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

A Study to Analyze Impact of Treatment Couch and Immobilization Devices on Surface Dose for Megavoltage Photon Beams

PURPOSE/OBJECTIVE

The purpose of this study is to investigate the effect of treatment couch and immobilization devices on surface dose for megavoltage photon beams.

MATERIAL/METHODS

Percentage surface dose (PSD) measurement was carried out in Elekta Synergy™ Linear accelerator using PTW Markus® Parallel plate ionization chamber of volume 0.05cm3 with water equivalent RW3 Slab phantom (PTW, Germany). The measurement depth was considered at 0.07mm. The reference PSD was measured at 0° gantry angle with 10×10cm2, 20×20cm2 and 30×30cm2 field sizes and 100cm SSD for 4MV, 6MV and 15MV photon beams. For comparison, PSD measurement was carried out at 180° gantry angle inclusion of treatment couch (TC), All in One positioning system (AIO - PS) and Vac lok Cushions (VLC).

RESULTS

Beam angle at 0°, for field sizes 10×10cm2, 20×20cm2 and 30×30cm2, the PSD was observed as 30.9%, 40.5%, 48.7% for 4MV; 23.7%, 33.8%, 42.2% for 6MV; and 17.0%, 29.6%, 38.6% for 15MV respectively. Beam angle at 180° with TC, an increase in PSD by maximum of 65.0% for 4MV, 64.9% for 6MV and 55.9% for 15MV as compared to 0° angle. The PSD increased when beam angle was 180° with TC and AIO - PS were 65.0% for 4MV, 67.4% for 6MV, and 60.9% for 15MV than 0° angle. Similarly, increased PSD for beam angle at 180° with TC and VLC were 66.8% for 4MV, 66.8% for 6MV and 61.3% for 15MV as compared to 0° angle.

CONCLUSION

For all three-photon energies, at 180° gantry angle, the PSD increased significantly in case of TC, VLC, and AIO - PS for all the field sizes as compared to gantry angle at 0°. It is necessary to consider TC, AIO - PS and VLC during dose calculation to ensure accuracy of patient treatment delivery.

Quantitative comparison of dosimetric data between the indigenous baseplate and commercially available carbon fiber baseplate for 6 and 15 MV photon energy

Background:

This study aims to design an indigenous baseplate (ID baseplate) that is economically viable and dosimetrically comparable for radiotherapy patient treatment. An ID baseplate was designed and manufactured using wood plastic composition materials that are readily available in the market and were compared dosimetrically with the commercially available carbon fiber baseplate (CF baseplate).

Materials and Methods:

Surface dose and beam attenuation properties of both the baseplates (ID and CF) were measured using a parallel plate chamber and compared with the dose calculated from the treatment planning system (TPS). Separate computer tomography images of both the baseplates were acquired by placing solid water phantoms. These images were used for surface dose calculation in the TPS and were validated with experimental measurements. Proper densities were assigned to the couch and baseplates to avoid uncertainties in dose calculations. All measurements were performed at field sizes 10 cm × 10 cm for 6 MV and 15 MV photon beams.

Results:

The percentage surface dose measured for the ID baseplate and CF baseplate was found to be matching for 6 MV beam (98.2% and 97%, respectively); however, for the 15 MV beam, the ID baseplate showed a higher surface dose of 98.6% compared to CF baseplate (87.4%). For the ID baseplate, the percentage difference in the surface dose between that TPS calculated value and the measured values were 1.6% and 1.4% for 6MV and 15MV, respectively. The ID baseplate showed higher beam attenuation than the CF baseplate by 2.2% for the 6MV beam and 3.4% for the 15MV beam when proper electron densities were not assigned. The difference between the TPS calculated dose and delivered dose was achieved within 3% after assigning proper electron density to the couch and baseplate.

Conclusions:

The ID baseplate has shown acceptable dosimetric results and can be an economically viable alternative to the commercially available CF baseplates. The manufacturing cost of the ID baseplate was ten times cheaper than the CF baseplate.

Dosimetric effects of supine immobilization devices on the skin in intensity-modulated radiation therapy for breast cancer: a retrospective study

Background

The dose perturbation effect of immobilization devices is often overlooked in intensity-modulated radiation therapy (IMRT) for breast cancer (BC). This retrospective study assessed the dosimetric effects of supine immobilization devices on the skin using a commercial treatment planning system.

