Phantom Scatter Factor Research Articles

Dosimetric comparison of 6 MV and 10 MV flattening filter-free beams using small diameter cone for trigeminal neuralgia radiosurgery

Abstract Purpose: To compare the dosimetric characteristics and treatment delivery efficiency of trigeminal neuralgia (TN) stereotactic radiosurgery (SRS) patients previously treated with a 6 MV-FFF (flattening filter-free; radiation beam obtained by removing the flattening filter) beam versus those re-planned with a 10 MV-FFF beam using a conical collimator on the TrueBeam Novalis STx linear accelerator. Methods: Eleven patients with TN previously treated with a 6 MV-FFF beam following the SRS protocol of 90 Gy in a single fraction were selected. Plans were recalculated using 10 MV-FFF beam, maintaining the same dose prescription and beam angle configuration used with 6 MV-FFF beam. The dose gradient, volumes receiving 20 and 10 Gy, maximum dose and dose to 10% of the brainstem were recorded for both the energies. Efficiency was assessed by the average monitor unit (MU) and time per arc. The 10 MV-FFF machine was configured in the treatment planning system (TPS) to measure the tissue phantom ratio (TPR), dose profiles and scatter factors using RAZOR, PTW-60012 diodes and EBT3 radiochromic films. Results: Compared to the 6 MV-FFF, the 10 MV-FFF plans exhibit average increments in dose gradient, volume of 20 Gy and volume of 10 Gy of 3.8, 17.1 and 17.8%, respectively. Average increases of 6.5 and 18.1% were obtained for maximum dose and dose to 10% of the brainstem, respectively. An average increase of 31 MU/arc was observed for the 10 MV-FFF plans, with a 40% reduction in treatment time per arc. The TPR for the 10 MV-FFF beams increased by 10%, and a penumbra width of 0.3 mm was observed. Scatter factor increments of 15, 13.5, 12.7 and 10.3% were observed for the 6 MV-FFF over the 10 MV-FFF for cones of 4, 5, 6 and 7 mm, respectively. Conclusions: In TN SRS, the utilisation of 10 MV-FFF beams reduces treatment duration but results in an increased brainstem radiation dose. To mitigate this increase in brainstem dose, it is necessary to adjust the isocentre position.

Independent monitor unit verification for dynamic flattened beam plans on the Halcyon linac.

Independent monitor unit verification (MUV) methods for the dynamic beam-flattening (DBF) technique have not been established. The purpose of this study was to clarify whether MU values for the DBF technique can be calculated using in-air and in-water output ratios (Sc and Scp ). Sc and Scp were measured in the DBF mode, and the phantom scatter factor (Sp ) was calculated. The difference between calculated and planned MUs with square and rectangle fields and clinical plans for different treatment sites was also evaluated. Sc values for the 4×4 to 24×24cm2 fields of the distal multi-leaf collimator (MLC) layer at 2-cm intervals were 0.887, 0.815, 0.715, 0.716, 0.611, 0.612, 0.511, 0.373, 0.374, 0.375, and 0.374, respectively. No collimator exchange effect was observed. Sc also depends slightly on the field size of the distal MLC layer. If the distal-MLC-layered field size was less than 20% of the corresponding MLC sequence size in the proximal MLC layer, Sc was affected by >1%, which was compensated using a correction factor (CF). Sp increased as the field sizes of the MLC sequence and distal MLC leaves increased. MUs calculated using measured Sc , Sp , and CF for square and rectangle fields agreed with planned MUs within ±1.2%. A larger difference (-1.5%) between calculated and planned MUs was observed for clinical plans, whereas differences in MUs were within 2MU for most fields (56 out of 64 fields). MU calculation for the DBF technique can be performed with Sc , Sp , and CF for independent MUV.

Determination of the Small-Field Output Factor for 6 MV Photon Beam Using EGSnrc Monte Carlo.

