Carbon Fiber Couch Research Articles

SU‐E‐T‐777: Use of Tennis Racket and Air Gap Between the Body and Carbon Fiber Couch for Skin Sparing in Radiation Therapy of Prone Breast

Purpose:To reduce the skin dose from the carbon fiber couch scatter in radiation treatment of breast cancer in the prone position. If this issue is not addressed, the prone breast touching the solid carbon fiber couch can absorb significant dose to the skin and cause the skin reaction.Methods:1. Use of “tennis racket” instead of the solid couch. To check this hypothesis, we measured the dose at the depth of 5 mm in solid water phantom placed on the couch, using a Farmer chamber. A plan for a patient with 6MV beams, gantry angles of 113 and 286 degrees Varian scale was used. It was found that treatment with “tennis racket” instead of the solid carbon fiber couch reduces the surface dose by 5–7%, depending on the beam direction. 2. Use of the air gap between the couch and the body was analyzed using radiochromic film on the surface of the solid water phantom 10 cm thick. Initially the phantom was placed on the couch with the film sandwiched in between. Two fields at the angles of 135 and 315 degrees were used. The measurements were repeated for the air gap of 2 and 5 cm and 6 and 15 MV beams.Results:It was found that a 2‐cm gap decreased the surface dose by 3% for a 6 MV beam and by 5.5% for a 15 MV beam. A 5‐cm gap reduced the dose by 9% for 6 MV and 13.5% for 15 MV.Conclusion:Use of both methods (combined if possible) can significantly reduce the surface dose in radiation therapy of the prone breast and possible skin reaction. We plan to explore dependence of the dose reduction upon the angle of incidence.

SU‐E‐T‐137: Attenuation of Carbon Fiber IGRT Couch Top for SBRT By Using a MapCheck 2 and Ion Chamber

Purpose:It is common to use posterior oblique beams in intensity‐modulated radiation therapy (IMRT) and volumetric‐modulated arc therapy (VMAT) for regular and stereotactic body radiotherapy (SBRT). Beam attenuation by the treatment couch is not negligible when the couch is in the beam portal. In this study, we have correlated relative dose versus beam angle through a MapCheck 2TM diode array and an ionization chamber measurement.Methods:A Varian TrueBeam linear accelerator equipped with the image‐guided radiation therapy (IGRT) carbon fiber couch was used for delivery of radiation with different photon energies of 6MV, 15MV and 6MV FFF. A MapCHECK 2TM diode array with a MapPhan was employed for the measurement at three field sizes (3 by 3, 5 by 5, and 10 by 10 cm). The independent measurement was performed with an ionization chamber placed at the center of an acrylic cylindrical phantom at the isocenter. The measured data with the MapCHECK 2TM diode array were averaged over the central 2×2 cm diodes for each measurement. The couch attenuation was deduced from the angular dependence with and without the couch involvement.Results:For the MapCHECK 2TM with MapPhan used only the obvious attenuation was observed at the beam angle between 5 and 10 degree from the angular dependence using a 6MV photon beam. The similar Result was obtained for the MapCHECK 2TM /MapPhan with the couch involvement. The couch attenuation was then deduced from the difference of these two sets of data measurements. Maximum couch attenuation was found up to 4.2% which was verified with the ion chamber measurement.Conclusion:A maximum attenuation of 4.2% was found for the carbon fiber couch top using the MapCHECK 2TM/MapPhan, which is consistent with the ion chamber measurement.