Methods

Forty women with BC were divided into four groups according to the type of primary surgery: groups A and B included patients with left and right BC, respectively, who received 50 Gy radiotherapy in 25 fractions after radical mastectomy, while groups C and D included patients with left and right BC, respectively, who received breast-conservation surgery (BCS) and 40.05 Gy in 15 fractions as well as a tumor bed simultaneous integrated boost to 45 Gy. A 0.2-cm thick skin contour and two sets of body contours were outlined for each patient. Dose calculations were conducted for the two sets of contours using the same plan. The dose differences were assessed by comparing the dose-volume histogram parameter results and by plan subtraction.

Results

The supine immobilization devices for BC resulted in significantly increased skin doses, which may ultimately lead to skin toxicity. The mean dose increased by approximately 0.5 and 0.45 Gy in groups A and B after radical mastectomy and by 2.7 and 3.25 Gy in groups C and D after BCS; in groups A–D, the percentages of total normal skin volume receiving equal to or greater than 5 Gy (V₅) increased by 0.54, 1.15, 2.67, and 1.94%, respectively, while the V₁₀ increased by 1.27, 1.83, 1.36, and 2.88%; the V₂₀ by 0.85, 1.87, 2.76, and 4.86%; the V₃₀ by 1.3, 1.24, 10.58, and 11.91%; and the V₄₀ by 1.29, 0.65, 10, and 10.51%. The dose encompassing the planning target volume and other organs at risk, showed little distinction between IMRT plans without and with consideration of immobilization devices.

Conclusions

The supine immobilization devices significantly increased the dose to the skin, especially for patients with BCS. Thus, immobilization devices should be included in the external contour to account for dose attenuation and skin dose increment.

Trial registration

This study does not report on interventions in human health care.

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.

Dosimetric Effects of Head and Neck Immobilization Devices on Multi-field Intensity Modulated Radiation Therapy for Nasopharyngeal Carcinoma

Background: In practice, the dose perturbation effect of head and neck immobilization devices is often overlooked in intensity-modulated radiation therapy (IMRT) for nasopharyngeal carcinoma (NPC). Purpose of this study is to verify and analyze the dosimetric effect of head and neck immobilization devices on NPC multi-field IMRT. Methods: Ten patients with nasopharyngeal carcinoma were randomly selected. Two sets of body contours were established for each patient. One set of body contours did not contain the immobilization device, and the other contour set included the immobilization device. For each patient, dose calculations were conducted for the two sets of contours using the same 9-field IMRT plan, which were recorded as Plan- and Plan+. The dose difference caused by the head and neck immobilization devices was assessed by comparing the dose-volume histogram (DVH) parameter results and by plan subtraction. The gafchromic EBT3 film and anthropomorphic phantom were used to verify the calculated doses. Results: The target coverage and average dose of Plan+ were lower than those of Plan- : the prescription dose coverage rates for PTVnx, PTVnd, PTV1 and PTV2 decreased by 2.4%, 9.9%, 1.5%, and 3.6%, respectively, and the mean doses were reduced by 0.9%, 1.9%, 1.1%, and 1.5%, respectively. Doses in the organs at risk showed no significant differences or slight reductions (the maximum reduction in mean dose was 1.7%). From the EBT3 measurements, the skin dose on the posterior neck was increased by approximately 53%. Conclusion: The attenuation and bolus effects of the head and neck immobilization device reduce dose coverage rate and average dose of the planning target volumes in nasopharyngeal carcinoma and lead to an increase in the skin dose. During treatment planning and dose calculation, the immobilization device should be included within body contour to account for the dose attenuation and skin dose increment.

Absorption ratio of treatment couch and effect on surface and build-up region doses

Aim

In this study, at different fields, energies and gantry angles, treatment couch and rails dose absorption ratio and treatment couch effect on surface and build-up region doses were examined.

Background

It is assumed that radiation attenuation is minimal because the carbon fiber couches have low density and it is not generally accounted for during treatment planning. Consequently, it leads to a major dosimetric mistake.

Materials and methods

Solid water phantom was used for relative dose measurement. The measurements were done using a Farmer ion chamber with 0.6 cc volume and a parallel plane ion chamber starting from surface with 1 mm depth intervals at 10 × 10 cm² field, SSD 100 cm. Measurements were taken for situations where the beams intersect the couch and couch rails.