Accuracy of ionization chamber (IC) to measure the scatter output factor (Scp) of a linear accelerator (linac) is crucial, especially in small field (<4 cm × 4 cm). The common IC volume of 0.6 cc is not adequate for small-field measurement and not all radiotherapy centers can afford to purchase additional IC due to the additional cost. This study aimed to determine the efficiency of the EGSnrc Monte Carlo (MC) to calculate the Scp for various field sizes including small field in Elekta Synergy (Agility multileaf collimator) linac. The BEAMnrc and DOSXYZnrc user codes were used to simulate a 6 MV linac model for various field sizes and calculate the radiation dose output in water phantom. The modeled linac treatment head was validated by comparing the percentage depth dose (PDD), beam profile, and beam quality (Tissue Phantom Ratio (TPR)20,10) with the IC measurement. The validated linac model was simulated to calculate the Scp consisting of collimator scatter factor (Sc) and phantom scatter factor (Sp). The PDD and beam profile of the simulated field sizes were within a good agreement of ±2% compared with the measured data. The TPR20,10 value was 0.675 for field size 10 cm × 10 cm. The Scp, Sc, and Sp simulated values were close to the IC measurement within ±2% difference. The simulation for Sc and Sp in 3 cm × 3 cm field size was calculated to be 0.955 and 0.884, respectively. In conclusion, this study validated the efficiency of the MC simulation as a promising tool for the Scp calculation including small-field size for linac.

Empirical Formalism for the Phantom Scatter Factor of Small Fields: Using Different Density Media

We present the Empirical Formula (EF) to calculate the phantom scatter factor, Sp, of small radiation fields under charge particle dis-equilibrium conditions. The Empirical Formula (EF) was verified by examining the calculated data with experimentally measured data utilizing the anthropomorphic phantom in twelve different combinations of beam entry and point location, where the value for Sp per tissue composition was within 3% in 8/12 cases, 5% in 1/12 cases, and 10% in 3/12 cases. Our results showed a good agreement with experimental data to less than 1% when the ion chamber was surrounded by the homogeneous tissue, whether lung, soft tissue, or bone. Indicating that the prediction of the equation is valid, and it can be reliably used for phantom scatter factor calculation for different homogeneous media under charge particle dis- equilibrium conditions.

Investigation of Estimation Accuracy of Phantom Scatter Factor by Clarkson Method Considering Depth in MLC Irregular Field

In monitor unit (MU) independent verification by calculation for irregular field (MLC field) using multileaf collimator in X-ray therapy, it has become common to use collimator scatter factor (Sc) and phantom scatter factor (Sp) instead of total scatter factor (Sc, p). It is usually expressed as Sc, p (A)=Sc (A)×Sp (A), and the field size A is considered but the depth d is not. Sc is data of in-air output, and measure with a mini-phantom at constant depth to remove electron contamination. On the other hand, Sp is obtained from measurement data of Sc, p and Sc, and can be expressed as Sc, p (d, A)=Sc (constant depth, A)×Sp (d, A) at an arbitrary depth d, thus Sp depends on the depth of Sc, p. Therefore, Sp needs to consider depth. In addition, a linear accelerator equipped with the tertiary MLC has two field sizes, that are collimator field by upper and lower collimators and MLC field by tertiary MLC below them. In MU independent verification by calculation, it is often used that the estimated value of Sp obtained by converting MLC field to equivalent square field and referring to data of Sp in square field. To convert the MLC field to equivalent square field, a conversion formula from sector radius r to equivalent square field L by Clarkson's sector integration (Clarkson method) is used. In this study, using 24 types of MLC fields to evaluate estimation accuracy due to the difference of conversion formula in Clarkson method, we estimated value of Sp using r=0.5611L of B-Clarkson method and using r=0.5580L of A-Clarkson method. And the difference with the measured value of Sp obtained by measuring Sc, p and Sc in the same MLC fields was compared. While, to evaluate estimation accuracy due to the different depths using these Clarkson methods, the difference between estimated value and measured value of Sp similarly obtained at depth of 5, 10 and 15 cm was compared. As results, estimated value of Sp using A-Clarkson method than using B-Clarkson method was close to measured value, and it was the same trend at depth of 5, 10 and 15 cm. Therefore, it was suggested that estimation accuracy of Sp by A-Clarkson method is higher than B-Clarkson method when verifying beams with different depths in MU independent verification by calculation for MLC field.

