Evaluation of the Sinmed Mastercouch® as replacement for a standard couch

Validation of the Elekta iBEAM Evo couchtop modeling in the Monaco treatment planning system

Recently, carbon fiber (CF) has prevailed as the primary material used in radiotherapy couchtops. Modern couchtops incorporate the CF sandwich design, in which 2 thin CF plates sandwich an air-equivalent polymeric foam. Developments in radiotherapy necessitate irradiation from posterior angles through the couchtop. However, the presence of the couchtop needs proper modeling in the treatment planning system (TPS) due to attenuation; otherwise, the tumor dose is reduced. In the current study, an effort was made with the intent of finding the optimum electron density (ED) values for Elekta's iBEAM Evo couchtop components (CF and Foam Core (FC)) for its proper modeling in Monaco TPS. Also, the attenuation of the beam due to the couchtop's presence was investigated. A cylindrical phantom with an ionization chamber positioned at the isocenter was utilized for the measurements. The phantom was placed centrally on the iBEAM Evo couchtop and was irradiated with an Elekta Infinity linear accelerator's 6, 10, and 15 MV photon beams. The gantry angle was set at 0o and from 120o to 180o with an increment of 10o. The same procedure was designed and followed in Monaco TPS. Measured and calculated dose values were compared by calculating percentage deviation (PD). Attenuation has also been calculated using the measurements of the experimental setup and the Monaco calculations. The values of ED that provided the optimum agreement between measured and Monaco-calculated dose values while minimizing PD were 0.55 g/cm3 for CF, and 0.1 g/cm3 for FC. The maximum values of PD for the beams of 6, 10, and 15 MV were -0.62%, +1,78%, and +2.35%, respectively, for a 5 × 5 cm2 field size. Furthermore, Monaco predicted attenuation from 1.83% to 6.26% (calculated values), while from the measurements, an attenuation from 1.44% to 5.75% (measured values) regarding the posterior angles was found. Thus, good agreement was verified between the TPS calculations and experimental measurements. Elekta's iBEAM Evo couchtop modeling was successfully validated in Monaco TPS. The couchtop's presence alters the patient's dose regarding irradiation from the posterior angles. Due to the attenuation of the beam, proper incorporation, modeling, and validation of the couchtop are necessary to improve the radiotherapy outcome and ensure that each patient receives the optimal treatment.

Commissioning of a RayStation structure template for the iBEAM evo Couchtop

Accurate radiotherapy treatment planning requires attenuation through the treatment couch to be accounted for in dose calculation. This is commonly performed by using contouring tools to add a virtual structure in the shape of the treatment couch and assigning the preferred absorption properties. The RayStation treatment planning system (TPS) allows users to assign a material that comprises both an elemental structure and a physical density. The selection of such parameters should be made so that modelled attenuation through the couch closely matches measured data. When these measurements involve the use of plastic phantoms and rotational beams, the validity of the data is dependent upon aspects of TPS and linear accelerator performance that can be difficult to quantify. A fundamental measure of couch attenuation using an ionisation chamber in water and perpendicular beam geometry that required no gantry movement was implemented to eliminate the identified uncertainties. This data was used to determine the combination of elemental composition and density assigned to a modelled couch structure that provided the most accurate representation of beam attenuation in this simple geometry. The preferred material was then validated using a cylindrical phantom and rotational beams. The findings were equivalent between the static gantry with water phantom and rotating gantry with cylindrical phantom. Of the elemental compositions investigated, it was possible to achieve suitable agreement with the measured data for each option provided the density was optimised. Choice of the elemental composition was not observed to be an important factor in achieving a good model.

The effect of carbon fibre treatment couch with and without immobilisation devices on radiotherapy dose calculation using three different planning algorithms and photon beam energies

Introduction:

The objective of radiotherapy immobilisation devices is to improve the reproducibility of patient positioning during treatment sessions. The inclusion of these devices in the treatment protocol may increase the skin dose. In practice, these devices are not systematically taken into account in the dose calculation.

