IGRT Couch Research Articles

Off-iso Winston-Lutz test on seven linear accelerators.

The aim of this study is to find optimal gantry, collimator, and couch angles for performing single isocenter, multiple target stereotactic radiosurgery (SIMT-SRS). Nineteen angle sets were tested across seven linear accelerators for radiation-isocenter coincidence and off-isocenter coincidence. The off-isocenter Winston-Lutz test was performed to evaluate the accuracy of isocenter alignment for each angle set, and optimal angle sets as well as maximum off-isocenter distance to target for each angle set was determined. The influence of simulated patient weight on off-iso Winston-Lutz test accuracy was also inspected. The SNC MultiMet-WL phantom and MultiMet-WL QA Software v2.1 were used for the direct measurement and analysis of the off-iso Winston-Lutz test (also referred to as Winston-Lutz-Gao test). A two-step method was developed to ensure precise initial placement of the target. Nineteen beams were delivered at 6X energy and 2×2cm field size to each of six targets on the MultiMet Cube with couch kicks at five cardinal angles (90°, 45°, 0°, 315°, and 270°). To reduce imaging uncertainty, only EPID was used in target alignment and test image acquisition. A total of 200Ibs (90.7kg) of weight was also used to mimic patient weight. All tests were performed on both the free table and the weighted table. For two new TrueBeam machines, the maximum offset was within the 1mm tolerance when the off-iso distance is less than 7cm. Two older VitalBeam machines exhibited unfavorable gantry, couch, and collimator (GCC) angle sets: Linac No. 3 at (0,90,0), (0,270,0) and Linac No. 4 at (0,45,45) and (0,90,0). The C-Series Linacs failed in the majority of GCC angle sets, with Linac No. 5 exhibiting a maximum offset of 1.53mm. Four of seven machines show a clear trend that offset increases with off-isocenter distance. Additionally, the IGRT table was less susceptible to the addition of simulated patient weight than the ExactCouch. Among the seven linear accelerators addressed, newer model machines such as the Varian TrueBeam were more precise than older models, especially in comparison to the C-Series Linacs. The newer machines are more suitable for delivering SIMT-SRS procedures in all GCC angle sets, and the results indicate that newer TrueBeams are capable of performing SIMT-SRS procedures at all angle sets for targets of off-iso distances up to 7cm. The trend that offset between the target center and radiation field center increases with off-iso distance, however, does not always hold true across machines. This may be comprised by the EPID's severe off-axis horn effect. Lastly, the IGRT couch was less susceptible to patient weight compared to ExactCouch in the off-isocenter Winston-Lutz test.

Dosimetric impact and modeling of prone breast positioning device and couch structures

In prone breast radiation, as the medial tangential beam usually passes through the immobilization board and couch, it is necessary to quantify the attenuation effect and the potential skin dose enhancement from these external structures. The prone breast board studied consists of an insert on which the contralateral breast rests and a board base indexed to the couch. Two different Varian couch systems were also studied. Transmission factors (TF) of the board were measured using a Farmer chamber at 4 cm depth. Couch TFs were measured using a thimble chamber centered in a cylindrical phantom. A custom support model was created in the treatment planning system (TPS). TFs were then computed in the TPS for comparison. Selected clinical plans were recomputed in the TPS incorporating external structures for target coverage evaluation. The correction for the attenuation effect in the TPS was also demonstrated. Skin dose effects were evaluated using a Markus parallel plate chamber with a 1 mm buildup cap. Measured insert TFs ranged 0.976 to 0.983 for 6 MV and 0.990 to 0.999 for 23 MV. Board base TFs ranged 0.979 to 0.985 for 6 MV and 0.989 to 0.998 for 23 MV. TPS values agreed within 0.9% and 0.5% for the insert and board base, respectively. Assigned Hounsfield units (HUs) providing the best agreement were 200, −100, and −900 for the insert, the board “base shell” and “base inside,” respectively. Varian Exact Couch and Exact IGRT Couch TFs varied with respect to couch angle, with minimum values of 0.837 and 0.956, respectively, for 6 MV. The clinical treatment volume (CTV) and whole breast receiving 95% of the prescription dose (CTV-V95 and WB-V95) of selected patients demonstrated reduced coverage due to attenuation of external structures. Close proximity to the base increased skin dose by up to 25% to 30%. Contacting the insert increased skin dose by 65% to 93% for 6 MV and 117% to 157% for 23 MV, respectively. Results have shown reduced coverage by attenuating external structures. Proper modeling of immobilization devices and couch structures in the TPS should be implemented for accurate dose calculation. Increased surface doses were observed due to direct contact to the insert or close proximity to the base. Further study is required to quantify such a skin dose enhancement effect and its correlation to clinically apparent skin effects and toxicity.