SU‐E‐T‐655: Quantifying the Dosimetric Characteristics of a Calypso Compatible Radiotherapy Treatment Couch with Angular Transmission Functions

Purpose:To quantified the dosimetric characteristics of a Calypso compatible radiotherapy couch using continuously scanned transmission factor as a function of gantry angle.Methods:Radiation dose as a function of gantry angle was measured with an ionization chamber at the center of a 30 cm diameter cylindrical phantom placed on Qfix kVue Calypso compatible couch. A 3‐D scanner was employed to continuously scan the dose during an arc rotation delivery. Nearly continuous transmission curves of 0.03° step size were obtained from IEC gantry angle 90° to180° for 6 MV and 15 MV photon beams at three couch longitudinal positions (due to longitudinally non‐uniform shape of the couch rails) with rails both at the center and on sides. The transmission curves were evaluated to assess the attenuation by the couch. They were also compared to those of a carbon fiber couch top.Results:The scanned transmission curves of the Calypso comptible couch showed fine details of the couch transmission as a function of gantry angle. The attenuation by the couch varied from 2.4% to 12.9% for the 6 MV photo beam and from 1.8% to 8.9% for the 15 MV photon beam depending on the gantry angle. The attenuation by the flat couch top is only approximately 2 to 3%, but by the rail is much more significant. Because the rails are not uniformly shaped in the longitudinal direction, attenuation by the rail for the same gantry angle varied with the rail longitudinal position. The transmission curves of the carbon fiber couch top are nearly identical to those of the Calypso compatible couch top.Conclusion:The Calypso compatible couch attenuates radiation by 2.4%–12.9% and 1.8%–8.9% for 6 MV and 15 MV photon beams, respectively. The attenuation by the rails is much more significant than the flat couch top.

The effect of carbon fiber couch on dose distribution of conformal intensity modulated plan

Objective To evaluate the effect of carbon fiber couch on dose distribution of radiotherapy planning and verification pass rate.Methods Establishing the carbon fiber treatment couch model in Pinnacle8.0m Treatment Planning system (TPS),and then this model was used to correct dose calculations of oblique fields in the treatment plans of 10 cases of nasopharyngeal carcinoma,10 cases of breast cancer and 10 cases of lung cancer and evaluate the effect of carbon fiber couch on the whole dose distribution of the plans.Then these plans were measured by three-dimensional dose verification equipment Delta4 to confirm the improvement extent of Gamma pass rate after considering the carbon fiber treatment couch.Results For the majority of plans,when the carbon fiber couch was taken into consideration,the target doses was significantly reduced (4772 cGy-7266 cGy vs.4859 cGy-7347 cGy,P=0.000-0.002) and the relative deviation of D95 was 1％ to 3％.Measurement results of Delta4 showed that Gamma pass rate (3 mm/3％ criteria) increased in all plans (96.4％-98.8％ vs.93.4％-97.3％,P =0.000),some of that were up to 5 percentage when the couch model was applied.Conclusions Target doses will be overestimated if the treatment couch is ignored in TPS measurement.,However it should arouse enough attention when the disease with smaller doses corresponding gradient. Key words: Carbon fiber treatment couch; Three-dimensional treatment planning; Dose distribution; γ passing rate

SU‐C‐16A‐01: In Vivo Source Position Verification in High Dose Rate (HDR) Prostate Brachytherapy Using a Flat Panel Imager: Initial Clinical Experience

Purpose:We report our initial clinical experience with a novel position‐sensitive source‐tracking system based on a flat panel imager. The system has been trialled with 4 prostate HDR brachytherapy patients (8 treatment fractions) in this initial study.Methods:The flat panel imaging system was mounted under a customised carbon fibre couch top assembly (Figure 1). Three gold fiducial markers were implanted into the prostate of each patient at the time of catheter placement. X‐ray dwell position markers were inserted into three catheters and a radiograph acquired to locate the implant relative to the imaging device. During treatment, as the HDR source dwells were delivered, images were acquired and processed to determine the position of the source in the patient. Source positions measured by the imaging device were compared to the treatment plan for verification of treatment delivery.Results:Measured dwell positions provided verification of relative dwell spacing within and between catheters, in the coronal plane. Measurements were typically within 2.0mm (0.2mm – 3.3mm, s.d. 0.8mm) of the planned positions over 60 dwells (Figure 2). Discrimination between larger dwell intervals and catheter differentiation were clear. This confirms important delivery attributes such as correct transfer tube connection, source step size, relative catheter positions and therefore overall correct plan selection and delivery. The fiducial markers, visible on the radiograph, provided verification of treatment delivery to the correct anatomical location. The absolute position of the dwells was determined by comparing the measured dwell positions with the x‐ray markers from the radiograph, validating the programmed treatment indexer length. The total impact on procedure time was less than 5 minutes.Conclusion:The novel, noninvasive HDR brachytherapy treatment verification system was used clinically with minor impact on workflow. The system allows verification of correct treatment delivery, free of most potential human related errors identified in ICRP 97.This research is supported by funding from the Australian Government Department of Health through Cancer Australia grant no. 616614.