Investigation of the Influence of the Conversion Method to Equivalent Square Field on the Depth Direction of Phantom Scatter Factor

In X-ray therapy, equivalent square field (side of equivalent square field) is important because it influences the accuracy of independent verification of monitor unit (MU) by calculation. To calculate the side of equivalent square field for rectangular fields, we often use a table of domestic standard measurement method (Day's method), or A/P method calculated by area-perimeter ratio. The sides of equivalent square fields of these methods are assumed to be unchanged by depth and energy, but there are reports that it is not valid. Therefore, the depth dependency of side of equivalent square fields of Day's method, A/P method, and area ratio correction (ARC) method was compared by measuring phantom scatter factors (Sp). From the analysis of Sp measured at different depths, the estimated value of Sp on the equivalent square side of the Day's method and A/P method had a depth dependency that the difference from the measured value was large when the measurement depth was deep. The estimated value of Sp on the equivalent square side of the ARC method had a small difference from the measured value even when the measurement depth was deep, and the depth dependency was small compared with the Day's method and the A/P method. Side of equivalent square field of ARC method had a smaller difference of depth dependency than in the case of Day's method and A/P method. Therefore, in the independent verification of MU for rectangular field, using the equivalent square side of the ARC method is better.

Beam characteristic of P.E collimators Add-on multileaf collimator

Introduction: Multileaf collimators (MLCs) are computer-controlled devices which are Useful and cost-effective tools for conformal therapy and intensity modulation radiotherapy (IMRT). Nowadays MLCs are important and standard configuration of the new medical linear accelerators which are substitute of conventional Blocks and has eliminated the difficulties of working with blocks. MLCs are available from many manufacturers and their designs differ in the way they couple with conventional jaws, number and width of leaves and in their physical and dosimetric characteristics, well understanding on the mechanics and dosimetric characteristics of MLC is the basic task of medical physicist. Materials and Methods: In this study some mechanical and dosimetric characteristics of a P.E. Collimators Add-On MLC which is installed on VARIAN CLINAC 2100 C/D are presented. Scatter factors (Sc, Sp, Scp), central axis depth dose and beam profiles were measured before and after installation of MLC. mechanical and Leaf Leakages for 6MV and 18MV beams were measured with Gafchromic EBT3 films. Results: radiation and light fields were match below the 2mm acceptable level based on AAPM reports. Average leaf leakage was noted 1.79% for 6MV and 1.98% for 18MV beams, respectively. No significant differences were found in beam profiles, collimator (Sc), phantom (Sp) and total (Sc, p) scatter factors after MLC installation. The percent depth dose data for all fields and both beam energies were within 1.6 % of the original data. Conclusion: Based on the Mechanical and dosimetric results which have been achieved from our different standard tests, we found no significant differences between before and after MLC installation Values on Clinac 2100 C/D Varian LINAC.

A multi-institutional study of independent calculation verification in inhomogeneous media using a simple and effective method of heterogeneity correction integrated with the Clarkson method.

In inhomogeneous media, there is often a large systematic difference in the dose between the conventional Clarkson algorithm (C-Clarkson) for independent calculation verification and the superposition-based algorithms of treatment planning systems (TPSs). These treatment site–dependent differences increase the complexity of the radiotherapy planning secondary check. We developed a simple and effective method of heterogeneity correction integrated with the Clarkson algorithm (L-Clarkson) to account for the effects of heterogeneity in the lateral dimension, and performed a multi-institutional study to evaluate the effectiveness of the method. In the method, a 2D image reconstructed from computed tomography (CT) images is divided according to lines extending from the reference point to the edge of the multileaf collimator (MLC) or jaw collimator for each pie sector, and the radiological path length (RPL) of each line is calculated on the 2D image to obtain a tissue maximum ratio and phantom scatter factor, allowing the dose to be calculated. A total of 261 plans (1237 beams) for conventional breast and lung treatments and lung stereotactic body radiotherapy were collected from four institutions. Disagreements in dose between the on-site TPSs and a verification program using the C-Clarkson and L-Clarkson algorithms were compared. Systematic differences with the L-Clarkson method were within 1% for all sites, while the C-Clarkson method resulted in systematic differences of 1–5%. The L-Clarkson method showed smaller variations. This heterogeneity correction integrated with the Clarkson algorithm would provide a simple evaluation within the range of −5% to +5% for a radiotherapy plan secondary check.