Analysis of setup uncertainties and determine the variation of the clinical target volume (CTV) to planning target volume (PTV) margin for various tumor sites treated with three-dimensional IGRT couch using KV-CBCT

The aim of this study was to determine the variation of patient setup uncertainties and clinical target volume (CTV) to planning target volume (PTV) margins for different tumor sites based on clinical data with three-dimensional IGRT couch using pre-treatment verification by KV-CBCT. In this study the tumor sites were divided in four categories: (1) brain (50 patients), (2) head and neck (H&N) (226 patients), (3) thorax (64 patients), and (4) pelvis (82 patients). The brain and head and neck patients were immobilized using thermoplastic three and five clamp mask and individual head-rest in the supine position. Using vac-loc had done the immobilization of thorax patients, and thermoplastic mask was used with feet-fix in the supine position for pelvis patients. All brain and H&N patients underwent first 3 days of pretreatment verification by KV-CBCT imaging and weekly one for remaining fractions, whereas for thorax and pelvis cases, patients underwent daily pretreatment verification by KV-CBCT images for the determination of overall distributions of setup error in the directions of anteroposterior, mediolateral, and craniocaudal, and CTV to PTV margins were analyzed with the help of van Herk’s, Stroom’s, and ICRU formulas. The maximum percentage displacement within ± 3 mm for brain, H&N, thorax, and pelvis cases were 96.5, 93.3, 90.9, and 88.3 in the direction of anterioposterior respectively. The maximum CTV to PTV margin, calculated from van Herk’s formula for the brain, H&N, thorax, and pelvis were 4.939 mm, 4.62 mm, 7.16 mm, and 5.763 mm in the craniocaudal axis respectively. The comparison of CTV to PTV margin in three consecutive years on 2016, 2017, and 2018 showed that for brain and head and neck cases, the margin initially had decreased by 1.03 mm and 1.11 mm, and then it had increased by 0.75 mm and 0.37 mm on 2018 with respect to 2017. But for thorax and pelvis cases, margins were gradually decreased on three consecutive years. For adequate target coverage, van Herk-calculated CTV to PTV margin are used in our institution. To reduce the setup uncertainties and CTV to PTV margin frequently, CBCT is essential. In the yearly comparison of CTV to PTV margin, we observed that for brain and H&N cases were not in descending order due to the differently deformation of head rests which were used for molding and treatment separately.

Investigation of the bolusing effect of the Varian ExactTM IGRT couch on flattened and flattening filter-free (FFF) photon beams of a Varian TrueBeam linac

The bolusing effect of radiotherapy treatment couches has been studied under various conditions for conventional flattened beams (cFF) but data for flattening filter-free (FFF) beams are still limited. The multiple commercial implementations of FFF technology in addition to the variation in couch-top types used in clinics lead to potential variation in build-up characteristics and therefore investigations should be vendor-specific. This study explores the relative changes in the build-up region caused by the Varian ExactTM IGRT couch insert (medium) for both cFF and FFF beams of nominal energies of 6- and 10 MV in a Varian TrueBeam v.2.5 linear accelerator. A small volume parallel plate chamber was used to measure surface dose data with and without the presence of the couch for a range of field sizes. Surface dose increases significantly with respect to dmax when the couch is present. Relative skin/surface dose (RSD) - defined as the ratio of the ionisation at 0.5 mm depth to the maximum ionisation along the reference axis — for static beams increases almost linearly with increasing field size for all beams investigated but the variation of the RSD with field size is larger for flattened beams as compared with FFF beams and is attributed to the fact that this effect is dominated by electron contamination. As beam quality becomes softer, the variation of RSD with field size becomes smaller and it seems that the presence of the couch has a smaller effect. TPS investigation of skin doses with and without the presence of the couch has shown that in rotational beams the presence of the couch produces much less exaggerated changes to skin dose due to the smearing effect in VMAT dose deposition however direct translation of relative results to actual clinical implications is challenging.

