M Aubin

Evaluation of ultrasound-based prostate localization for image-guided radiotherapy.

To evaluate the use of the ultrasound-based BAT system for daily prostate alignment. Prostate alignments using the BAT system were compared with alignments using radiographic images of implanted radiopaque markers. The latter alignments were used as a reference. The difference between the BAT and marker alignments represents the displacements that would remain if the alignments were done using ultrasonography. The inter-user variability of the contour alignment process was assessed. On the basis of the marker alignments, the initial displacement of the prostate in the AP, superoinferior, and lateral direction was -0.9 +/- 3.9, 0.1 +/- 3.9, and 0.2 +/- 3.4 mm respectively. The directed differences between the BAT and marker alignments in the respective directions were 0.2 +/- 3.7, 2.7 +/- 3.9, and 1.6 +/- 3.1 mm. The occurrence of displacements >/=5 mm was reduced by a factor of two in the AP direction after the BAT system was used. Among eight users, the average range of couch shifts due to contour alignment variability was 7, 7, and 5 mm in the antero-posterior (AP), superoinferior, and lateral direction, respectively. In our study, the BAT alignments were systematically different from the marker alignments in the superoinferior, and lateral directions. The remaining random variability of the prostate position after the ultrasound-based alignment was similar to the initial variability. However, the occurrence of displacements >/=5 mm was reduced in the AP direction. The inter-user variation of the contour alignment process was significant.

A diamond target for megavoltage cone-beam CTa)

PURPOSE To determine the properties of a megavoltage cone-beam CT system using the unflattened beam from a sintered diamond target at 4 and 6 MV. METHODS A sintered diamond target was used in place of a graphite target as part of an imaging beam line (an unflattened beam from a graphite target) installed on a linear accelerator. The diamond target, with a greater density than the graphite target, permitted imaging at the lower beam energy (4 MV) required with the graphite target and the higher beam energy (6 MV) conventionally used with the tungsten/stainless steel target and stainless steel flattening filter. Images of phantoms and patients were acquired using the different beam lines and compared. The beam spectra and dose distributions were determined using Monte Carlo simulation. RESULTS The diamond target allowed use of the same beam energy as for treatment, simplifying commissioning and quality assurance. Images acquired with the diamond target at 4 MV were similar to those obtained with the graphite target at 4 MV. The slight reduction in low energy photons due to the higher-Z sintering material in the diamond target had minimal effect on image quality. Images acquired at 6 MV with the diamond target showed a small decrease in contrast-to-noise ratio, resulting from a decrease in the fraction of photons in the beam in the energy range to which the detector is most sensitive. CONCLUSIONS The diamond target provides images of a similar quality to the graphite target. Diamond allows use of the higher beam energy conventionally used for treatment, provides a higher dose rate for the same beam current, and potentially simplifies installation and maintenance of the beam line.

Mégavoltage cone-beam CT : récents développements et applications cliniques pour la radiothérapie conformationnelle avec modulation d'intensité

The Megavoltage cone-beam (MV CBCT) system consists of a new a-Si flat panel adapted for MV imaging and an integrated workflow application allowing the automatic acquisition of projection images, cone-beam CT image reconstruction, CT to CBCT image registration and couch position adjustment. This provides a 3D patient anatomy volume in the actual treatment position, relative to the treatment isocenter, moments before the dose delivery, that can be tightly aligned to the planning CT, allowing verification and correction of the patient position, detection of anatomical changes and dose calculation. In this paper, we present the main advantages and performance of this MV CBCT system and summarize the different clinical applications. Examples of the image-guided treatment process from the acquisition of the MV CBCT scan to the correction of the couch position and dose delivery will be presented for spinal and lung lesions and for head and neck, and prostate cancers.

Soft tissue visualization using a highly efficient megavoltage cone beam CT imaging system

Recent developments in two-dimensional x-ray detector technology have made volumetric Cone Beam CT (CBCT) a feasible approach for integration with conventional medical linear accelerators. The requirements of a robust image guidance system for radiation therapy include the challenging combination of soft tissue sensitivity with clinically reasonable doses. The low contrast objects may not be perceptible with MV energies due to the relatively poor signal to noise ratio (SNR) performance. We have developed an imaging system that is optimized for MV and can acquire Megavoltage CBCT images containing soft tissue contrast using a 6MV x-ray beam. This system is capable of resolving relative electron density as low as 1% with clinically acceptable radiation doses. There are many factors such as image noise, x-ray scatter, improper calibration and acquisitions that have a profound effect on the imaging performance of CBCT and in this study attempts were made to optimize these factors in order to maximize the SNR. A QC-3V phantom was used to determine the contrast to noise ratio (CNR) and f50 of a single 2-D projection. The computed f50 was 0.43 lp/mm and the CNR for a radiation dose of 0.02cGy was 43. Clinical Megavoltage CBCT images acquired with this system demonstrate that anatomical structures such as the prostate in a relatively large size patient are visible using radiation doses in range of 6 to 8cGy.