An Analytical Method to Calculate Phantom Scatter Factor for Photon Beam Accelerators.

IntroductionOne of the important input factors in the commissioning of the radiotherapy treatment planning systems is the phantom scatter factor (Sp) which requires the same collimator opening for all radiation fields. In this study, we have proposed an analytical method to overcome this issue.MethodsThe measurements were performed using Siemens Primus Plus with photon energy 6 MV for field sizes from 5×5cm2 to 40×40cm2. Phantom scatter factor was measured through the division of total scatter output factors (Scp), and collimator scatter factor (Sc).ResultsThe mean percent difference between the measured and calculated Sp was 1.00% and -3.11% for 5×5, 40×40 cm2 field size respectively.ConclusionThis method is applicable especially for small fields used in IMRT which, measuring collimator scatter factor is not reliable due to the lateral electron disequilibrium.

SU‐F‐T‐66: Characteristics of Electron Beams From Varian Trubeam

Purpose:The purpose of this study was to compare the electron beam data between Truebeam and 2300ix Varian accelerators for percent depth dose for broad beam and small circular cutouts, cone factors, head scatter factor as a function of cone size and SSD, phantom scatter factor, blocking factor, distance factor and virtual source position.Methods:Measurements were performed for Truebeam and 2300ix Varian accelerators. The main energies used were: 6, 9, 12, 16 and 20 MeV. PDD was measured at SSD = 100 cm for open beam and small circular cutouts (r = 0.5, 1.0, 1.5, 2.0, 3.0, 4.0 and 6.6cm) for different energies. Measurements to determine the head scatter factor (H) were done as a function of radius for six representative energies and five cone sizes (6, 10, 15, 20 and 25cm2). The phantom scatter factor (PSF) is defined as the ratio of blocking factor in water at reference depth and head scatter factor in air. PSF was measured as a function of radius and electron energy. Distance factor was measured for all energies and cones for three SSD's (100, 110 and 120cm).Results:The percent depth dose (PDD) was measured for small cutouts of radius r = 0.5, 1.0, 1.5, 2.0, 3.0, 4.0, 5.0 and 5.6cm. Blocking factor (BF) was measured for Truebeam and 2300ix accelerators, for different circular cutouts and energies for a 10×10 cone. Cone factors were compared between the two accelerators for different energies and applicator sizes.Conclusion:Cone factors measured for the two accelerator types differ by up to 5% for the largest applicator size. Blocking factors differs by up to 3%, with the largest variation for the smallest field size (0.5cm). Distance factor for different SSD's differ by up to 4.5%.

SU‐F‐J‐12: Dosimetrical Characteristics of a 2.5 MV Megavoltage Photon Beam

Purpose:To evaluate the accuracy of modeling scatter factor (SF) and primary off‐axis ratio (POAR) for 2.5 MV megavoltage photon beams using an empirical model, which can be used as a dose calculation model for low energy EPID imaging.Methods:Scatter photon parameters were calculated for 2.5 MV photon beam from a Varian TrueBeam by fitting the product of depth dose, PDD, and the phantom scatter factor, Sp, for a range of square field sizes between 2 and 40 cm and depths between 0 and 30 cm. The model is then applied to off‐axis profiles measured for a slit field (3cm ×35 cm) for 5 depths: 0.6, 5, 10, and 20, 30 cm. Primary off‐axis ratio (POAR) values were determined based on the fitting to data using the empirical model.Results:The fitting to PDD*Sp has a maximum error of 14%, mostly near surface, where backscattering of photons dominate and has a standard deviation of 4.7%. The attenuation coefficient and beam‐hardening coefficient of the beam is determined to be 0.095 1/cm and 0.0038 1/cm, respectively. POAR for 2.5 MV has a peak in the center, corresponding to a beam without sufficient material to flatten the beam.Conclusion:This work illustrates a deficiency of the current empirical model to fit SF for a 2.5 MV photon beams. Most of the errors are near the surface region and an improved model that incorporate better modeling of backscattering can be developed to improve the model accuracy.

