obtained with a single sliding window delivery. Using three gap widths, this results in 147 DLS values for each evaluated MLC. Results: The DLS for three MLCs associated with three Varian(TM) linear accelerators has been measured using three sliding window gap sizes (5mm, 10mm and 15mm) and 49 detectors for each measurement. The results are averaged for each MLC along with associated standard deviations. A more detailed characterization of the DLS‐MLC can be obtained by plotting the DLS as a function of position along the main axis perpendicular to the direction of leaf motion. This 1‐D DLS distribution contains information associated with the effect of inter‐leaf and intra leaf transmission on the DLS. Conclusions: Using a diode array for the direct measurement of the DLS provides a quick and simple way to determine the DLS in 1‐D. The process can be repeated on a routine basis as part of a Quality Assurance program to monitor the behavior of the MLC system over the course of its use.

MO‐D‐BRB‐11: Evaluation of the Accuracy of Eclipse AAA Modeling for Flatten Filter Free Beam Used for SBRT

Purpose: To evaluate the Eclipse analytical anisotropic algorithm (AAA) modeling of flattening filter free (FFF) megavoltage photon beams for a new Varian linear accelerator, TruebeamSTx, TM, used for Gated SBRT. Materials and Methods: With the new Varian TruebeamSTx, Eclipse AAA was configured by using the measured dosimetric data. The TruebeamSTx can operate 6 and 10 MV photon beams free of the flattening filter (FFF). FFF photon beams are especially suitable for use of SBRT since they can operate at high dose rate (1400MU/min for 6MVFFF, 2400MU/min for 10MVFFF). Gated IMRT, RapidArc and gated RapidArc have routinely been used for SBRT, using the FFF photon beams. The accuracy of treatment plans with field sizes as small as 1×1 sq. cm are evaluated by measurement performed using films, ion chamber, and also with a Delta4 measurement system (ScandiDos). For gated treatments, measurements were done on a Respiratory Gating Platform (Standard Imaging) with motion simulating the actual patient breathing pattern. Results: The dosimetric leaf gap of the HD120MLC was adjusted to 1.7 mm with leaf transmission set to 0.015 so the calculated isodose distribution has the best fit with the measurement. The comparison between Eclipse and measured isodose distribution show an agreement of better than 95% using a gamma index of (3%/3mm). For gated RapidArc, the difference of FWHM of the measured and calculated profiles is found to be less than 0.2 mm with target size larger than 3 cm in diameter, and less than 1 mm for 1 cm diameter target size. Conclusions: The Eclipse AAA (V8.9) is found to be within 1mm accuracy for target volume as small as 1 cm in diameter. No major difference of the isodose distribution produced between the WFF and the FFF beams.

SU‐E‐T‐432: Independent Calculation of Modulation Factors Based on Comprehensive MLC and Head Scatter Modeling to Streamline IMRT 2nd Check Process

Purpose: To extend a previously described technique of modeling 2‐D fluence from any MLC leaf sequence, and develop an independent and accurate calculation of modulation factors required for pre‐treatment QA of IMRT plans. This calculation is intended to replace ion chamber measurements and thus offers a quicker and more stream‐lined verification process. Methods: In‐house software developed in MATLAB was used to calculate each IMRT fieldˈs modulation factor, defined as the ratio of the dose with MLC to the corresponding open field dose (without MLC) at a given reference point. For IMRT fields, the program uses the leaf sequence file for each dynamic field to calculate 2‐D maps of the primary fluence modulation and the timings of leaf openings. Fluence modeling includes beam horn characterization and head scatter effects. Commissioning values of leaf transmission and the dynamic leaf gap for our Varian Millenium MLCs are used to incorporate the effects of transmission and rounded leaf‐end design on the fluence modulation. Interleaf leakage and tongue‐and‐groove effects are modeled based on the timing map and the shape (including penumbra) of the interleaf leakage and tongue‐and‐groove characterized from 2‐D EPID images. Once calculated, the IMRT fluence is convolved with a Monte‐Carlo (EGSnrcMP) pencil‐beam kernel to generate a dose at the reference point depth, and divided by the open field dose to obtain the calculated modulation factor. Results: Results from 18 IMRT plans (121 fields), including treatments from pelvis, prostate, head and neck, and anal canal cancer, show excellent agreement between calculated and ion‐chamber measured modulation factors. The mean discrepancy is −0.3%, with standard deviation of 1.2%, where calculation times are 55 seconds per field on one PC core. Conclusions: An accurate and streamlined calculation method for determining modulation factors has been developed to replace tedious ion chamber measurements needed for pre‐treatment IMRT QA.