P28. A set of HU for “Exact IGRT” couch in Eclipse TPS

Introduction The planning system Eclipse contains a couch model library. The objective of this study is to determine the HU parameters to apply for the Exact IGRT couch. The validity of these parameters was also examined with a comparison of the impact of the couch on the calculated and measured dose. Materials and methods Couch modeling of the “Exact IGRT” is customizable with two adjustable parameters by user: the Hounsfield units of the internal structure and those of external structure. A CT scan image was acquired and HU of these structures was measured. The obtained set of HU was used for dose calculation with couch modeling. Impact of the presence of the dose of 6 MV and 18 MV direct beams was evaluated with a calculation performed with Acuros XB V11 (Varian) with and without the couch. Results are compared to the measure. The thin and thick models were considered. Results Hounsfield units of the internal structure were measured at −945 ± 5. Those of the external structure were measured at −300 ± 20. Differences between the calculated dose with and without thin couch modeling were respectively 1.8% and 1.0% for the 6 MV and 18 MV beams. For the thick model, differences were 2.4% and 1.4%. During measurements, the difference was established at 1.8% and 1.1% for the thin part of couch (landmark H4) and 2.1% and 1.0% or the thick part of couch (landmark H0). Conclusions The results show a good correlation between calculated and measured dose difference. Geometrical model build-in the planning system Eclipse associated with the determined HU enable a correct modeling of treatment couch. The planning system Eclipse contains a couch model library. The objective of this study is to determine the HU parameters to apply for the Exact IGRT couch. The validity of these parameters was also examined with a comparison of the impact of the couch on the calculated and measured dose. Couch modeling of the “Exact IGRT” is customizable with two adjustable parameters by user: the Hounsfield units of the internal structure and those of external structure. A CT scan image was acquired and HU of these structures was measured. The obtained set of HU was used for dose calculation with couch modeling. Impact of the presence of the dose of 6 MV and 18 MV direct beams was evaluated with a calculation performed with Acuros XB V11 (Varian) with and without the couch. Results are compared to the measure. The thin and thick models were considered. Hounsfield units of the internal structure were measured at −945 ± 5. Those of the external structure were measured at −300 ± 20. Differences between the calculated dose with and without thin couch modeling were respectively 1.8% and 1.0% for the 6 MV and 18 MV beams. For the thick model, differences were 2.4% and 1.4%. During measurements, the difference was established at 1.8% and 1.1% for the thin part of couch (landmark H4) and 2.1% and 1.0% or the thick part of couch (landmark H0). The results show a good correlation between calculated and measured dose difference. Geometrical model build-in the planning system Eclipse associated with the determined HU enable a correct modeling of treatment couch.

Radiotherapy couches: is kevlar an obstacle? Attenuation study of three different tabletops

AbstractIntroductionTreatment tabletops are usually made of carbon fibre due to its high mechanical strength and rigidity, low specific density, extremely light and regularly considered radiotranslucent. Our clinic acquired a Calypso 4D Localization System where electromagnetic (EM) frequencies to detect implanted transponders in the patient are used. Carbon fibre is an electrical conductive material which interferes with EM frequencies. Therefore, in order to be able to use the Calypso System the carbon fibre tabletop in the treatment room must be replaced. It is our goal to determine the attenuation of the new, non-carbon fibre, tabletop in treatment delivery.Materials and MethodsMeasurements were performed using an ionisation chamber inserted in a slab phantom positioned at the isocenter for 6, 10 MV, 6 and 10 flattening filter free (FFF) MV photon beams. These measurements were performed with and without tabletop for 0°, 30° and 60° beam angle for a True Beam STx linac, for 5×5 cm2 and 10×10 cm2 field size beams. The attenuation was calculated for each measurement for each tabletop.ResultsAt 0° incidence on the Exact IGRT Couch, the measured attenuation for 10×10 cm2 was 2·8 and 2·1% for 6 and 10 MV beams, respectively. For the same field size was measured 3·3 and 2·6% attenuation for 6 and 10 FFF MV beams, respectively. At the same incidence and regarding the other tabletops, the calculated attenuation is lower. For 10×10 cm2 field, there is 2·0, 1·4, 2·1 and 2·6% attenuation for 6, 10 MV, 6 and 10 FFF MV energy beams on the kVueTM Universal Couch. For the KvueTM Calypso® Couch 10×10 cm2 irradiation field, the measurements were 1·6, 1·3, 1·9 and 1·5%, respectively. This tendency is observed for all gantry angles.DiscussionThe attenuation outputs were definitely higher for the Varian Exact IGRT Couch when compared with the kVue tabletops. The attenuation measurements for the kVue tabletops were closer to each other. Nevertheless kVueTM Calypso® Varian tabletop showed smaller mean attenuation of the beams than kVueTM Universal Tip Insert for all measurements.ConclusionsThere was no loss in treatment quality administration due to beam attenuation in the tabletop when tabletops were exchanged because of Calypso system integration. There is no need to change between kVue tabletops whenever there is a regular treatment or a Calypso System guided treatment.