SU-F-T-582: Small Field Dosimetry in Radiosurgery Collimators with a Stealth Chamber

Purpose: The extraction of a reference signal for measuring small fields in scanning mode can be problematic. In this work we describe the use of a transmission chamber in small field dosimetry for radiosurgery collimators and compare TMR curves obtained with stereotactic diode and microionization chamber. Methods: Four radiosurgery cones of diameters 5, 10, 12.5, and 15mm supplied by Elekta Medical were commissioned in a 6MV FFF beam from an Elekta Versa linac. A transmission chamber manufactured by IBA (Stealth chamber) was attached to the lower part of the collimators and used for PDD and profile measurements in scanning mode with a Scanditronix stereotactic diode. It was also used for centering the stereotactic diode in the water tank to measure TMR and output factors, by integrating the signal. TMR measurements for all collimators and the OF for the largest collimator were also acquired on a polystyrene PTW 29672 phantom with a PTW PinPoint 3D chamber 0.016 cm3 volume. Results: Measured TMR with diode and microionization chamber agreed very well with differences larger than 1% only for depths above 15cm, except the smaller collimator, for which differences were always smaller than 2%. Calculated TMR were significantly different (up to 7%) from measured TMR. The differences are attributed to the change in response of the diode with depth, because the effective field aperture varies with depth. Furthermore, neglecting the ratio of phantom-scatter factors in the conversion formula also contributes to this difference. OF measured with diode and chamber showed a difference of 3.5%. Conclusion: The transmission chamber overcomes the problem of extracting a reference signal and is of great help for small field commissioning. Calculating TMR from PDD is strongly discouraged. Good agreement was found when comparing measurements of TMR with stereotactic diode in water with measurements with microionization chamber in polystyrene.

Dosimetric properties of equivalent-quality flattening filter-free (FFF) and flattened photon beams of Versa HD linear accelerator.

This study presents the basic dosimetric properties of photon beams of a Versa HD linear accelerator (linac), which is capable of delivering flattening filter‐free (FFF) beams with a beam quality equivalent to the corresponding flattened beams based on comprehensive beam data measurement. The analyzed data included the PDDs, profiles, penumbra, out‐of‐field doses, surface doses, output factors, head and phantom scatter factors, and MLC transmissions for both FFF and flattened beams of 6 MV and 10 MV energy from an Elekta Versa HD linac. The 6MVFFF and 10MVFFF beams had an equivalent mean energy to the flattened beams and showed less PDD variations with the field sizes. Compared with their corresponding flattened beams, Dmax was deeper for FFF beams for all field sizes; the ionization ratio variations with the field size were lower for FFF beams; the out‐of‐field doses were lower and the penumbras were sharper for the FFF beams; the off‐axis profile variations with the depths were lesser for the FFF beams. Further, the 6MVFFF and 10MVFFF beams had 35.7% and 40.9% less variations in output factor with the field size, respectively. The collimator exchange effect was reduced in the FFF mode. The head scatter factor showed 59.1% and 73.6% less variations, on average, for the 6MVFFF and 10MVFFF beams, respectively; the variations in the phantom scatter factor were also smaller. The surface doses for all beams increased linearly with the field size. The 6MVFFF and 10MVFFF beams had higher surface doses than the corresponding flattened beams for field sizes of up to 10×10cm2 but had lower surface doses for larger fields. Both FFF beams had lower average MLC transmissions than the flattened beams. The finding that the FFF beams were of equivalent quality to the corresponding flattened beams indicates a significant difference from the data on unmatched FFF beams.PACS number(s): 87.56.bd, 87.55.Qr

SU‐E‐T‐93: Activation of Psoralen at Depth Using Kilovoltage X‐Rays: Physics Considerations in Implementing a New Teletherapy Paradigm