TU‐D‐9A‐01: TG176: Dosimetric Effects of Couch Tops and Immobilization Devices

The dosimetric impact from devices external to the patient is a complex combination of increased skin dose, reduced tumor dose, and altered dose distribution. Although small monitor unit or dose corrections are routinely made for blocking trays, ion chamber correction factors, or tissue inhomogeneities, the dose perturbation of the treatment couch top or immobilization devices are often overlooked. These devices also increase surface dose, an effect which is also often ignored or underestimated. These concerns have grown recently due to the increased use of monolithic carbon fiber couch tops which are optimal for imaging for patient position verification but cause attenuation and increased surface dose compared to the ‘tennis racket’ style couch top they often replace. Also, arc delivery techniques have replaced stationary gantry techniques which cause a greater fraction of the dose to be delivered from posterior angles. A host of immobilization devices are available and used to increase patient positioning reproducibility, and these also have attenuation and skin dose implications which are often ignored. This report of Task Group 176 serves to present a survey of published data that illustrates the magnitude of the dosimetric effects of a wide range of devices external to the patient. The report also provides methods for modeling couch tops in treatment planning systems so the physicist can accurately compute the dosimetric effects for indexed patient treatments. Both photon and proton beams are considered. A discussion on avoidance of high density structures during beam planning is also provided. An important aspect of this report are the recommendations we make to clinical physicists, treatment planning system vendors, and device vendors on how to make measurements of skin dose and attenuation, how to report these values, and for the vendors, an appeal is made to work together to provide accurate couch top models in planning systems.Learning Objectives: What are the dosimetric effects of couch tops What are the dosimetric effects of immobilization devices How can one model couch tops in the treatment planning system How can one measure attenuation and surface dose changes due to external devices

Dosimetric effects caused by couch tops and immobilization devices: Report of AAPM Task Group 176

The dosimetric impact from devices external to the patient is a complex combination of increased skin dose, reduced tumor dose, and altered dose distribution. Although small monitor unit or dose corrections are routinely made for blocking trays, ion chamber correction factors, e.g., accounting for temperature and pressure, or tissue inhomogeneities, the dose perturbation of the treatment couch top or immobilization devices is often overlooked. These devices also increase skin dose, an effect which is also often ignored or underestimated. These concerns have grown recently due to the increased use of monolithic carbon fiber couch tops which are optimal for imaging for patient position verification but cause attenuation and increased skin dose compared to the "tennis racket" style couch top they often replace. Also, arc delivery techniques have replaced stationary gantry techniques which cause a greater fraction of the dose to be delivered from posterior angles. A host of immobilization devices are available and used to increase patient positioning reproducibility, and these also have attenuation and skin dose implications which are often ignored. This report of Task Group 176 serves to present a survey of published data that illustrates the magnitude of the dosimetric effects of a wide range of devices external to the patient. The report also provides methods for modeling couch tops in treatment planning systems so the physicist can accurately compute the dosimetric effects for indexed patient treatments. Both photon and proton beams are considered. A discussion on avoidance of high density structures during beam planning is also provided. An important aspect of this report are the recommendations the authors make to clinical physicists, treatment planning system vendors, and device vendors on how to make measurements of surface dose and attenuation and how to report these values. For the vendors, an appeal is made to work together to provide accurate couch top models in planning systems.