Material and methods:

In this study, the dosimetric effects of the carbon fibre couch iBEAM Evo Extension 415, with and without three different immobilisation devices (a Klarity Breastboard R610-2ECF, a Bionix Butterfly Board and CIVCO Vac-Lok vacuum bag), were calculated and evaluated on the dose calculation for conformal three-dimensional radiation therapy. The measurements were carried out by comparing the measured dose with the one calculated for three different algorithms, FFT convolution, fast superposition and superposition algorithms, which are implemented in Xio treatment planning system (TPS).

Results:

Dosimetric tolerance levels have been respected for specific dose calculations, which do not include the fibre couch with or without immobilisation devices. Errors of up to 8% in the dose calculation were obtained for the beams passing through the fibre couch and the breast board base support region.

Conclusion:

According to the significant attenuation differences of the beam by the fibre couch and immobilisation devices, it was concluded that ignoring the device in the dose calculation can change patient’s skin and target doses. The fibre couch and immobilisation device should be included within external body contour to account for the TPS calculation algorithms dose attenuation.

Couch modeling optimization for tomotherapy planning and delivery

We sought to validate new couch modeling optimization for tomotherapy planning and delivery. We constructed simplified virtual structures just above a default setting couch through a planning support system (MIM Maestro, version 8.2, MIM Software Inc, Cleveland, OH, USA). Based on ionization chamber measurements, we performed interactive optimization and determined the most appropriate physical density of these virtual structures in a treatment planning system (TPS). To validate this couch optimization, Gamma analysis and these statistical analyses between a three‐dimensional diode array QA system (ArcCHECK, Sun Nuclear, Melbourne, FL, USA) results and calculations from ionization chamber measurements were performed at 3%/2 mm criteria with a threshold of 10% in clinical QA plans. Using a virtual model consisting of a center slab density of 4.2 g/cm³ and both side slabs density of 1.9 g/cm³, we demonstrated close agreement between measured dose and the TPS calculated dose. Agreement was within 1% for all gantry angles at the isocenter and within 2% in off‐axis plans. In validation of the couch modeling in a clinical QA plan, the average gamma passing rate improved approximately 0.6%–5.1%. It was statistically significant (P < 0.05) for all treatment sites. We successfully generated an accurate couch model for a TomoTherapy TPS by interactively optimizing the physical density of the couch using a planning support system. This modeling proved to be an efficient way of correcting the dosimetric effects of the treatment couch in tomotherapy planning and delivery.

Photon beam attenuation characteristics of three commercial radiation therapy treatment couch-tops

Aim

The purpose of the study was to investigate the detailed angularly dependent attenuation characteristics of three different commercial couch-tops: Varian IGRT, Qfix kVue Standard and Qfix kVue Dose Max couch-tops used in radiation therapy.

Materials and methods

The attenuation of photon beams by the treatment couch-tops was measured using a farmer chamber inserted at the centre of a 16 cm diameter cylindrical acrylic phantom for five different photon energies: 6 MV, 6FFF MV, 10 MV, 10FFF MV and 15 MV photon beams. The Varian IGRT couch-top has three different thicknesses thus attenuation measurements were done at the three different longitudinal locations. Measurements were made with the sliding support rails of the Qfix kVue Standard and Qfix kVue Dose Max couch-tops at both ‘rails-in’ and ‘rails-out’ positions. All measurements were taken for several projections through 360° movement of the gantry and for two different field sizes; 5×5 cm2 and 10×10 cm2.

Results and findings

The results indicate that the maximum attenuation of the Varian IGRT couch-top at the thin, medium and thick portions are 5·1, 5·7 and 8·9%, respectively, the Qfix kVue Standard couch with the rails-in and rails-out are 11·2 and 13·7%, respectively, and Qfix kVue Dose Max couch-top with rails-in and rails-out are 9·7 and 13·8%, respectively. The results from this study can be used to account for the couch-top attenuation during radiation treatment planning of patients treated with these couch-tops.

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

Introduction

Treatment 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 Methods

Measurements 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.

Results

At 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.

Discussion

The 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.

Conclusions

There 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.

