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The British Journal of Radiology, 82 (2009), 855–859 SHORT COMMUNICATION Simple methods to reduce patient dose in a Varian cone beam CT system for delivery verification in pelvic radiotherapy 1 P ROXBY, MSc, 1,2T KRON, and 1J CRAMB, MSc PhD, 3 F FOROUDI, MD, 1,2 A HAWORTH, PhD, 1 C FOX, PhD, 1 A MULLEN, MSc 1 Peter MacCallum Cancer Centre, Department of Physical Sciences, St Andrews Place, East Melbourne, Victoria 3002, RMIT University, Department of Applied Physics, Melbourne and 3Peter MacCallum Cancer Centre, Department of Radiation Oncology, St Andrews Place, East Melbourne, Victoria 3002, Australia 2 ABSTRACT. Cone-beam computed tomography (CBCT) is a three-dimensional imaging modality that has recently become available on linear accelerators for radiotherapy patient position verification. It was the aim of the present study to implement simple strategies for reduction of the dose delivered in a commercial CBCT system. The dose delivered in a CBCT procedure (Varian, half-fan acquisition, 650 projections, 125 kVp) was assessed using a cylindrical Perspex phantom (diameter, 32 cm) with a calibrated Farmer type ionisation chamber. A copper filter (thickness, 0.15 mm) was introduced increasing the half value layer of the beam from 5.5 mm Al to 8 mm Al. Image quality and noise were assessed using an image quality phantom (CatPhan) while the exposure settings per projection were varied from 25 ms/80 mA to 2 ms/2 mA per projection. Using the copper filter reduced the dose to the phantom from approximately 45 mGy to 30 mGy at standard settings (centre/periphery weighting 1/3 to 2/3). Multiple CBCT images were acquired for six patients with pelvic malignancies to compare CBCTs with and without a copper filter. Although the reconstructed image is somewhat noisier with the filter, it features similar contrast in the centre of the patient and was often preferred by the radiation oncologist because of greater image uniformity. The X-ray shutters were adjusted to the minimum size required to obtain the desired image volume for a given patient diameter. The simple methods described here reduce the effective dose to patients undergoing daily CBCT and are easy to implement, and initial evidence suggests that they do not affect the ability to identify soft tissue for the purpose of treatment verification. Cone beam computed tomography (CBCT) is a threedimensional imaging modality that has recently become available on linear accelerators used for radiotherapy treatment [1, 2]. The systems commercially available include megavoltage imaging [3] as well as kilovoltage imaging systems [2, 4]. For the latter, linear accelerators are equipped with a diagnostic imaging set (X-ray tube and flat panel imaging device) mounted at 90 ˚ to the treatment beam on the gantry. CBCTs are reconstructed from projections acquired during rotation of the gantry by >180˚. The maximum speed of rotation is 6˚ per second, a restriction imposed by the International Electrotechnical Commission to reduce the risk of serious collisions in rotating open gantry systems [5]. The typical distance between the focal spot and the detector is 150 cm, as illustrated in Figure 1. Given the high cost of large flat panel devices, most commercial systems Address correspondence to: Tomas Kron, PhD, Principal Research Physicist, Peter MacCallum Cancer Centre, Department of Physical Sciences, Locked Bag 1, A’Beckett St, Victoria 8006, Australia. E-mail: [email protected] The present work was supported in part by a research collaborative agreement between the Peter MacCallum Cancer Centre and Varian Medical Systems. The British Journal of Radiology, October 2009 Received 19 December 2007 Revised 21 October 2008 Accepted 21 October 2008 DOI: 10.1259/bjr/37579222 ’ 2009 The British Institute of Radiology feature a panel width in the order of 40 cm. With the panel centred on the axis of the X-ray beam, this would only allow reconstruction of a cross-sectional image with a diameter of 25 cm or less. However, if the detector panel is moved laterally to one side and the X-ray beam collimated in the same way, it is possible to acquire images in half fan mode [6, 7]. This requires a 360 ˚ rotation of the gantry, which will yield at least 180˚ of projections for every voxel in the image. A small overlap of information in the centre of the image at the medial aspect of the off-centre flat panel device can be used for calibration. In order to facilitate the two different modes of image acquisition (full fan: small field of view (FOV); half fan: large FOV), the system allows the mounting of two different bow tie filters. For radiotherapy treatment delivery, CBCT constitutes an excellent method to verify that the patient to be treated is in the same position as they were for treatment planning [4, 6, 8]. As most radiotherapy treatment planning makes use of a diagnostic CT scan, the verification CBCT reproduces the same geometry imaged with the same modality as the treatment plan. In addition to external contours, the CBCT