Accounting for the effects of carbon fibre treatment couches in the XiO treatment planning system

An investigation was conducted to determine the impact of two carbon fibre couch tops on treatment planning and to establish a method by which these couch tops may be accounted for in the XiO treatment planning system. In this study, the effects of the Civco Universal and the Medical Intelligence iBeam evo carbon fibre couch tops were investigated. Customised templates were developed in Focal to mimic the effects of these patient support devices and each template was then included in calculations performed by the XiO treatment planning system. The accuracy of modelling the couch tops in this manner was investigated and it was shown that, while using the customised couch templates, XiO modelled the increase in the surface dose due to treating through the couch tops at 180° to 1.0% ± 1.7%. The attenuation due to the presence of the couches was modelled to within 0.5% ± 0.4% for angles that pass through the flat central region of the couch tops and to within 1.4% ± 1.2% for angles that passed through their rapidly varying edges. When the couch templates were not included in XiO, the calculated dose at depth was recorded to be, in some cases, up to 5.3% ± 1.0% more than corresponding measured values. It was concluded that using the method described in this study it is possible to accurately model the effect of carbon fibre couch tops in the XiO treatment planning system.

EP-1475: The significance of carbon fiber couch top attenuation in a 6MV VMAT beam

EP-1475: The significance of carbon fiber couch top attenuation in a 6MV VMAT beam

SU‐E‐T‐179: Evaluating Treatment Couch Dosimetric Effects in Spine SBRT Treatment Planning

Purpose: To evaluate the dosimetric effects of the treatment couch on skin dose and field attenuation for Spine SBRT. Methods: Measurements were acquired with an Elekta Synergy S in a 10.4 cm x 10.4 cm open 6 MV field with a Marcus parallel plate ionization chamber for varying thickness of water equivalent plastic. Measurements were repeated with and without the Hexapod carbon fiber treatment couch. A program was written to superimpose a CT scan of the treatment couch onto the patient planning CT. Dose calculations were performed with and without the treatment couch in a solid water phantom in Raystation Treatment planning software and compared to the measured values. All doses are in reference to Dmax of the open field without couch. Results: The measurements displayed a 69% higher dose (93% versus 24%) at the surface with the addition of the couch due to the electron contamination from the couch. Our beam model in Raystation underestimates the surface dose by 10% and agreement is achieved at 7 mm and within 4% at 4 mm depth. At depths greater than Dmax, the couch introduced an attenuation factor of 2%. Conclusion: A carbon fiber couch significantly affects the surface dose for beams that pass through the couch. The couch introduces electron contamination that increases the dose by 69% at the surface. High doses to the skin can cause skin erythema. Couch dose enhancement combined with high dose fractions and several beams passing through the couch for spine SBRT treatments should be accurately modeled to evaluate the actual dose delivered to the skin.

Confirmation of Skin Doses Resulting from Bolus Effect of Intervening Alpha-cradle and Carbon Fiber Couch in Radiotherapy