[Dose impact of a carbon fiber couch for stereotactic body radiation therapy of lung tumors]

The aim of this study was to measure the dose attenuation caused by a carbon fiber radiation therapy table (Imaging Couch Top; ICT, BrainLab) and to evaluate the dosimetric impact of ICT during stereotactic body radiation therapy (SBRT) in lung tumors. The dose attenuation of ICT was measured using an ionization chamber and modeled by means of a treatment planning system (TPS). SBRT was planned with and without ICT in a lung tumor phantom and ten cases of clinical lung tumors. The results were analyzed from isocenter doses and a dose-volume histogram (DVH): D95, Dmean, V20, V5, homogeneity index (HI), and conformity index (CI). The dose attenuation of the ICT modeled with TPS agreed to within ±1% of the actually measured values. The isocenter doses, D95 and Dmean with and without ICT showed differences of 4.1-5% for posterior single field and three fields in the phantom study, and differences of 0.6-2.4% for five fields and rotation in the phantom study and six fields in ten clinical cases. The dose impact of ICT was not significant for five or more fields in SBRT. It is thus possible to reduce the dose effect of ICT by modifying the beam angle and beam weight in the treatment plan.

A validation of carbon fiber imaging couch top modeling in two radiation therapy treatment planning systems: Philips Pinnacle3 and BrainLAB iPlan RT Dose

BACKGROUND

Carbon fiber (CF) is now the material of choice for radiation therapy couch tops. Initial designs included side metal bars for rigidity; however, with the advent of IGRT, involving on board imaging, new thicker CF couch tops without metal bars have been developed. The new design allows for excellent imaging at the expense of potentially unacceptable dose attenuation and perturbation.

OBJECTIVES

We set out to model the BrainLAB imaging couch top (ICT) in Philips Pinnacle(3) treatment planning system (TPS), to validate the already modeled ICT in BrainLAB iPlan RT Dose treatment planning system and to compute the magnitude of the loss in skin sparing.

RESULTS

Using CF density of 0.55 g/cm(3) and foam density of 0.03 g/cm(3), we demonstrated an excellent agreement between measured dose and Pinnacle(3) TPS computed dose using 6 MV beam. The agreement was within 1% for all gantry angle measured except for 120°, which was 1.8%. The measured and iPlan RT Dose TPS computed dose agreed to within 1% for all gantry angles and field sizes measured except for 100° where the agreement was 1.4% for 10 cm × 10 cm field size. Predicted attenuation through the couch by iPlan RT Dose TPS (3.4% - 9.5%) and Pinnacle(3) TPS (2% - 6.6%) were within the same magnitude and similar to previously reported in the literature. Pinnacle(3) TPS estimated an 8% to 20% increase in skin dose with increase in field size. With the introduction of the CF couch top, it estimated an increase in skin dose by approximately 46 - 90%. The clinical impact of omitting the couch in treatment planning will be dependent on the beam arrangement, the percentage of the beams intersecting the couch and their angles of incidence.

CONCLUSION

We have successfully modeled the ICT in Pinnacle(3) TPS and validated the modeled ICT in iPlan RT Dose. It is recommended that the ICT be included in treatment planning for all treatments that involve posteriors beams. There is a significant increase in skin dose that is dependent on the percentage of the beam passing through the couch and the angle of incidence.

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

Impact of 6MV photon beam attenuation by carbon fiber couch and immobilization devices in IMRT planning and dose delivery