can be used to determine the location of the target and critical normal structures, which allows optimisation of radiotherapy 855 P Roxby, T Kron, F Foroudi et al Figure 1. Cone beam CT imaging geometry for two patients of different sizes. Sup, superior; Inf, inferior. delivery for every treatment day. Consequently, the most comprehensive way of utilising this technology is to acquire an image for each fraction of the treatment. CT has been identified as a major contributor to the overall radiation burden to populations in modern society [9]. While there are clear benefits for the patient from this image guidance procedure, it also results in a significant additional dose to the patient, a fact that has led to a recent report of task group 75 of the American Association of Physicists in Medicine [10]. In the case of daily CT scanning, the dose requires careful justification as it may be large, not only in the region of the target but also in the surrounding normal structures. Therefore, it is not surprising that dose delivered in megavoltage [3, 11] and kilovoltage [12–14] CBCT has received consideration recently. There are several methods to control and reduce radiation dose in CT (e.g. compare at www.impactscan. org). They range from adding filtration to reduced field sizes, exposure factors and number of projections. The reduction of the number of projections is complicated in the present version of the Varian CBCT software (version 2.0; Varian Medical Systems, Palo Alto, CA) and was not considered. Similarly, a variation in tube potential was not studied. The system utilises 125 kVp as a default, which is the accelerating potential used as the standard factory setting. The selection of a smaller tube potential may improve the contrast but will generally increase the radiation dose to patients. Finally, the Varian system employs a single focused grid to reduce scatter at the detector system. While the removal of the grid may allow a reduction of dose by maintaining the photon flux at the detector, the scatter to primary ratio is already large in kilovoltage CBCT systems and a further increase would result in more artefacts and image noise [2, 15]. It was the aim of the present study to determine and implement simple strategies for reduction of the dose delivered in CBCT following the general principle of dose optimisation, i.e. to maintain exposures as low as reasonably practical, included in the recommendations of the International Commission on Radiological Protection [16]. This was to be performed in the specific context of the 856 commercial Varian CBCT system available at our institution. Several methods of dose reduction were to be investigated. These were: (i) the use of an additional 0.15 mm thick copper filter on the tube side of the supplied bow tie filter, (ii) coning down using available collimation so that only the volume of interest is irradiated and (iii) reduction of the beam current (mA) per projection. Methods and materials CBCT system All measurements were performed on a CBCTequipped Varian Trilogy linear accelerator with CBCT 2.0 software (Varian Medical Systems). The system uses a CsI flat panel device with an active area of 30 6 40 cm2. In our institution, a standard focus to detector distance of 150 cm is employed for all imaging. A single focused grid is mounted directly above the detector system. The system acquires 650 images over a 360˚ gantry rotation with a few degrees added at both the start and the end of the scan. The factory standard settings (Varian Medical Systems) are 125 kVp with 80 mA and 25 ms per projection, which is identical to the parameters used by Song et al [14] in their recent study. With these settings, the half value layer (HVL) of the beam is 4.6 mm Al as assessed with a Unfors XI multimeter (Unfors Instruments, Sweden). For CBCT acquisition, an aluminium (Al) bow tie filter is employed that brings the HVL at the thinnest part of the filter up to 5.5 mm Al. This is comparable with a HVL of 5.7 mm Al reported in the literature [14]. One full rotation takes 60 s and the typical frequency of projections is up to 12 projections per second, resulting in nearly 2 projections per degree. In our clinic, we use the default Shepp–Logan convolution with modified Blackman window filter for CBCT reconstruction. Other filters (Square, Hamming, Bartlett and Welsh) are available in the software and were tested but no significant improvement in image quality was found. Images were routinely reconstructed The British Journal of Radiology, October 2009 Short communication: Dose reduction in cone beam CT with a 2.5 mm slice width and spacing, with a pixel matrix of 512 6 512. The system allows the slice width to be decreased to 1 mm. This was tested but not routinely used clinically because of increased reconstruction times. All experiments and patient images were acquired using half fan CBCT with a nominal FOV between 35 cm and 50 cm in diameter. Dose reduction strategies Three methods were studied to reduce the dose: 1. A copper filter (thickness, 0.15 mm) was mounted behind the bow tie filter of the X-ray tube. 2. For accurate CBCT reconstruction, it is required to have all parts of the patient to be imaged in the FOV of all projections. Therefore, owing to beam divergence, the width of the cone beam in the superior– inferior direction depends upon the required scan length, L, and the thickness of the patient, T. This is illustrated in Figure 1. From geometric principles, the X-ray defining blades, b, were set as: b~ððL=2Þ|100Þ=ð100{ðT=2ÞÞz0:25 cm ð1Þ 2. The equation applies to a focus to axis distance of 100 cm and the addition of 0.25 cm allows for a small safety margin. 