SU-FF-J-81: Clinical Integration of a MV Conebeam CT System for Image-Guided Treatment

Purpose: To perform the integration of a newly developed image‐guidance system and to describe the main advantages and performance of the first Megavoltage Conebeam CT (MV CBCT) system. Method and Materials: The MV CBCT system, consisting of a new a‐Si flat panel adapted for MV imaging and an integrated workflow application allowing the automatic acquisition of projection images, conebeam CTimage reconstruction,CT to CBCTimage registration and couch position adjustment was recently introduced in clinic. Template protocols can be used for the acquisition of CBCTimages at different dose ranging from 1 to 60 M.U. Geometrical calibration, gain image adjustment and defect pixels correction procedures are performed off‐line. Results: For a typical case, 200 projection portal images and a total exposure of 5 to 8 M.U. are acquired with the 6 MV beam in 45 seconds and the 256×256×256 MV CBCTimage is reconstructed less than two minutes after the start of the acquisition. Examples of the image‐guided treatment process including the acquisition of projections images, the reconstruction of the MV CBCTimage and its registration with the planning CT, followed by the couch position correction and dose delivery will be presented. Conclusion: MV CBCT provides a 3D patient anatomy volume in the actual treatment position, relative to the treatment isocenter, moments before the dose delivery, that can be tightly aligned to the planning CT, allowing verification and correction of the patient position. Research supported by Siemens Oncology Care Systems.

TH‐D‐L100J‐05: Quality Assurance of Megavoltage Cone‐Beam CT

Purpose: Megavoltage Cone‐Beam CT (MVCBCT) is now widely used in radiation therapy. The objective of this work was to evaluate the stability of MVCBCT and define a quality assurance protocol. Methods and Materials: Two MVCBCT systems were followed for a period of 4 months. The systems were fully calibrated (geometry, CT♯ scaling factor and flat panel corrections) and analyzed daily on the first week, weekly for a month and monthly thereafter. Images of a gold seed placed at the machine isocenter were used to track the positional accuracy and stability of the system. An image quality phantom was used to monitor the stability with respect to CT♯, contrast‐to‐noise ratio (CNR), spatial resolution, noise and uniformity. A graphical user interface was developed with Matlab to automatically analyze the image quality. For each day of analysis,images were reconstructed using the calibrations obtained that day and the oldest calibrations available to investigate how frequently calibration is needed. Results: The stability of all measurements over the 4‐month period was excellent. The reconstructedgold seed position relative to isocenter was better than 1 mm for a period of 4 months. Small variations in image quality were observed between the two systems. The inserts mean CNR were 12.4±0.7 and 12.2±0.7 for the two systems. Using the initial calibrations resulted in slightly more variability in the measurements. Based on the measurements and our experience of the last 5 years, we generated a practical list of possible artifacts occurring with MVCBCT. Conclusion: Monthly calibration of the MVCBCT system is largely sufficient. Imaging a small fiducial on a daily or weekly basis may also be warranted to detect geometric misalignments caused by sudden mechanical failures. Based on the measurements, a system performance baseline for MVCBCT has been defined. Conflict of Interest: Research sponsored by Siemens OCS.

TH‐D‐ValB‐05: Evaluation of Image Quality in Megavoltage Digital Tomosynthesis

Purpose: We report on the characteristics of Megavoltage Cone Beam Digital Tomosynthesis (MVCB DTS) and its potential clinical application for imaging of pulmonary lesions. Method and Materials: MVCB CT refers to the reconstruction of a 3D image from a set of 2D projections, acquired using a medical linear accelerator equipped with an electronic portal imaging device(EPID). A typical MVCB CT scan is acquired over a 200 degrees arc. In the case of MVCB DTS, the angular range is limited to reduce the acquisition time. This limited angular range affects the image quality of the reconstructed tomograms. To study the image quality as a function of the angular range, phantom measurements were performed and data from a head and neck patient were analyzed. The image quality was analyzed in terms of effective slice thickness, shape distortion and contrast sensitivity. MVCB DTS of the lung was performed on patients, with localized and diffuse lesions. Results: The image quality and the capability to distinguish overlaid structures decreases with decreasing angular range: a 20 degrees arc DTS results in a slice thickness of 2.7cm (vs. 1mm), a ratio of the vertical to lateral diameter of a sphere of 0.15, and a reduced contrast sensitivity. The acquisition is faster than MVCB CT. It takes 5–10 seconds for arcs of 20–40 degrees, compared to 45 seconds for a 200 degrees arc. In lungimages, the faster acquisition results in a reduced blur due to the respiratory motion. Conclusion: This study indicates some potential advantages of DTS for imaginglung patients in the treatment position. Compared with EPID, DTS provides 3D information and better soft tissue contrast. Compared with CBCT, DTS allows shorter acquisition times, compatible with breath holding. Conflict of Interest: Research sponsored by Siemens OCS.