Purpose:Psoralen is a UV‐light activated anti‐cancer biotherapeutic used for treating skin lesions (PUVA) and advanced cutaneous T‐cell lymphoma (ECP). To date psoralen has not been used to treat deep seated tumors due to difficulty in generating UV‐light at depth. We recently demonstrated psoralen activation at depth by introducing energy converting particles that absorb kV x‐ray radiation and re‐emit UV‐light. Our in‐vitro work found that 0.2–1Gy using 40–100kVp x‐rays combined with psoralen and particles can induce a substantial apoptotic response beyond that expected from the sum of individual components. In preparation for a phase I clinical trial of canine companion animals, we address the physics and dosimetry considerations for applying this new teletherapy paradigm to an in‐vivo setting.Methods:The kV on‐board imaging (OBI) system mounted on a medical linear accelerator (Varian) was commissioned to deliver the prescribed dose (0.6Gy) using 80 and 100kVp. Dosimetric measurements included kVp, HVL, depth dose, backscatter factors, collimator and phantom scatter factors, field size factors, and blade leakage. Absolute dosimetry was performed following AAPM TG61 recommendations and verified with an independent kV dose meter. We also investigated collimated rotational delivery to minimize skin dose using simple dose calculations on homogeneous cylindrical phantoms.Results:Single beam delivery is feasible for shallow targets (&lt;5cm) without exceeding skin tolerance, while a rotational delivery may be utilized for deeper targets; skin dose is ∼75% of target dose for 80kVp collimated rotational delivery to a 3cm target within a 20cm phantom. Heat loading was tolerable; 0.6Gy to 5cm can be delivered before the anode reaches 75% capacity.Conclusion:KV teletherapy for Psoralen activation in deep seated tissue was successfully commissioned for a Varian OBI machine for use in a phase I clinical trial in canines. Future work will use Monte Carlo dosimetry to investigate dose in presence of bone.Research funded by Immunolight LLC. H. Walder, Z. Fathi, &amp; W. Beyer are employees of Immunolight LLC which holds a patent on the technology. Drs. Adamson and Oldham are consultants to Immunolight LLC.

SU‐E‐T‐687: Scatter Factors Comparison of 6MV FFF and Energy Matched 6MV EqFFF

Purpose:To compare the Collimator scatter factor (Sc), Phantom scatter factor (Sp) and Total scatter factors (Sc,p) of 6MV FFF and energy matched 6MV FFF (6MV eqFFF)Methods:The flattening filter and primary collimator are the major sources of producing the scattered radiation. In this study, the field sizes from 5 × 5 cm2 to 40 × 40 cm2 compared for 6MV FFF and 6MV eqFFF. We measured Sc,p with CC 13 chamber at the depth of 10 g/cm2 using IBA blue water phantom and Sc measured with CC 13 chamber at the depth of 10 g /cm2 using columnar phantom (TG 74) for 6MV FFF and 6MV eqFFF x‐ray beams from a Siemens — ARTISTE(6MV eqFFF ) and Varian True beam (6MV FFF) linear accelerator. The Sp values derived from the Sc,p and Sc Values.Results:All the values of Sc,p, Sc and Sp are normalized to 10 × 10 cm2 field size. The measured values of Sc,p for 6MV FFF and 6MV eqFFF varies from 0.9404 to 1.0760 and 0.9690 to 1.0283 respectively. The Sc values for 6MV FFF and 6MV eqFFF varies from 0.9880 to 1.133 and 0.9882 to 1.0075 respectively. The Sp values for 6MV FFF and 6MV eqFFF varies from 0.9518 to 1.0619 and 0.9806 to 1.0206 respectively. From the measured values, we found that Sc, Sc,p and Sp factors of 6MV FFF less than 10 × 10cm2 field is similar to the 6MV eqFFF. For greater than 10 × 10 cm2 field size Sc, Sc,p and Sp factors higher for 6MV FFF than 6MV eqFFF.Conclusion:The energy matched 6MV FFF (6MV eqFFF) photon beam producing lesser Scatter radiation compare to the conventional 6MV FFF photon beams for field size more than 10 × 10 cm2.

SU-E-T-554: Comparison of Electron Disequilibrium Factor in External Photon Beams for Different Models of Linear Accelerators