3. Exposure settings were varied between the factory setting of 25 ms/80 mA and a minimum setting of 2 ms/2 mA per projection. region of interest of 100 pixels) was evaluated in three homogeneous cylindrical water phantoms of 7–30 cm in diameter and height extending beyond the FOV of the CBCT image (.20 cm). Patient studies Patients CBCT images were acquired as part of an in-house clinical trial open for bladder and post-prostatectomy patients. A total of six patients were enrolled in this feasibility study. Patients who gave informed consent had CBCTs taken daily throughout the first week of treatment. Images were saved and used for off-line evaluation only. The first three images were acquired with Varian standard settings (80 mA, 25 ms per projection) and the bow tie filter only (no copper filter). Image quality was assessed by asking the clinician whether the CBCT dataset was adequate for outlining of the bladder, rectum and prostate (where applicable in bladder patients). If the image quality of the first three images was deemed to be acceptable, the copper filter was introduced. An additional two CBCTs were acquired and the image quality assessed. If the introduction of the copper filter did not affect the ability of the clinician to outline structures, weekly images were taken with CBCT using the copper filter. Results and discussion Phantom studies Phantom studies Dose assessment Doses were measured at the centre and periphery (1.2 cm from the edge) of a 32 cm diameter cylindrical Perspex phantom with a thickness of 9.5 cm. It should be noted that the limited thickness of computed tomography dose index (CTDI) phantoms can significantly underestimate the contribution of scatter [17]. As such, the dose measurements should be seen as indicative only; however, the dose comparisons between irradiation with and without the filter, which are the main topic of the paper, are not affected. A 0.6 cm3 Farmer type ionisation chamber was used for the dose measurements in conjunction with a NE 2670 electrometer (NE Technology, UK). Calibration factors were derived from values for superficial/orthovoltage radiotherapy treatment beams traceable to the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA). Introduction of a copper filter For the default technique settings, the dose in the CTDI phantom was 23.7 mGy at the isocentre and 53.3 mGy at the periphery. Using a weighting of 1/3 to 2/3, this results in a weighted CTDI of approximately 45 mGy. This can be compared with a value of 54 mGy reported for the same procedure in the literature [14]. Using the copper filter of 0.15 mm thickness increased the HVL of the beam to approximately 8 mm Al. According to Amer et al [13], the Elekta XVI system uses a copper filter of 0.127 mm thickness. Therefore, it is not surprising that the beam quality reported by Song et al [14] for the Elekta XVI system is 7 mm Al at 120 kVp, which compares well with the HVL of 8 mm Al for 125 kVp found in our study for a slightly thicker copper filter. The introduction of the copper filter not only hardens the beam to correct a potential under filtering [15], but also reduced the dose to the phantom from approximately 45 mGy to 30 mGy at standard settings. As expected for a harder radiation beam, the ratio between dose in the periphery and the one in the centre was reduced. The effect of the copper filter on image quality can be seen in Figure 2, which shows patient images with and without the additional filtration. Image quality Image quality was assessed using a CatPhan (The Phantom Laboratory, Salem, NY) CT QA phantom with spatial and contrast resolution objects. In order to mimic the effect of large patients, additional scatter material (4 cm of solid water) was added in a triangular structure around the phantom. This brings the average thickness of the phantom to approximately 30 cm. The effect of different materials was studied using a disk phantom with different materials. In addition to this, noise (expressed as the standard deviation of CT numbers in a The British Journal of Radiology, October 2009 Reduction of beam current The effect of reduction of beam current (mA) and exposure time (ms) per projection can be seen in Figure 3 for different phantom materials and sizes. The figures 857 P Roxby, T Kron, F Foroudi et al (a) (b) (c) Figure 2. CT images for the same bladder cancer patient (as Figure 1) taken (a) at planning (Philips Brilliance, 16 slice) and cone beam