SU‐FF‐I‐13: Dose Delivered to Patients for Megavoltage Cone‐Beam CT Imaging

Purpose: Megavoltage Cone‐Beam CT (MVCBCT) has recently been introduced in the clinic to improve patient alignment prior to dose delivery. The objective of this research was to evaluate the dose delivered to patients for MVCBCT acquisition. We also studied the possibility of making simple plan modifications to compensate for the dose delivered by daily MVCBCT imaging.Method and Materials: Because MVCBCT uses the treatment beam, conventional CT scans (pelvis and head and neck patients) were imported in a treatment planning system (Phillips, Pinnacle) to simulate an MVCBCT acquisition. To validate the dose obtained from Pinnacle, a simple water‐equivalent cylindrical phantom with spaces for MOSFETs and an ion chamber was used to measure the actual dose delivered during MVCBCT. Results: The MVCBCT dose delivered to the phantom, calculated from Pinnacle, was within 3% to all the MOSFET measurement points. The difference between Pinnacle and the ion chamber was 0.2%. For a typical MVCBCT (arc: 270° to 110°) the delivered dose forms an anterior‐posterior gradient. Head and neck patients receive dose ranging from 0.7 to 1.2 cGy per MVCBCT monitor unit (MU). The range is 0.6 to 1.2 cGy per MVCBCT MU for pelvis patients. The total dose for daily positioning using MVCBCT can be reduced and made uniform by alternating between two opposed imaging arcs. Dose‐volume histograms of a compensated plan for a pelvis patient imaged with 10 MU MVCBCTs for 40 fractions show no additional dose to the target and small increases at low doses. Conclusion: Given that clinical MVCBCTs are currently performed at doses ranging from 2–15 MU, simple plan modifications, such as reducing the total number of MU, can be used to nearly eliminate the dose used for daily positioning. Results for other body sites will also be presented. Conflict of Interest: Research sponsored by Siemens OCS.

SU‐FF‐J‐60: Effectiveness of MVCBCT for Patients with Implanted High‐Z Material

Purpose: To exploit the penetrability of high‐energy photons of Megavoltage ConeBeam CT system (MVCBCT) to obtain 3D images of the anatomy in presence of “Non‐compatible CT” objects made of high‐Z material. Methods and Material: A new MVCBCT system integrated onto a clinical Linac was used to acquire 3D images of different phantoms and patients. The grey levels of different electron density inserts (lung to dense bone) in a CT phantom were measured with and without the presence of a small Cerrobend rod (10×15mm) on a regular CT and with the MVCBCT. MVCBCT of a Rando phantom with implanted gold markers and tooth fillings as well as patients with dental implants or with gold markers implanted in prostate were also obtained. Results: The presence of the Cerrobend object in the CT phantom scanned with a regular CT creates strong artifacts around the object and disturbs the quality of the entire image, modifying the Hounsfield numbers by an average of 10%, even 15 cm away from the rod. The grey levels of the density inserts in the CT phantom remain unchanged within 3% in presence of the Cerrobend rod for MVCBCT. Similarly, gold markers appear with the typical star pattern artifact on CTimages where a well‐defined dot is seen on MVCBCT. The tooth fillings in the MVCBCT Rando phantom do not disturb the soft tissue information around the teeth. Conclusion: Compared to the kV energy range, the presence of high‐Z material has relatively little impact on image quality of MVCBCT. Therefore, MVCBCT can complement missing information for planning or patient position verification purposes when high‐Z materials such as gold markers, tooth fillings, dental implants or hip prostheses are present. Clinical examples of each of these items will be presented. Conflict of Interest: Siemens supports this Research.

WE‐D‐I‐6B‐01: Soft Tissue Visualization Using a Highly Efficient Megavoltage Cone Beam CT Imaging System

Purpose: Recent developments in two‐dimensional x‐ray detector technology have made volumetric Cone Beam CT(CBCT) a feasible approach for integration with conventional medical linear accelerators. The requirements of a robust image guidance system for radiation therapy include the challenging combination of soft tissue sensitivity with clinically reasonable doses. Previously, low contrast objects have generally not been perceptible with MV energies due to relatively poor signal to noise ratio (SNR) performance. We have developed an improved imagingsystem that is optimized for MV CBCT and acquire CBCTimages containing soft tissuecontrast using a 6MV x‐ray beam. Method and Materials: Many factors, such as imagenoise, x‐ray scatter, improper calibration and acquisitions have a profound effect on the imaging performance of CBCT. In this study attempts were made to optimize these factors in order to maximize the SNR. A QC‐3V and contrast/resolution phantoms were used to determine the contrast to noise ratio (CNR) and f50 of a single 2‐D projection and the contrast and spatial resolution of the reconstructed images. Results: The computed f50 was 0.43 lp/mm and the CNR for a radiationdose of 0.02cGy was 43. Relative electron density as low as 1% can be resolved with clinically reasonable radiationdoses. Clinical Megavoltage CBCTimages acquired with this system demonstrate that anatomical structures such as the prostate and optic nerves are visible using radiationdoses in range of 4 to 8cGy. Conclusion: We have developed an imagingsystem that is optimized for MV and acquire Megavoltage CBCTimages containing soft tissuecontrast using a 6MV x‐ray beam and irradiation doses in range of 4 to 8cGy. This system can also be used for routine portal imaging applications without risk of saturation for high dose/high energy treatment/verification imaging, or dosimetry applications. Conflict of Interest : Sponsored by Siemens.