Multiple fields in IMRT and optimization allow conformal dose to the target and reduced dose to the surroundings and the regions of interest. Thus we can escalate the dose to the target to achieve better tumor control with low morbidity. Orientation of multiple beams can be achieved by i) different gantry angles, ii) rotating patient's couch isocentrically. In doing so, one or more beam may pass through different materials like the treatment couch, immobilization cast fixation plate, head and neck rest or any other supportive device. Our observations for 6MV photon beam on PRIMUS-KXE2 with MED-TEC carbon fiber tabletop and 10 × 10 cm² field size reveals that the maximum dose attenuation by the couch was of the order of 2.96% from gantry angle 120-160°. Attenuation due to cast fixation base plate of PMMA alone was of the order of 5.8-10.55% at gantry angle between 0 and 90°. Attenuation due to carbon fiber base plate alone was 3.8-7.98%. Attenuation coefficient of carbon fiber and PMMA was evaluated and was of the order of 0.082 cm⁻¹ and 0.064 cm⁻¹ respectively. Most of the TPS are configured for direct beam incidence attenuation correction factors only. Whereas when the beam is obliquely incident on the couch, base plate, headrest and any other immobilization device get attenuated more than the direct beam incidence. The correction factors for oblique incidence beam attenuation are not configured in most of the commercially available treatment planning systems. Therefore, such high variations in dose delivery could lead to under-dosage to the target volume for treatments requiring multiple fields in IMRT and 3D-CRT and need to be corrected for monitor unit calculations.

Evaluating and modeling of photon beam attenuation by a standard treatment couch

The purpose of this study was to evaluate beam attenuation by treatment couch and build a treatment couch model in TPS to check for beam–couch intersection at the planning stage and deal with beam attenuation by treatment couch in dose calculation. In this study, a standard treatment couch, Siemens ZXT couch, has been incorporated into Pinnacle3 8.0 TPS, based on an existing TPS tool, model‐based segmentation (MBS). This was done by generating the couch's model from contours of the couch, together with the density information. Both the geometric and dosimetric accuracy of the couch model were evaluated. The test of beam–couch intersection prediction showed good agreement between predicted and measured results, and the differences were within 1° gantry rotation. For individual posterior oblique beams, the attenuation by metallic frames and PMMA couch top could reach nearly as high as 60% and 10%, respectively. For several posterior oblique beams (180°, 220°, 235°) that attenuated by the PMMA couch top, the calculated and measured dose distributions were compared. The dose differences at central axis were within 1%, and almost all points agreed with the calculations when the DD and DTA criteria of 3%/3 mm were adopted. The difference between calculated and measured attenuation factors were within 0.5%. This study demonstrates that the couch model created by MBS, which contains geometric and density information of the couch, can be used to detect the beam–couch intersection, and also is able to provide an accurate representation of the couch top attenuation properties in patient dose calculation.

PACS numbers: 87.55.D‐, 87.55.Gh, 87.55.km

Determination of the photon beam attenuation by the Brainlab imaging couch: angular and field size dependence

Highly attenuating radiation treatment couches are no longer useful in the present era of radiotherapy utilizing IMRT and IGRT. Carbon fibers couch tops with its high tensile strength and low density present a useful alternative. The objective of the current study was to quantify the attenuation of megavoltage photons through a Brainlab imaging couch top and headrest at various angles and field sizes. At normal incidence, the couch attenuated 6 MV photons by 4.9% and 3.4% for 5 x 5 cm(2) and 10 x 10 cm(2) field sizes respectively. The headrest on the other hand only attenuated 6 MV photons by 2.5% and 1.6% respectively. There was no significant attenuation of the 18 MV beam by either the couch or the headrest. We found the attenuation to be dependent on the gantry angle, with the highest attenuation recorded at 1200. At this angle, the couch attenuated the 6 MV photon beam by 10% and 8.3% for the 5 x 5 cm(2) and 10 x 10 cm(2) field sizes respectively. Similarly, 18 MV photon beam was attenuated by 3.6% and 3.4% for the 5 x 5 cm(2) and 10 x 10 cm(2) field sizes at 1200 gantry angle. The highest attenuation for the headrest on the other hand occurred at 110 degrees gantry angle. For the 6 MV photon beam the headrest attenuation at this angle was 6.3% and 5.6% for the 5 x 5 cm(2) and 10 x 10 cm(2) field sizes respectively. Similarly for the 18 MV the attenuation was 2.3% and 2.1% 5 x 5 cm(2) and 10 x 10 cm(2) field sizes respectively. It apparent that the use of the Brainlab imaging couch and headrest in IMRT with posterior beams will results in significant decrease in the dose delivered to the target.