Physical and dosimetric characteristic of high‐definition multileaf collimator (HDMLC) for SRS and IMRT

Physical and dosimetric characteristics of HDMLC were studied for SRS6, 6, and 10 MV X‐rays from Novalis Tx. This in‐built tertiary collimator consists of 60 pairs (32×0.25 cm; 26×0.5 cm and 2×0.7 cm) of leaves. Properties of HDMLC studied included alignment, readout and radiation field congruence, radiation penumbra, accuracy and reproducibility of leaf position and gap width, static and dynamic leaf shift, tongue‐and‐groove effect, leaf transmission and leakage, leaf travel speed, and delivery of dynamic conformal arc and IMRT. All tests were performed using a calibrated ionization chamber, film dosimetry and DynaLog file analysis. Alignment of leaves with isocenter plane was better than 0.03 cm at all gantry and collimator positions. The congruence of HDMLC readout and radiation field agreed to within ± 0.03cm for filed sizes ranging from 1×1 to 20×20 cm2. Mean 80% to 20% penumbra width parallel (perpendicular) to leaf motion was 0.24±0.05(0.21±0.02) cm, 0.37±0.12(0.29±0.07) cm, and 0.51±0.13(0.43±0.07) cm for SRS6, 6, and 10 MV X‐rays, respectively. Circular field penumbra was comparable to corresponding square field. Average penumbra of 1×20 cm2 field was effectively constant over off‐axis positions of up to 12 cm with mean value of 0.16 (± 0.01)cm at 1.5 cm depth and 0.38 (± 0.04)cm at 10 cm depth. Minimum and maximum effective penumbra along the straight diagonal edge of irregular fields increased from 0.3 and 0.32 cm at 70° steep angle to 0.35 and 0.56 cm at 20° steep angle. Modified Picket Fence test showed average FWHM of 0.18 cm and peak‐to‐peak distance of 1.99 cm for 0.1 cm band and 2 cm interband separation. Dynamic multileaf collimation (DMLC) output factor remained within ± 1% for 6 MV and ± 0.5% for 10 MV X‐rays at all gantry positions, and was reproducible within ± 0.5% over a period of 14 months. The static leaf shift was 0.03 cm for all energies, while dynamic leaf shift was 0.044 cm for 10 MV and 0.039 cm for both SRS6 and 6 MV X‐rays. The dose depression and corresponding tongue‐and‐groove size were 24% and 0.17 cm for 6 MV and 19% and 0.20 cm for 10 MV X‐rays. Average transmission through HDMLC was 1.09%, 1.14% and 1.34% for SRS6, 6 and 10 MV X‐rays. Analysis of DynaLog files for leaf speed test in arc dynamic mode, delivery test of dynamic conformal arc, and step‐and‐shoot and sliding window IMRT showed at least 95% or more of the error counts had misplacements < 0.2cm, with maximum root mean square (RMS) error value calculated at 0.13 cm. Accurate and reproducible leaf position and gap width, and less leakage and small consistent penumbra over the fields demonstrate HDMLC suitable for high‐dose resolution SRS and IMRT.PACS number: 87.56.N‐, 87.55.Qr, 87.50.cm, 87.55.de, 87.53.Ly

Flattening filter free beams in SBRT and IMRT: Dosimetric assessment of peripheral doses