Modeling of couch transmission in the RayStation treatment planning system

PurposeTo present our methods and results regarding the modeling of a carbon fiber couch (Varian Exact IGRT) in the RayStation treatment planning system (TPS). MethodsThree geometrical-models (GMs) were implemented in the TPS to represent the three different regions of the couch (thick, medium and thin). The materials and densities of each GM component were tuned to maximize the agreement between measured and calculated attenuations. Moreover, a couch computed-tomography (CT) scan was acquired and dosimetrically compared with the GMs. For validation, plan-specific quality assurance (QA) of VMAT plans (TG-119 cases, 5 prostate and 5 H&N clinical cases) was performed by comparing measured dose distributions with doses computed with and without including the GMs in the TPS. ResultsCouch attenuations up to 4.3% were measured (energy: 6MV). Compared to couch CT, GMs could be modified to optimize the agreement with measurements and reduce dependence on the dose grid resolution. For both couch CT and GM, absolute deviations between measured and calculated attenuations were within 1.0%. When including the GMs in plan-specific QA, global 2%/2mm γ-pass rates showed an average improvement of 4.8% (p-value<0.001, max +18.6%). The couch reduced the mean dose to targets by up to 2.4% of the prescribed dose for prostate cases and up to 1.4% for H&N cases. ConclusionsRayStation accurately considers the implemented couch GMs replicating measured attenuations within an uncertainty of 1.0%. Materials and densities are proposed for the Varian Exact IGRT couch. The results obtained justify introducing couch GMs in clinical routine.

O.6 - Scripting a Varian IGRT couch on Pinnacle for treatment planning purposes

O.6 - Scripting a Varian IGRT couch on Pinnacle for treatment planning purposes

SU‐C‐207‐04: Reconstruction Artifact Reduction in X‐Ray Cone Beam CT Using a Treatment Couch Model

Purpose:to mitigate artifacts induced by the presence of the RT treatment couch in on‐board CBCT and improve image qualityMethods:a model of a Varian IGRT couch is constructed using a CBCT scan of the couch in air. The model is used to generate a set of forward projections (FP) of the treatment couch at specified gantry angles. The model couch forward projections are then used to process CBCT scan projections which contain the couch in addition to the scan object (Catphan phantom), in order to remove the attenuation component of the couch at any given gantry angle. Prior to pre‐processing with the model FP, the Catphan projection data is normalized to an air scan with bowtie filter. The filtered Catphan projections are used to reconstruct the CBCT with an in‐house FDK algorithm. The artifact reduction in the processed CBCT scan is assessed visually, and the image quality improvement is measured with the CNR over a few selected ROIs of the Catphan modules.Results:Sufficient match between the forward projected data and the x‐ray projections is achieved to allow filtering in attenuation space. Visual improvement of the couch induced artifacts is achieved, with a moderate expense of CNR.Conclusion:Couch model‐based correction of CBCT projection data has a potential for qualitative improvement of clinical CBCT scans, without requiring position specific correction data. The technique could be used to produce models of other artifact inducing devices, such as immobilization boards, and reduce their impact on patient CBCT images.