Purpose: The dose in the buildup region of a photon beam is usually determined by the transport of the primary secondary electrons and the contaminating electrons from accelerator head. This can be quantified by the electron disequilibrium factor, E, defined as the ratio between total dose and equilibrium dose (proportional to total kerma), E = 1 in regions beyond buildup region. Ecan be different among accelerators of different models and/or manufactures of the same machine. This study compares E in photon beams from different machine models/ Methods: Photon beam data such as fractional depth dose curve (FDD) and phantom scatter factors as a function of field size and phantom depth were measured for different Linac machines. E was extrapolated from these fractional depth dose data while taking into account inverse-square law. The ranges of secondary electron were chosen as 3 and 6 cm for 6 and 15 MV photon beams, respectively. The field sizes range from 2x2 to 40x40 cm2. Results: The comparison indicates the standard deviations of electron contamination among different machines are about 2.4 - 3.3% at 5 mm depth for 6 MV and 1.2 - 3.9% at 1 cm depth for 15 MV for the same field size. The corresponding maximum deviations are 3.0 - 4.6% and 2 - 4% for 6 and 15 MV, respectively. Both standard and maximum deviations are independent of field sizes in the buildup region for 6 MV photons, and slightly decreasing with increasing field size at depths up to 1 cm for 15 MV photons. Conclusion: The deviations of electron disequilibrium factor for all studied Linacs are less than 3% beyond the depth of 0.5 cm for the photon beams for the full range of field sizes (2-40 cm) so long as they are from the same manufacturer.

SU-E-T-74: Commissioning of the Elekta VersaHD Linear Accelerator

Purpose: To present the commissioning process of recently-released Elekta VersaHD linear accelerator, equipped with Agility 160-leaf multileaf collimator and flattening-filter free (FFF) photon modes. Methods: In addition to routine QA procedures, we adopted an EPID-based method to perform the table rotation and Winston-Lutz tests, and a novel multiradiation isocenter alignment check. The beam data acquired include photon percent-depth dose (PDD) of 6X, 6XFFF, 10X, 10XFFF, and 15X in the field size from 2×2 to 40×40cm{sup 2}, profiles, collimator and phantom scatter factors (Sc and Sp), wedge factor, electron (6, 9, 12, and 15MeV) PDD and profiles, cone and cutout factors, and virtual SSD. Validation measurements were carried out in water tank to evaluate the accuracy of beam modeling by the Pinnacle planning system. End-to-End test and IMRT QA were performed to validate the overall delivery accuracy. A theoretical model has also been used to extract the primary dose ratio and off-axis beam softening effects by fitting photon beam profile measurements. Results: The PDDs of FFF beams with field size 10×10cm{sup 2} at 10cm depth, 100cm SSD were intentionally adjusted within 1% of the non-FFF beams. The photon profiles of 30×30cm{sup 2} at 10cm depth between non-FFF and FFF beams aremore » very different, OAR(10)=0.74 and 0.63, respectively, for 6XFFF and 10XFFF. The collimator and phantom scatter factors of FFF beam demonstrated smaller variation with field sizes. The EPID-based method demonstrated the maximum deviation between the table rotation axis and radiation isocenter is within 1mm, and the radiation isocenters are within 0.4mm relative to that of 6X. The validation measurement shows less than 2% deviation between the measurement and Pinnacle modeling for most of the test conditions. Conclusion: To the best of our knowledge, this is the first study reporting the Elekta VersaHD commissioning experience, which can be a valuable reference for the radiotherapy community.« less

SU‐E‐T‐505: CT‐Based Independent Dose Verification for RapidArc Plan as a Secondary Check

Purpose:To design and develop a CT‐based independent dose verification for the RapidArc plan and also to show the effectiveness of inhomogeneous correction in the secondary check for the plan.Methods:To compute the radiological path from the body surface to the reference point and equivalent field sizes from the multiple MLC aperture shapes in the RapidArc MLC sequences independently, DICOM files of CT image, structure and RapidArc plan were imported to our in‐house software. The radiological path was computed using a three‐dimensional CT arrays for each segment. The multiple MLC aperture shapes were used to compute tissue maximum ratio and phantom scatter factor using the Clarkson‐method. In this study, two RapidArc plans for oropharynx cancer were used to compare the doses in CT‐based calculation and water‐equivalent phantom calculation using the contoured body structure to the dose in a treatment planning system (TPS).Results:The comparison in the one plan shows good agreement in both of the calculation (within 1%). However, in the other case, the CT‐based calculation shows better agreement compared to the water‐equivalent phantom calculation (CT‐based: ‐2.8% vs. Water‐based: ‐3.8%). Because there were multiple structures along the multiple beam paths and the radiological path length in the CT‐based calculation and the path in the water‐homogenous phantom calculation were comparatively different.Conclusion:RapidArc treatments are performed in any sites (from head, chest, abdomen to pelvis), which includes inhomogeneous media. Therefore, a more reliable CT‐based calculation may be used as a secondary check for the independent verification.