Purpose Recently, there has been a growing interest in operating medical linear accelerators without a flattening filter. Due to reduced scatter, leaf transmission and radiation head leakage a reduction of out-of-field dose is expected for flattening filter free beams. The aim of the present study was to determine the impact of unflattened beams on peripheral dose for advanced treatment techniques with a large number of MUs. Material and methods An Elekta Precise linac was modified to provide 6 and 10 MV photon beams without a flattening filter. Basic beam data were collected and implemented into the TPS Oncentra Masterplan (Nucletron). Leakage radiation, which predominantly contributes to peripheral dose at larger distances from the field edge, was measured using a Farmer type ionisation chamber. SBRT (lung) and IMRT (prostate, head&neck) treatment plans were generated for 6 and 10 MV for both flattened and unflattened beams. All treatment plans were delivered to the relevant anatomic region of an anthropomorphic phantom which was extended by a solid water slab phantom. Dosimetric measurements were performed with TLD-700 rods, radiochromic films and a Farmer type ionisation chamber. The detectors were placed within the slab phantom and positioned along the isocentric longitudinal axis. Results Using unflattened beams results in a reduction of treatment head leakage by 52% for 6 and 65% for 10 MV. Thus, peripheral doses were in general smaller for treatment plans calculated with unflattened beams. At about 20 cm distance from the field edge the dose was on average reduced by 23 and 31% for the 6 and 10 MV SBRT plans. For the IMRT plans (10 MV) the average reduction was 16% for the prostate and 18% for the head&neck case, respectively. For all examined cases, the relative deviation between peripheral doses of flattened and unflattened beams was found to increase with increasing distance from the field. Conclusions Removing the flattening filter lead to reduced peripheral doses for advanced treatment techniques. The relative difference between peripheral doses of flattened and unflattened beams was more pronounced when the nominal beam energy was increased. Patients may benefit by decreased exposure of normal tissue to scattered dose outside the field.

Study of Interleaf Leakage and Leaf Transmission in Elekta Synergy S Beam Modulator

Study of Interleaf Leakage and Leaf Transmission in Elekta Synergy S Beam Modulator

SU‐GG‐T‐277: Dosi Metric Comparison of the New Siemens 160 MLC and the Varian Millennium 120 MLC

Purpose: Recent models of multileaf collimators (MLC) have smaller leaf widths and increased target conformality. The new Siemens 160 MLC and the Varian Millennium 120 MLC models both have individual leaf widths of 5 mm projected at isocenter. This study quantifies and compares the physical penumbra and transmission of the Siemens 160 MLC and the Varian 120 MLC for 6 and 10MV photon beams. Method and Materials: Penumbra was measured for 5×5 cm2 and 10×10 cm2 field sizes both on‐axis and off‐axis at depths ranging from 0.5 cm to 10 cm depth. Measurements were performed using the new Sun Nuclear Edge™ diode detector in a Scanditronix‐Wellhoffer scanning tank and confirmed with Kodak EDR2 film. The average of interleaf and intraleaf transmission and end leaf transmission were measured with Kodak EDR2 film and confirmed with a 0.6cc farmer type ion chamber. The films were scanned using a Vidar VXR 16‐bit scanner. The penumbra was determined for inplane and crossplane profiles. Results: The average central and off‐axis penumbra was 4.0 mm and 4.9 mm for the Siemens MLC and 4.3 mm and 4.5 mm for the Varian MLC, respectively. The penumbra increases with energy, field size, and depth for both MLC designs. The average of interleaf and intraleaf transmission was determined to be 0.7% for the Siemens MLC and 1.9% for the Varian MLC. End leaf transmission averaged 19.1% for the Siemens MLC and 30.3% for the Varian MLC. Conclusion: The Siemens MLC demonstrates a sharper off‐axis penumbra. while the Varian MLC demonstrates a sharper off‐axis penumbra. Penumbra for the Siemens MLC is on average 0.5 mm sharper on one side of the inplane profile. Due to the design and role of the Siemens MLC as a primary collimator, transmission was less through the Siemens MLC leaves than the Varian MLC.