SU‐E‐P‐39: Characterization of Dose Impact On Different Beam Fields Size Attenuation for Elekta IGRT Couch

Purpose:Quantification of the dosimetric impact of the Elekta IGRT treatment couch in different beam field sizes. Established the relationship of relative dose versus beam angle at different beam field sizes.Methods:Measurements of couch attenuation were performed at gantry angles from 180° to 120°, using a 0.125cc semiflex ionization chamber, isocentrically placed in the center of a homogeneous cylindric sliced RW3 phantom for 6 photon beams. Measurements were performed at six different field sizes (3×3, 5×5, 7×7,10×10, 12×12 and 15×15 cm2). The phantom were positioned at the center of the couche,100 MU were delivered at every gantry angle. The dose difference to the ion chamber was determined.Results:For oblique fields with 6 MV photons at the same gantry angle the attenuation coefficient value from the lagest to the smallest the order is field size 7 cm2,5 cm2,10 cm2,12 cm2,15 cm2 and 3 cm2. The biggest couch attenuation by up to 4.15% at the gantry angle of 140°for the field size of 7 cm2, while for the field size of 3 cm2 the couch attenuated value only 3.5%. The other field size couch attenuation values are between the couch attenuated value of field size of 7 cm2 and 3 cm2Conclusion:The recommended treatment couch attenuation measured beam field size is 10×10 or 12×12 cm2. When measured using the beam field size 3×3 cm2 the tested value will be lower, while measuerd using the beam field size 7×7 cm2 the tested value will be higer than the recommended beam field size. This should be noted when modeling the treatment couch in the treatment planning system.

IMRT and RapidArc commissioning of a TrueBeam linear accelerator using TG-119 protocol cases.

The purpose of this study is to evaluate the overall accuracy of intensity‐modulated radiation therapy (IMRT) and RapidArc delivery using both flattening filter (FF) and flattening filter‐free (FFF) modalities based on test cases developed by AAPM Task Group 119. Institutional confidence limits (CLs) were established as the baseline for patient specific treatment plan quality assurance (QA). The effects of gantry range, gantry speed, leaf speed, dose rate, as well as the capability to capture intentional errors, were evaluated by measuring a series of Picket Fence (PF) tests using the electronic portal imaging device (EPID) and EBT3 films. Both IMRT and RapidArc plans were created in a Solid Water phantom (30 × 30 × 15 cm3) for the TG‐119 test cases representative of normal clinical treatment sites for all five photon energies (6X, 10X, 15X, 6X‐FFF, 10X‐FFF) and the Exact IGRT couch was included in the dose calculation. One high‐dose point in the PTV and one low‐dose point in the avoidance structure were measured with an ion chamber in each case for each energy. Similarly, two GAFCHROMIC EBT3 films were placed in the coronal planes to measure planar dose distributions in both high‐ and low‐dose regions. The confidence limit was set to have 95% of the measured data fall within the tolerance. The mean of the absolute dose deviation for variable dose rate and gantry speed during RapidArc delivery was within 0.5% for all energies. The corresponding results for leaf speed tests were all within 0.4%. The combinations of dynamic leaf gap (DLG) and MLC transmission factor were optimized based on the ion chamber measurement results of RapidArc delivery for each energy. The average 95% CLs for the high‐dose point in the PTV were 0.030 ± 0.007 (range, 0.022–0.038) for the IMRT plans and 0.029 ± 0.011 (range, 0.016–0.043) for the RapidArc plans. For low‐point dose in the avoidance structures, the CLs were 0.029 ± 0.006 (range, 0.024–0.039) for the IMRT plans and 0.027 ± 0.013 (range, 0.017–0.047) for the RapidArc plans. The average 95% CLs using 3%/3 mm gamma criteria in the high‐dose region were 5.9 ± 2.7 (range, 1.4–8.6) and 3.9 ± 2.9 (range, 1.5–8.8) for IMRT and RapidArc plans, respectively. The average 95% CLs in the low‐dose region were 5.3 ± 2.6 (range, 1.2–7.4) and 3.7 ± 2.8 (range, 1.8–8.3) for IMRT and RapidArc plans, respectively. Based on ion chamber, as well as film measurements, we have established CLs values to ensure the high precision of IMRT and RapidArc delivery for both FF and FFF modalities.PACS number: 87

SU‐F‐BRE‐10: Methods to Simulate and Measure the Attenuation for Modeling a Couch Top with Rails for FFF Treatment Delivery On the Varian Edge Linac