In this study, we verified the treatment planning calculations of skin doses with the incorporation of the bolus effect due to the intervening alpha-cradle (AC) and carbon fiber couch (CFC) using radiochromic EBT2 films. A polystyrene phantom (25 × 25 × 15 cm(3)) with six EBT2 films separated by polystyrene slabs, at depths of 0, 0.1, 0.2, 0.5, 1, 1.4 cm, was positioned above an AC, which was ~1 cm thick. The phantom and AC assembly were CT scanned and the CT-images were transferred to the treatment planning system (TPS) for calculations in three scenarios: (A) ignoring AC and CFC, (B) accounting for AC only, (C) accounting for both AC and CFC. A single posterior 10 × 10 cm(2) field, a pair of posterior-oblique 10 × 10 cm(2) fields, and a posterior IMRT field (6 MV photons from a Varian Trilogy linac) were planned. For each radiation field configuration, the same MU were used in all three scenarios in the TPS. Each plan for scenario C was delivered to expose a stack of EBT2 films in the phantom through AC and CFC. In addition, in vivo EBT2 film measurement on a lung cancer patient immobilized with AC undergoing IMRT was also included in this study. Point doses and planar distributions generated from the TPS for the three scenarios were compared with the data from the EBT2 film measurements. For all the field arrangements, the EBT2 film data including the in vivo measurement agreed with the doses calculated for scenario (C), within the uncertainty of the EBT2 measurements (~4%). For the single posterior field (a pair of posterior-oblique fields), the TPS generated doses were lower than the EBT2 doses by 34%, 33%, 31%, 13% (34%, 31%, 31%, 11%) for scenario A and by 27%, 25%, 22%, 8% (25%, 21%, 21%, 6%) for scenario B at the depths of 0, 0.1, 0.2, 0.5 cm, respectively. For the IMRT field, the 2D dose distributions at each depth calculated in scenario C agree with those measured data. When comparing the central axis doses for the IMRT field, we found the TPS generated doses for scenario A (B) were lower than the EBT2 data by 35%, 34%, 31%, 16% (29%, 26%, 23%, 10%) at the depths of 0, 0.1, 0.2, 0.5 cm, respectively. There were no significant differences for the depths of 1.0 and 1.4 cm for all the radiation fields studied. TPS calculation of doses in the skin layers accounting for AC and CFC was verified by EBT2 film data. Ignoring the presence of AC and/or CFC in TPS calculation would significantly underestimate the doses in the skin layers. For the clinicians, as more hypofractionated regimens and stereotactic regimens are being used, this information will be useful to avoid potential serious skin toxicities, and also assist in clinical decisions and report these doses accurately to relevant clinical trials/cooperative groups, such as RTOG.

Carbon-Fiber Couch Modeling Optimization for Monaco VMAT Planning and Delivery

Carbon-Fiber Couch Modeling Optimization for Monaco VMAT Planning and Delivery

Poster - Thur Eve - 58: The clinical significance of couch top modeling in VMAT and IMRT treatment planning.

In recent years, carbon fibre couch tops have become popular in radiation therapy with marketed attributes such as high tensile strength, light weight, and relative radiotranslucence. We have performed a detailed analysis of the iBEAM evo couch top using two models created in the Pinnacle v9.0 treatment planning system. The first model is a single ROI, and its thickness and shape was determined based on attenuation factor measurements in cylindrical phantoms. The second model is a thin shell made of two ROIs with exact outer dimensions as the real couch top. The couch top attenuation was found to range from 2% to 6.5% for a 6 MV beam energy. The two models were found to match the attenuation factor measurements to within 4.2% and 1.7%, respectively. The clinical impact of the couch top models was examined in 4 VMAT and 4 IMRT prostate plans by comparing mean and maximum point doses, DVHs, and measurements (VMAT plans only) using Gamma analysis. A very small, but clinically insignificant difference was observed between the two couch top models for both VMAT and IMRT plans. The lack of a couch top model in treatment planning was also examined and found to result in ∼ 1% average underdose to the PTV. This study suggests that the iBEAM evo couch top is relatively radiotranslucent for typical VMAT and IMRT delivery as long as the number of control points with extremely oblique beam angles is relatively small.

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.

Increased beam attenuation and surface dose by different couch inserts of treatment tables used in megavoltage radiotherapy

The use of solid carbon fiber table materials in radiotherapy has become more common with the implementation of image‐guided radiotherapy (IGRT), since the solid materials give less imaging artifacts than the so‐called tennis racket couchtops. The downside of the solid carbon fiber couch inserts is that they increase the beam attenuation, resulting in increased surface doses and inaccuracies in determine the dose in the patient. The purpose of this study was to evaluate the interaction of 6 and 15 MV photons with eight different couch inserts. The presented results enable direct comparison of the attenuation properties of the studied couchtops. With a direct posterior beam the maximum attenuations reach 3.6% and 2.4% with 6 and 15 M V, respectively. The measured maximum attenuation by a couchtop with an oblique gantry angle was 10.8% and 7.4% at 6 and 15 MV energies, respectively. The skin‐sparing effect was decreased substantially with every couchtop. The highest increases in surface doses were recorded to be four‐ and threefold, as compared to the direct posterior open field surface doses of 6 and 15 MV, respectively. In conclusion, the carbon fiber tabletops decrease the skin‐sparing effect of megavoltage photon energies. The increased beam attenuation and skin doses should be taken into account in the process of treatment planning.PACS number: 07.90.+c