CT (CBCT) images at treatment time (c) with and (b) without a copper (Cu) filter. For illustration purposes, all images are displayed with the same window (2000) and level (0). show the standard deviation of the CT numbers in small areas of the image (100 voxels in a volume of approximately 10 6 10 6 2.5 mm3) for different materials and homogeneous water phantoms of different diameters. As can be seen in Figure 3b, for a dose reduction of 50% from the standard settings of 80 mA and 25 ms per projection, the noise increases significantly only for the largest phantom size. The CatPhan phantom experiments showed a small increase in noise (six rather than seven of the 1% contrast test objects were visible) with 50% of the default beam current. Spatial resolution was assessed using the line pair tool embedded in the Catphan phantom. The results for two different observers are shown Figure 4. In order to assess the impact of increased scatter, a separate set of experiments was performed with the CatPhan phantom set in a triangular structure of solid water slabs of 4 cm thickness. This is termed ‘‘large phantom’’ in Figure 4 and it can be seen that the spatial resolution is slightly reduced in this case. Patient studies Introduction of a copper filter Multiple CBCT images were acquired for six patients with pelvic malignancies to compare CBCTs with (n53) and without (n53) a copper filter. Although the reconstructed image is somewhat noisier with the filter, it features similar contrast in the centre of the patient and was scored at least of equal image quality by the attending clinician. The improved image uniformity was seen as an advantage. This is demonstrated in Figure 2. The subjective ability to outline relevant structures, such as the bladder and rectum, was not affected by the introduction of the filter. It has recently been shown by Moore et al [18] that filters in CBCT can be further optimised using zonal filters, which result in dose reduction and an improved image quality. Adjustment of X-ray field size It is good practice to reduce the irradiated length of the patient to the area required for reconstruction of images. Owing to beam divergence, this is affected also by the 858 Figure 3. (a) Cone beam CT (CBCT) number as a function of exposure settings for the CBCT acquisition for water and Perspex. As can be seen from the increasing standard deviation for the p ix el values , a diff erence of 100 Hounsfield units (HU) can clearly be distinguished down to approximately 200 mA ms per projection. (b) Standard deviation (SD) of CT numbers in homogeneous water phantoms of different diameters as a function of beam current and exposure time per projection. The phantoms extend over the whole width of the cone beam. In addition to this, values for lung and bone are shown in a 20 cm diameter slab phantom. The British Journal of Radiology, October 2009 Short communication: Dose reduction in cone beam CT easy to implement and appear not to affect the ability to identify soft tissue for the purpose of treatment verification obtained from the images. References Figure 4. Spatial resolution assessed by two operators using a line pair tool in a CatPhan phantom. The results are shown for the phantom with and without additional scatter material (4 cm each side of the phantom). patient diameter, as can be seen in Figure 1. A table of shutter settings was developed that ensures that the minimum possible section of the patient is irradiated. The table is specific to a focus to detector (FDD) distance of 150 cm and includes a geometric ‘‘safety margin’’ of 0.25 cm in both the superior and inferior direction, as can be seen in Equation 1. The shutter settings are manually adjusted for each individual patient undergoing CBCT acquisition in our institution. The dose reduction is particularly noticeable for thin patients. Similar tables can easily be developed from geometric principles for different panel dimensions and FDDs. Reduction of beam current Following the phantom study, there appears to be scope to reduce the beam current for smaller patients. Image quality was just acceptable with the larger patients and default beam current settings when improved owing to increased beam hardening using the copper filter. As such, we intend to reduce the beam current only for patients with an average diameter of less than 30 cm as part of a future study. If image quality is found to be insufficient, the dose will be increased after two suboptimal images. It is beyond the scope of the present study to prove objectively that a reduction of exposure settings (mAs) does not alter the ability of operators to act upon the images in adaptive radiotherapy. Elucidating the required receiver operating characteristics is planned as future work. Conclusions It is generally accepted that CT scanning is a ‘‘high dose’’ diagnostic procedure. Diagnostic CT scanners have developed over the past few years to include a large number of dose reduction schemes (e.g. compare at www.impactscan.org). 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