Results

Dose absorption ratio of carbon fiber couch obtained at gantry angle of 180° was 1.52%, 0.69%, 0.33% and 0.25% at different field sizes for 6 MV. For 15 MV, this ratio was 0.95%, 0.27%, 0.20% and 0.05%. The absorption ratio is between 3.4% and 1.22% when the beams intersect with couch rails. The couch effect increased surface dose from 14% to 70% for 6 MV and from 11.34% to 53.03% for 15 MV.

Conclusions

The results showed that the carbon fiber couch increased surface dose during posterior irradiation. Therefore, the skin-sparing effect of the high energy beams was decreased. If the effect of couch is not considered, it may cause significant differences at dose which reaches the patient and may cause tissue problems such as erythema.

Effects of Siemens TT-D carbon fiber table top on beam attenuation, and build up region of 6 MV photon beam

AIM

This study deals with Monte Carlo simulations of the effects which the 550 TXT carbon fiber couch can have on the relevant parameters of a 6 MV clinical photon beam in three field sizes.

BACKGROUND

According to the reports issued by the International Commission on Radiation Units and Measurements (ICRU), the calculated dose across a high gradient distribution should be within 2% of the relative dose, or within 0.2 cm of the isodose curve position in the target volume. Nowadays, the use of posterior oblique beam has become a common practice. It is clear that, in radiotherapy, the presence of the couch affects the beam intensity and, as a result, the skin dose.

MATERIALS AND METHODS

Firstly, Siemens linear accelerator validation for 6 MV photon beam was performed, and satisfactory agreement between Monte Carlo and experimental data for various field sizes was observed. Secondly, the couch transmission factor for the reference field size and depth was computed, and the skin dose enhancement by the couch was assessed.

RESULTS

The largest impact of the carbon fiber couch effect was observed for the 5 × 5 cm2 field size. Such evaluation has not been reported for this couch before.

CONCLUSION

Despite providing minimal attenuation for the primary radiation, the assumption that carbon fiber couches are radiotranslucent is not valid, and the effects of couches of this type on the transmission factor, and on the skin dose should be carefully investigated for each field size and depth.

Patient-related quality assurance with different combinations of treatment planning systems, techniques, and machines : A multi-institutional survey

PURPOSE

This project compares the different patient-related quality assurance systems for intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) techniques currently used in the central Germany area with an independent measuring system.

MATERIALS AND METHODS

The participating institutions generated 21 treatment plans with different combinations of treatment planning systems (TPS) and linear accelerators (LINAC) for the QUASIMODO (Quality ASsurance of Intensity MODulated radiation Oncology) patient model. The plans were exposed to the ArcCHECK measuring system (Sun Nuclear Corporation, Melbourne, FL, USA). The dose distributions were analyzed using the corresponding software and a point dose measured at the isocenter with an ionization chamber.

RESULTS

According to the generally used criteria of a 10 % threshold, 3 % difference, and 3 mm distance, the majority of plans investigated showed a gamma index exceeding 95 %. Only one plan did not fulfill the criteria and three of the plans did not comply with the commonly accepted tolerance level of ±3 % in point dose measurement.

CONCLUSION

Using only one of the two examined methods for patient-related quality assurance is not sufficiently significant in all cases.

Generation and verification of QFix kVue Calypso-compatible couch top model for a dedicated stereotactic linear accelerator with FFF beams

This study details the generation, verification, and implementation of a treatment planning system (TPS) couch top model for patient support system used in conjunction with a dedicated stereotactic linear accelerator. Couch top model was created within the TPS using CT simulation images of the kVue Calpyso-compatible couchtop (with rails). Verification measurements were compared to TPS dose prediction for different energies (6 MV FFF and 10 MV FFF) and rail configurations (rails in and rails out) using: 1) central axis point-dose measurements with pinpoint chamber in water-equivalent phantom at 42 gantry angles for various field sizes (2 × 2 cm², 4 × 4 cm², 10 × 10 cm²); and 2) Gafchromic EBT3 film parallel to beam in acrylic slab to assess changes in surface and percent depth doses in PA geometry. To assess sensitivity of delivered dose to variations in patient lateral position, measurements at central axis using the pinpoint chamber geometry were taken at lateral couch displacements of 2, 5, and 10 mm for 6 MV FFF. The maximum percent difference for point-dose measurements was 3.24% (6 MV FFF) and 2.30% (10 MV FFF). The average percent difference for point-dose measurements was less than 1.10% for all beam energies and rail geometries. The maximum percent difference between calculated and measured dose can be as large as 13.0% if no couch model is used for dose calculation. The presence of the couch structures also impacts surface dose and PDD, which was evaluated with Gafchromic film measurements. The upstream shift in the depth of dose maximum (dmax) was found to be 10.5 mm for 6 MV FFF and 5.5 mm for 10 MV FFF for 'Rails In' configuration. Transmission of the treatment beam through the couch results in an increase in surface dose (absolute percentage) of approximately 50% for both photon energies (6 MV FFF and 10MV FFF). The largest sensitivity to lateral shifts occurred at the lateral boundary of the rail structures. The mean magnitude (standard deviation) of the deviation between shifted and centered measurements over all field sizes tested was 0.61% (0.61%) for 2 mm shifts, 0.46% (0.67%) for 5 mm shifts, and 0.86% (1.46%) for 10 mm shifts.