SU-E-T-756: The Verification of Accuaracy in Dose Distribution Using Superpositon and Convolution Algorithm including Couch for TomoDirect

Purpose: The purpose of this study is to verify the accuracy of superposition and convolution(S/C) algorithm for the dose distribution which would be perturbed by the carbon fiber couch(CFC) focused on the photon attenuation. Methods: we perform the experiment using TomoDirect that irradiates every 15 degrees from the incident angle with 1, 2.5 and 5 cm jaw‐spacing. We measure the delivered dose to cylindrical cheese phantom to compare with planned one from TPS. Results: What we have found is quite interesting and important clinically. The percentage difference between delivered and planned dose up to 5\% in specific gantry angle, which is our preliminary experiment. Based on the result, the effect of CFC was not efficiently implemented in TPS. Conclusions: From this preliminary works, we can conclude that the effect of CFC is not sufficiently implemented to TPS for TomoDirect in certain gantry angle. Also for the issue of the verification of S/C algorithm from the TPS of TomoDirect, it is working properly unless x—ray does pass through CFC partially.

Carbon fiber couch effects on skin dose for volumetric modulated arcs

The purpose of this study is to evaluate the dosimetric effect of carbon fiber couches (CFCs) on delivered skin dose as well as to explore potential venues for its minimization for volumetric modulated arc (VMAT) treatments. A carbon fiber couch (BrainLab) was incorporated in Pinnacle treatment planning system (TPS) by autocontouring. A retrospective investigation on five lung and five prostate patient plans was performed. Targets and organs at risk (OARs), together with a 0.3 cm thick skin contour interfacing the CFC, were outlined in each plan. For each patient, two VMAT plans were generated: a single arc with 6 MV photon energy and two or three arcs with 18 MV photon energy for the posterior arc(s) and 6 MV energy for the anterior arc (mixed energy plans). Both plans for each patient case were normalized such that 95% of the PTV was covered by the same prescription dose, ranging from 7600 to 7800 cGy. For each patient, the prescription doses were escalated to the maximum allowed by the OAR constraints. CFC bolus effects on skin doses were tallied by the highest dose to 1% of skin volume. With the utilization of higher energy photons for the posterior arcs, the statistically significant differences in skin dose between the two plans were as high as 34% of the prescribed dose, where surface doses changed on average from 3800 to 2940 cGy for 6 MV and mixed energy plans, respectively. In addition, skin doses in excess of 68% and 80% of the prescription doses for mixed and 6 MV energy plans, respectively, were observed in individual cases. The presented findings indicate that mixed energy VMAT plans would result in a substantial skin sparing of more than approximately 34% compared to VMAT plans with only 6 MV arc(s). Additionally, the high skin doses in some cases (81% of the prescription dose) suggest that in hypofractionated SRS/SRT treatments, the carbon fiber couch effects on skin doses need to be evaluated when arc delivery is considered as a treatment option.

Characterization of dose impact on IMRT and VMAT from couch attenuation for two Varian couches

In intensity‐modulated radiation therapy (IMRT) and volumetric‐modulated arc therapy (VMAT), the use of posterior oblique beams has become common. Beam attenuation by the treatment couch is not negligible when the couch is in the beam portal. In this study, we established the relationship of relative dose vs. beam angle for two Varian 21EX linacs, one equipped with the Exact couch (standard couch) with sliding side support rails, and the other equipped with the Exact image‐guided radiation therapy (IGRT) carbon fiber couch. Measurements were performed using an ion chamber placed at the center of an acrylic cylindrical phantom positioned at the linac isocenter for 6 MV and 18 MV photon beams. Measurements were performed at three different field sizes (3×3,5×5, and 10×10 cm2, and were repeated with the phantom positioned at different longitudinal locations on the couches. To evaluate beam attenuation by the standard couch in a clinical setting, two test IMRT plans and two test VMAT plans on the standard couch were delivered. The plans were generated with the sliding rails at the “in” position and delivered with the rails at both “in” and “out” positions. The dose difference to the ion chamber was determined. For oblique fields with 6 MV photons, the standard couch attenuated the radiation beam by up to 26.8%, while the carbon fiber IGRT couch attenuated the beam by up to 4.1%. In the clinical evaluation, the highest dose difference between rails set at the “in” and “out” positions was 2.6% in the IMRT case and 2.1% in the VMAT case. The magnitude of potential dose difference has been quantified and could be used for a quick estimation of dose difference due to couch attenuation in IMRT and VMAT.PACS number: 87.56.‐v