Purpose:To measure attenuation for modelling of the KVue Couchtop for 6X and 10X FFF SRS/SBRT treatmentMethods:Treatment planning simulation studies were done using 6X FFF beams to estimate the dosimetric impact of KVue couchtops (including the Q‐Fix IGRT [carbon fiber] and Calypso [nonconductive Kevlar material]) with a structure model obtained from a research workstation (Eclipse, advanced planning interface (API) v13). Prior to installation on the Varian Edge linac, the couchtop along with (Kevlar) rails were CT scanned with the rails at various positions. An additional scan with the couchtop 15cm above the CT table top was obtained with 20cm solid water to facilitate precised/indexed data acquisition. Measurements for attenuation were obtained for field sizes of 2, 4 and 10 cm2 at 42 gantry angles including 6 pairs of opposing fields and other angles for oblique delivery where the beams traversed the couchtop and or rails. The delivery was fully automated with xml scripts running in developer mode. The results were then used to determine an accurate structure model for AAA (Eclipse v11) planning of IMRT and RapidArc delivery.Results:The planning simulation relative dose attenuation for oblique entry was not significantly different than the Exact IGRT or BrainLab iBeam couch except that the rails added 6% additional attenuation. The relative attenuation measurements for PA, PA (rails: inner position), oblique, oblique (rails: outer position), oblique (rails: inner position) were: −2.0%, −2.5%, −15.6%, −2.5%, −5.0% for 6X FFF and −1.4%, −1.5%, −12.2%, − 2.5%, −5.0% for 10X FFF with slight decrease in attenuation versus field size. A Couch structure model (with HU values) was developed. Calculation compared to measurement showed good agreement except for oblique (rails: outer position) where differences approached a magnitude of 6%.Conclusion:A model of the couch structures has been developed accounting for attenuation for FFF beams.

SU-E-T-543: Beam Attenuation Characteristics of Treatment Couches.

Measure photon beam attenuation characteristics of the Varian EXACT, Varian IGRT treatment couches and two CIVCO Universal extensions; and assess accuracy of Varian's Eclipse treatment planning system in modeling the beam attenuation of each couch. Dose Measurements were taken using a Farmer ion chamber inside a cylindrical acrylic phantom. They were taken for two photon energies (6MV and 15MV), at three different field sizes, for various gantry angles through the couch. EXACT couch was tested both with the adjustable rails pushed to the outside of the couch, and pushed to the center of the couch. IGRT couch was divided by thickness into three sections. Effects on surface dose were measured using a Tissue Maximum Ratio or Percent Depth Dose curve. CIVCO extensions were CT scanned for inclusion in the planning system. All measurements were modeled in the Eclipse treatment planning system for comparison. Maximum attenuation was measured at 6MV with the smallest field for all couch setups. The EXACT couch produced a maximum attenuation of 19.2% and a surface dose increase of 47% of Dmax through the couch. Maximum attenuation for the IGRT couch and both CIVCO extensions were between 6-8%. Through the couch, the IGRT couch showed a surface dose increase of 29% of Dmax,; the CIVCO extensions produced a surface dose increase of 33% and 7% of Dmax. Maximum difference between measured and planned dose was 7%, for the EXACT couch, 1.9% for the IGRT couch and 2.4% for both CIVCO extensions. These treatment couches were shown to affect delivered and surface dose. Including the couch structure during treatment planning can account for the couch attenuation in most cases, but some attenuation values are underestimated in the planning software. Largest effects are for oblique treatment angles using low energy and small field sizes.

A Monte Carlo model of the Varian IGRT couch top for RapidArc QA

The objectives of this study are to evaluate the effect of couch attenuation on quality assurance (QA) results and to present a couch top model for Monte Carlo (MC) dose calculation for RapidArc treatments. The IGRT couch top is modelled in Eclipse as a thin skin of higher density material with a homogeneous fill of foam of lower density and attenuation. The IGRT couch structure consists of two longitudinal sections referred to as thick and thin. The Hounsfield Unit (HU) characterization of the couch structure was determined using a cylindrical phantom by comparing ion chamber measurements with the dose predicted by the treatment planning system (TPS). The optimal set of HU for the inside of the couch and the surface shell was found to be respectively −960 and −700 HU in agreement with Vanetti et al (2009 Phys. Med. Biol. 54 N157–66). For each plan, the final dose calculation was performed with the thin, thick and without the couch top. Dose differences up to 2.6% were observed with TPS calculated doses not including the couch and up to 3.4% with MC not including the couch and were found to be treatment specific. A MC couch top model was created based on the TPS geometrical model. The carbon fibre couch top skin was modelled using carbon graphite; the density was adjusted until good agreement with experimental data was observed, while the density of the foam inside was kept constant. The accuracy of the couch top model was evaluated by comparison with ion chamber measurements and TPS calculated dose combined with a 3D gamma analysis. Similar to the TPS case, a single graphite density can be used for both the thin and thick MC couch top models. Results showed good agreement with ion chamber measurements (within 1.2%) and with TPS (within 1%). For each plan, over 95% of the points passed the 3D gamma test.