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 cm3) 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 cm2 field, a pair of posterior-oblique 10 × 10 cm2 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.

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.

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

Two years experience with quality assurance protocol for patient related Rapid Arc treatment plan verification using a two dimensional ionization chamber array

PURPOSE

To verify the dose distribution and number of monitor units (MU) for dynamic treatment techniques like volumetric modulated single arc radiation therapy-Rapid Arc-each patient treatment plan has to be verified prior to the first treatment. The purpose of this study was to develop a patient related treatment plan verification protocol using a two dimensional ionization chamber array (MatriXX, IBA, Schwarzenbruck, Germany).

METHOD

Measurements were done to determine the dependence between response of 2D ionization chamber array, beam direction, and field size. Also the reproducibility of the measurements was checked. For the patient related verifications the original patient Rapid Arc treatment plan was projected on CT dataset of the MatriXX and the dose distribution was calculated. After irradiation of the Rapid Arc verification plans measured and calculated 2D dose distributions were compared using the gamma evaluation method implemented in the measuring software OmniPro (version 1.5, IBA, Schwarzenbruck, Germany).

RESULTS

The dependence between response of 2D ionization chamber array, field size and beam direction has shown a passing rate of 99% for field sizes between 7 cm×7 cm and 24 cm×24 cm for measurements of single arc. For smaller and larger field sizes than 7 cm×7 cm and 24 cm×24 cm the passing rate was less than 99%. The reproducibility was within a passing rate of 99% and 100%. The accuracy of the whole process including the uncertainty of the measuring system, treatment planning system, linear accelerator and isocentric laser system in the treatment room was acceptable for treatment plan verification using gamma criteria of 3% and 3 mm, 2D global gamma index.

CONCLUSION

It was possible to verify the 2D dose distribution and MU of Rapid Arc treatment plans using the MatriXX. The use of the MatriXX for Rapid Arc treatment plan verification in clinical routine is reasonable. The passing rate should be 99% than the verification protocol is able to detect clinically significant errors.

Assessment and management of radiotherapy beam intersections with the treatment couch

The impact of the treatment couch on a radiotherapy plan is rarely fully assessed during the treatment planning process. Incorporating a couch model into the treatment planning system (TPS) enables the planner to avoid or dosimetrically evaluate beam-couch intersections. In this work, we demonstrate how existing TPS tools can be used to establish this capability and assess the accuracy and effectiveness of the system through dose measurements and planning studies. Such capabilities may be particularly relevant for the planning of arc therapies.Treatment couch top models were introduced into a TPS by fusing their CT image sets with the patient CT dataset. Regions of interest characterizing couch elements were then imported and assigned appropriate densities in the TPS. Measurements in phantom agreed with TPS calculations to within 2% dose and 1 degrees gantry rotation. To clinically validate the model, a retrospective study was performed on patient plans that posed difficulties in beam-couch intersection during setup. Beam-couch intersection caused up to a 3% reduction in PTV coverage, defined by the 95% of the prescribed dose, and up to a 1% reduction in mean CTV coverage. Dose compensation strategies for IMRT treatments with beams passing through couch elements were investigated using a four-field IMRT plan with three beams passing through couch elements. In this study, ignoring couch effects resulted in point dose reductions of 8 +/- 3%.A methodology for incorporating detailed couch characteristics into a TPS has been established and explored. The method can be used to predict beam-couch intersections during planning, potentially eliminating the need for pretreatment appointments. Alternatively, if a beam-couch intersection problem arises, the impact of the couch can be assessed on a case-by-case basis and a clinical decision made based on full dosimetric information.

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.