６軸カウチの線量補正に関する検討

We investigated experimentally and clinically the influence of a six degree (6D) carbon fiber couch on conventional radiation therapy. We used 4, 6 and 10 MV X-rays and compared dose distributions based on correction methods, i.e. monitor unit (MU) addition, including computed tomography (CT) couch, and the couch modeling. Additionally, we evaluated the clinical value of dosimetric correction for the 6D couch in 30 patients treated with multi-field irradiation. In the phantom study, the maximum difference of isocenter doses attributable to the 6D couch was 5.1%; the difference was reduced with increasing X-ray energy. Although the isocenter dose based on each correction method was precise within ±1%, MU addition underestimated the surface dose. In the clinical study, the maximum difference of isocenter doses attributable to the 6D couch was 2.7%. The correction methods for the 6D couch provide for highly precise treatment planning. However, the clinical indication of complicated correction methods should be considered for each institution or each patient, because the influence of the 6D couch was reduced with multi-field irradiation.

Dosimetric characterization of the iBEAM evo carbon fiber couch for radiotherapy

This study characterizes the dosimetric properties of the iBEAM evo carbon fiber couch manufactured by Medical Intelligence and examines the accuracy of the CMS XiO and Nucletron Oncentra Masterplan (OMP) treatment planning systems for calculating beam attenuation due to the presence of the couch. To assess the homogeneity of the couch, it was CT scanned at isocentric height and a number of signal intensity profiles were generated and analyzed. To simplify experimental procedures, surface dose and central axis depth dose measurements were performed in a solid water slab phantom using Gafchromic film for 6 and 10 MV photon beams at gantry angles of 0 degree (normal incidence), 30 degrees, and 60 degrees with an inverted iBEAM couch placed on top of the phantom. Attenuation measurements were performed in a cylindrical solid water phantom with an ionization chamber positioned at the isocenter. Measurements were taken for gantry angles from 0 degree to 90 degrees in 10 degrees increments for both 6 and 10 MV photon beams. This setup was replicated in the XiO and OMP treatment planning systems. Dose was calculated using the pencil beam, collapsed cone, convolution, and superposition algorithms. The CT scan of the couch showed that it was uniformly constructed. Surface dose increased by (510 +/-0)% for a 6 MV beam and (600 +/- 20)% for a 10 MV beam passing through the couch at normal incidence. Obliquely incident beams resulted in a higher surface dose compared to normally incident beams for both open fields and fields with the couch present. Depth dose curves showed that the presence of the couch resulted in an increase in dose in the build up region. For 6 and 10 MV beams incident at 60 degrees, nearly all skin sparing was lost. Attenuation measurements derived using the ionization chamber varied from 2.7% (0 degree) to a maximum of 4.6% (50 degrees) for a 6 MV beam and from 1.9% (0 degree) to a maximum of 4.0% (50 degrees) for a 10 MV beam. The pencil beam and convolution algorithms failed to accurately calculate couch attenuation. The collapsed cone and superposition algorithms calculated attenuation within an absolute error of +/- 1.2% for 6 MV and +/- 0.8% for 10 MV for gantry angles from 0 degree to 40 degrees. Some differences in attenuation were observed dependent on how the couch was contoured. These results demonstrate that the presence of the iBEAM evo carbon fiber couch increases the surface dose and dose in the build up region. The inclusion of the couch in the planning scan is limited by the field of view employed and the couch height at the time of CT scanning.