SU-E-T-733: Effect of IGRT Couch and Alpha-Cradle on Dose Distributions in the Buildup Region: Comparison of EBT2 Film Data and TPS Calculation

Purpose: To evaluate current treatment planning system (TPS) dose calculation algorithm for radiation fields with normal and oblique incidence through alpha‐cradle and IGRT couch using radiochromic EBT2 film dosimetry. Methods: A polystyrene phantom (25×25×15cm3) with seven EBT2 films separated by polystyrene slabs, at depths of 0, 1, 2, 5, 10, and 14mm, was positioned above an alpha‐cradle (AC), which was ∼1cm thickness. Such phantom and AC assembly were CT scanned and the CT‐images were transferred to the TPS for calculations in three scenarios: (1) ignoring AC and couch, (2) accounting for AC only, (3) accounting for both AC and IGRT couch. A single PA field and two posterior‐oblique fields (6MV photons, 10×10cm2 from a Varian Trilogy) were planned in the TPS. Each beam was delivered to expose a stack of EBT2 films in the phantom. All films were scanned using an Epson 10000XL flatbed scanner with 72 dpi pixel resolution. The pixel values were converted to doses based on an established calibration curve. Point doses and planar distributions generated from the TPS for the three scenarios were compared with the measurements. Results: The EBT2 film data agree with the doses calculated by our TPS for scenario (3), within the uncertainty of the EBT2 data. The TPS generated doses were lower than the EBT2 doses by 34%, 32%, 31%, 12% for scenario (1) and by 26%, 23%, 22%, 7% for scenario (2) at the depths of 0, 1, 2, and 5mm, respectively. There were no significant differences for the depths of 10 and 14mm. Conclusions: TPS calculation of doses in the buildup region accounting for AC and IGRT couch top was validated by EBT2 film data. Ignoring the presence of AC and/or IGRT couch in TPS calculation would significantly underestimate the doses in the buildup region.

SU-E-T-52: A Monte Carlo Model of the Varian IGRT Couch for RapidArc

Purpose: To evaluate the impact of the Varian IGRT treatment couch on quality assurance results and to present a Monte Carlomodel of the IGRT couch that can be included on both a phantom and patient CT dataset. Methods: Three clinical RapidArc treatment plans were generated for various tumor sites. The correct set of Hounsfield Units for each part of the couch structure was determined experimentally. For each patient three verification plans were produced, one including the thick model of the couch, one including the thin model of the couch and one without any couch included. Monte Carlo dose calculations were performed in a water equivalent phantom for each of these plans. Various values of the physical densities for the couch shell were used. Results were compared to measurements using a calibrated Farmer ionization chamber with an active volume of 0.6 cm3. Comparison of RapidArc and Monte Carlo 3D doses was performed using a 3 dimensional Gamma‐factor analysis with a 3%/3mm DTA criteria. Results: Good agreement was observed between chamber measurements and the treatment planning system for each verification plans. Differences as high as 2.6% was observed between the calculated dose by the planning system with and without the couch. The optimal physical density to model the couch shell was determined to be 0.65g/cm3 giving the best agreement with both chamber measurements and the planning system with Gamma values below 1 in over 95% of the points considered for all plans. Conclusions: This work indicates the possibility to accurately model the Varian IGRT couch for Monte Carlo RapidArc simulations and to apply it on patient CT dataset.

SU‐E‐J‐163: Tracking Interfractional Patient Positioning Consistency When a Robotic Couch Is in Use