An analytical formalism to calculate phantom scatter factors for flattening filter free (FFF) mode photon beams

Phantom Scatter Factors, Sp in the Khan formalism (Khan et al 1980 J. Radiat. Oncol. Biol. Phys. 6 745–51) describe medium-induced changes in photon-beam intensity as a function of size of the beam. According to the British Journal of Radiology, Supplement 25, megavoltage phantom scatter factors are invariant as a function of photon-beam energy. However, during the commissioning of an accelerator with flattening filter free (FFF) photon beams (Varian TrueBeamTM 6-MV FFF and 10-MV FFF), differences were noted in phantom scatter between the filtered beams and FFF-mode beams. The purpose of this work was to evaluate this difference and provide an analytical formalism to explain the phantom scatter differences between FFF-mode and the filtered mode. An analytical formalism was devised to demonstrate the source of phantom scatter differences between the filtered and the FFF-mode beams. The reason for the differences in the phantom scatter factors between the filtered and the FFF-mode beams is hypothesized to be the non-uniform beam profiles of the FFF-mode beams. The analytical formalism proposed here is based on this idea, taking the product of the filtered phantom scatter factors and the ratio of the off-axis ratio between the FFF-mode and the filtered beams. All measurements were performed using a Varian TrueBeamTM linear accelerator with photon energies of 6-MV and 10-MV in both filtered and FFF-modes. For all measurements, a PTW Farmer type chamber and a Scanditronix CC04 cylindrical ionization were used. The in-water measurements were made at depth dose maximum and 100 cm source-to-axis distance. The in-air measurements were done at 100 cm source-to-axis distance with appropriate build-up cap. From these measurements, the phantom scatter factors were derived for the filtered beams and the FFF-mode beams for both energies to be evaluated against the phantoms scatter factors calculated using the proposed algorithm. For 6-MV, the difference between the measured and the calculated FFF-mode phantom scatter factors ranged from −0.34% to 0.73%. The average per cent difference was −0.17% (1σ = 0.25%). For 10-MV, the difference ranged from −0.19% to 0.24%. The average per cent difference was −0.17% (1σ = 0.13%). An analytical formalism was presented to calculate the phantom scatter factors for FFF-mode beams using filtered phantom scatter factors as a basis. The overall differences between measurements and calculations were within ±0.5% for 6-MV and ±0.25% for 10-MV.

SU‐E‐T‐393: A Study of Varian Linac Beam Data Equivalency for IMRT

Purpose: The purpose of this study was to compare the photon beam data for four Varian (2300iX) accelerators in six categories: beam quality, phantom scatter factor, output ratio in air, primary off axis ratio, electron disequilibrium factor and benchmark measurements including dose per MU ratio at selected points and a few IMRT plans. Methods: We compared the beam quality of these accelerators, characterizing the attenuation and beam hardening coefficients and percent depth dose (PDD) at 10cm depth. The phantom scatter factor (Sp) and output ratio in air (Sc) were measured in a full phantom and a miniphantom, respectively, at depth of 10cm and SAD = 100cm as a function of field sizes. Results: PDD agreed within 1% and the attenuation coefficients agreed within 3% for the same energy. The output ratio in air Sc is almost machine independent but was energy dependent and agreed within 0.3%. For the same nominal energy, the phantom scatter factor (Sp) measured at 10cm depth agreed within 1.4% regardless of the accelerator. We delivered 3 IMRT plans (prostate, H&amp;N and lung) to study the difference in dose delivery among the four linacs. The dose delivered was compared for depths of 5, 10 and 20cm and to the calculated dose. All measurements were performed using a 2D diode array and analyzed using the distance to agreement (DTA) criteria of 3% and 3mm. The agreement among linear accelerators is much better than the agreement between measurement and calculation, and are better than 3.5% for all IMRT cases studied. Conclusion: Accelerators from the Varian manufacturer, with the same nominal energy and model, can be treated as identical to within 1.8% for conventional treatment as well as for IMRT treatment, provided that caution is given to the dosimetrical characteristics for OAR and small field dosimetry.