SU-GG-T-271: Dosimetric Properties of Flattening Filter Free Photon Beams from a New Clinical Accelerator

Purpose: To determine the dosimetric properties of flattening filter free (FFF) megavoltage photon beams for a new Varian linear accelerator, Trilogy MX. Materials and methods: A new Varian linac consists of 6, 10 and 15 MV photon beams equipped with flattening filters (WFF), and 6 and 10 MV photon beam free of the flattening filter (FFF). Dosimetric data including depth dose, profiles, output factors, phantom scatter factors, and surface doses operated under the FFF mode were examined. Measurements of leaf transmission, leakage radiation, and off axis beam quality were also performed. Treatment plans using the FFF beams were evaluated to determine differences between WFF and FFF beams. Results: Depth of dose maxima for photon beams with WFF and FFF were compared. It was found that for FFF beams, dmax was shallower. The PDDs for the FFF beams were found to be lower than WFF beams. Surface doses were found to be larger for FFF. The flatness for a 10×10 cm2 field is 6.1 % (FFF) versus 2.5 % (WFF) for a 6MV at a depth of 10 cm. Sc,p were found to be higher for FFF beams for field sizes < 10×10 cm2, but lower otherwise. No significant dose distribution differences were observed between IMRT treatment plans using the FFF or WFF beams. Conclusions: The results indicate that the physical characteristic of the FFF photon beams are not dramatically different from the WFF photon beams, except for uniformity across the field. However, FFF mode does result in a softer beam due to reduced beam hardening. The increased dose rate for FFF beams will reduce treatment time. Conflicts of interest: Research funding from Varian Medical Systems.

The effect of electron collimator leaf shape on the build-up dose in narrow electron MLC fields

Previously, we have found that the build-up dose from abutting narrow electron beams formed with unfocussed electron multi-leaf collimator (eMLC) steal leaves was higher than with the respective open field. To investigate more closely the effect of leaf material and shape on dose in the build-up region, straight, round (radius 1.5 cm) and leaf ends with a different front face angle of α (leaf front face pointing towards the beam axis at an angle of 90 − α) made of steel, brass and tungsten were modelled using the BEAMnrc code. Based on a treatment head simulation of a Varian 2100 C/D linac, depth–dose curves and profiles in water were calculated for narrow 6, 12 and 20 MeV eMLC beams (width 1.0 cm, length 10 cm) at source-to-surface distances (SSD) of 102 and 105 cm. The effects of leaf material and front face angle were evaluated based on electron fluence, angle and energy spectra. With a leaf front face angle of 15°, the dose in the build-up region of the 6 MeV field varied between 91 and 100%, while for straight and round leaf shapes the dose varied between 89 and 100%. The variation was between 94 and 100% for 12 and 20 MeV. For abutting narrow 6 MeV fields with total field size 5 × 10 cm2, the build-up doses at 5 mm depth for the face angle 15° and straight and round leaf shapes were 96% and 86% (SSD 102 cm) and 89% and 85% (SSD 105 cm). With higher energies, the effect of eMLC leaf shape on dose at 5 mm was slight (3–4% units with 12 MeV) and marginal with 20 MeV. The fluence, energy and angle spectra for total and leaf scattered electrons were practically the same for different leaf materials with 6 MeV. With high energies, the spectra for tungsten were more peaked due to lower leaf transmission. Compared with straight leaf ends, the face angle of 15° and round leaf ends led to a 1 mm (for 6 MeV) and between 1 and 5 mm (12 and 20 MeV at a SSD of 105 cm) decrease of therapeutic range and increase of the field size, respectively. However, profile flatness was better for abutting 6 MeV beams with round (2.5%) and face angle 15° leaves (3.0%) compared to straight leaf shape (5.2%). The eMLC leaves with a face angle of 15° resulted in a marked increase in the build-up dose for the single narrow eMLC beam and thus in the dose in the build-up region from matched abutting fields.