Purpose: To track interfractional patient positioning consistency during a course of stereotactic radiotherapy using BrainLAB frameless immobilization and localization system, with ExacTRAC IGRT and Robotic couch. Methods: One relationship between the BrainLAN iPLAN coordinates and the Varian couch parameters was developed so that three translational couch positions were incorporated into Aria R&amp;V system as part of the plan parameters. Therapists use these positions to verify patientˈs initial set‐up. After ExacTRAC calculates 3 shifts and 3 rotations of a patientˈs current position to the expected position based on the registration of a pair of x‐ray images with DRR, the robotic couch can be driven to place the target at Linac isocenter. However, since the rotational axes for pitch and roll of the robotic couch are not coincident with the Linac isocenteral axes, the couch positions will be changed to allow for pitch and roll corrections. Current R&amp;V system only records 3 translational and 1 couch angle (no pitch and roll), and the interfractional couch position difference can be &gt; 8 cm vertically. A second relationship was developed between the recorded translational parameters and the actual target position, taking into account the pitch and roll from the ExacTRAC system, so that the interfractional patient position can be tracked. Results: The differences between the estimated and actual translational couch positions are &lt; 2 mm on average for 28 patients. The interfractional couch position variance is &lt; 1.5 mm for 3 patients (10 fractions per patient). Discussions: If the EaxcTRAC IGRT system is mis‐calibrated to a wrong position other than Linac isocenter, the x‐ray image registration system cannot in itself detect the error. Therefore, this patient position tracking methodology can be a very important safety tool for preventing errors, so that the treatment team can deliver stereotactic radiation at a higher confidence level.

SU-E-T-541: A Clinical Approach to Account for the Attenuation of Varian Exact IGRT Couch Top for Spine Stereotatic Radiosurgery Patients

Purpose: The purpose of this work is to develop an approach to account for the attenuation of Varian Exact IGRT couch top for spine cancer patients in spine stereotatic radiosurgery. Methods: Varian Exact IGRT couch which has carbon fiber plates sandwiched with a plastic foam core was used for this study. Transmission of 6MV x‐rays through the couch was measured at various oblique beam angles. Based on the measured values, we modeled the couch top as a stored organ in Pinnacle treatment planning system (TPS) for spine stereotatic radiosurgery (SSRS) patients. A planner can load the virtual couch top and override the density when designing a treatment plan to account for the couch attenuation explicitly. The dosimetric impact of the couch top on DVHs was investigated in 3 representative patients. Results: At normal incidence, percentage of the attenuation of the Exact IGRT couch for a 6MV beam was measured to be 2.40% and 2.68% for field size of and , respectively. The attenuation was found to be gantry angle dependent, with maximum attenuation 3.90% at 120° for field size of and 4.25% at 130° for , respectively. By using a fixed density of 1.05 within the thin carbon fiber layers, the calculated attenuation for the virtual couch top were 2.38% and 2.94% for field, 4.15%, and 4.57% for field, respectively, which agreed well with the measurement. Without considering the couch attenuation, the average loss of target coverage was 7.1% in three representative cases we analyzed. Conclusions: The attenuation of Varian Exact IGRT Couch Top should not be ignored when using the couch top for SSRS patients. A clinical approach was developed to account for the attenuation. A calibrated virtual couch model can be used to account for the attenuation in the treatment planning process.

SU-FF-T-258: The Effect of Treatment Couch Attenuation On Radiation Dose for Megavoltage Photon Beams

Purpose: To investigate the attenuation of two types of linac couches and to evaluate its impact in radiation therapy with megavoltage photon beams. Methods: The relationship of relative dose versus beam angle was established for two Varian 21EX linacs, equipped with Exact couch (standard couch) with sliding side rails and Exact IGRT couch, respectively. An ion chamber was placed at the center of a solid water cylindrical phantom positioned at the linac isocenter for 6MV and 18MV photon beams. Measurements were repeated with the phantom positioned at different locations on the couches with two different field sizes (5×5 and 10×10 cm2). An IMRT GYN treatment plan was also delivered with standard couch, rails set at in and out position. Results: For standard couch, the relative dose error caused by couch attenuation could be up to 26.8% for 6MV photon beams with sliding rails in the beam path and 13.3% with the rails not in the beam path. For the IGRT couch, the error could be up to 4.8%. Due to the architecture of the couches, the dose error is different for different disease sites; and is sensitive to radiation gantry angle for standard couch. Less dose error was observed for 18MV compared to 6MV beams with identical setup. For an IMRT GYN patient, two posterior oblique 6MV beams (out of 9 beams) accounts for 17.3% of the total MU, and the dose difference is 2.5% for rails set at in and out position. Conclusion:Dose error introduced by the couch attenuation could be an issue for patient treated with oblique/posterior photon beams. Couch parameters should be included in the planning system. For linacs with standard couch, beam angles should be carefully selected to avoid the sliding rails. IGRT couch introduces less couch attenuation in general compared to standard couch.