Megavoltage Cone-beam CT Research Articles

A New Scatter Correction Method for KV and MV Cone Beam CT

Kilo-voltage(KV) and mega-voltage(MV) cone beam computed tomography (CBCT) play an important role in image-guided adaptive radiotherapy. However, the cupping effect, introduced by Compton scatter contribution due to a large cone angle of CBCT acquisition, significantly degrades the CBCT image quality and hampers its clinical application. The objective of this study is to develop a new CBCT scatter correction method in the image domain. The hypotheses of the proposed method are that the image intensity of different kind of materials in CBCT image should be globally uniform and the cupping effect can be described as a bias field, a low frequency signal across the whole 3-D images. Based on above assumptions, we proposed a maximum a posteriori probability (MAP) framework to estimate the bias field contribution. The well-known fuzzy C-mean model is extended to define the likelihood function of the CBCT image. A markov random field model is used to describe the prior probability of the mixture defined in FCM model. The bias field is characterized by a Gaussian prior probability with a banded covariance matrix representing a low-pass filter. An iterated conditional mode (ICM)-like approach is utilized to minimize the objective function. The corrected CBCT image can be obtained by subtracting the bias field from original CBCT image. The performance of correcting the cupping effect of CBCT image with the proposed MAP framework was tested using both contrast phantoms and clinical CBCT images. In the cylindrical shape-contrast phantom the higher density material is surrounded by the lower density material. In the contrast phantom study, the intensities of higher density material in both KV and MV CBCT images are even lower than that of lower density material due to the cupping effect. The corrected CBCT images showed almost same intensity distribution as the original contrast phantom images. In a real patient study, it is clearly shown that there is a shadow inside the brain soft tissue in the original brain KV and MV CBCT images. The corrected CBCT images significantly improved the uniformity of intensity distribution of brain soft tissue. The proposed image-based scatter correction method showed a promising result to reduce the cupping effect commonly encountered in KV and MV CBCT image.

Contrôle qualité d’un système d’imagerie cone beam mégavoltage

Purpose This paper presents the development of a protocol for quality control of a megavoltage cone beam CT imaging system (MVCB) mounted on a Siemens Oncor 6 MV linear accelerator. Materials and methods Several parameters were controlled on the MVCB system: (1) the initial geometric calibration of the system; (2) the quality of the images (geometric distortion, uniformity, spatial resolution, low contrast resolution) for various protocols; (3) the correspondence between the intensity of voxels and electronic densities; (4) the dose delivered when achieving a MVCB. These tests were done mainly with two cylindrical phantoms specific to the quality control (QC) of a MVCB system, supplied by Siemens, and with the Catphan 600 phantom (The Phantom Laboratory) and Quasar™ Multipurpose Body phantom (Modus Medical Devices Inc). Results The results of the quality control of the images were within the tolerances. The use of the Catphan 600 phantom was inadequate for the QC of MVCB images. These tests also highlighted the need to correct the MVCB images for the “cupping artefact” for dose calculation purpose. Conclusion The initial characteristics of the MVCB imaging system were established. Such testing also provided the assessment of the influence of various parameters on the image quality as well as the associated dose delivered during their acquisition, and emphasized the corrections needed to use MVCB images for dose calculation.

Sci-Thurs PM: Delivery-12: Correction and calibration of megavoltage cone-beam CT images for the calculation of the dose of the day

To show that accurate dose calculations can be achieved with megavoltage cone-beam CT (MVCBCT) images of head-and-neck (H&N) and prostate sites, allowing the verification of the daily dose distribution received by these patients. Corrections for the cupping and missing data artifacts seen on MVCBCT images were developed for both H&N and pelvic imaging. MVCBCT images of six H&N and two prostate patients were acquired weekly during the course of their treatment. Several regions of interest were contoured including: the prostate and rectum and the spinal cord and parotids. Dose calculation was performed with the MVCBCT images using the plan beams. Variations from treatment plan dosimetric endpoints were analyzed. Dose calculations with kVCT and corrected MVCBCT images of the H&N (pelvic) regions show standard deviations of 1.9% (0.6%). The mean dose to the right parotid of H&N patients had an average increase of 18% during treatment. The maximum dose to 1% of the spinal cord went up by 2% on average. For prostate patients on one fraction the dose received by 95% of the prostate diminished by 3%. One patient had an average increase of 3.6% of the maximum dose received by 1% of the rectum. MVCBCT can be used to verify daily dose distributions for H&N and prostate patients. An increase in the mean dose to normal tissues was observed during H&N treatment. Underdosage of the prostate and the dosimetric consequences of volume changes in rectum and bladder were observed. Research supported by Siemens.

SU-GG-T-81: Issues in Image-Guided Radiotherapy (IGRT) and Re-Planning with Megavoltage Cone Beam CT (MVCT)

Purpose: In this study, the MVCT dose and its distributions are determined with ion chamber and film measurements in phantom. The effect of the imagingdose for a number of disease sites is investigated. The application of MVCT in re‐planning is also addressed. Method and Materials: A solid water phantom is CT scanned and the arc rotation of MVCT is simulated in the Pinnacle planning system. A 0.6cc ion chamber is used to measure the MVCT dose at the center of the phantom. Dose distributions are determined with Kodak XV films and compared with calculation. To investigate the effect of imagingdose, DVHs for the target and critical organs are compared with and without correcting for the MVCT dose for four diseases sites (lung, H&N, mediastinum and prostate). To assess the potential application of MVCT in re‐planning, dose distributions on the planning CT of a Rando phantom are compared with those calculated on the MVCT image set. Image fusion is also performed between the CT study and its MVCT image set. Results: MVCT dose varies linearly with MU. The imagingdose cannot be completely corrected for in practice. For example in an H&N case, the dose/fraction may be adjusted to account for the MVCT dose. 3–5% additional doses to the left eye and the left parotid are still resulted. In IGRT and re‐planning applications, distributions calculated on both the planning CT and the MVCT agree to within 1%. However, erroneous contours occur on the MVCT image set because of the difference in slice thickness between the two studies. Conclusion: While the imagingdose can be determined easily for MVCT, it cannot be corrected completely in treatment planning. For re‐planning application, the MVCT slice thickness should be the same as the planning CT.Conflict of Interest: Research supported partially by Siemens.

SU‐GG‐J‐54: Comparison of Head and Neck Localization with Portal Imaging and Megavoltage Cone‐Beam Computed Tomography

Purpose: To compare portal imaging (PI) and mega‐voltage cone‐beam CT (MV‐CBCT) for head and neck (HN) cancer patient localization. Method and Materials: The treatment couch shifts were recorded for 30 HN cancer patients localized weekly using anterior‐posterior and right‐lateral portal images, and for 11 patients localized using MV‐CBCT, either daily (n=4) or twice weekly (n=7). A total of 214 and 184 shifts were obtained for PI and MV‐CBCT, respectively, in the left‐right (LR), superior‐inferior (SI) and anterior‐posterior (AP) directions, along with the magnitude of the 3D shifts. The percentage of shifts that are equal to 0 in any direction, that is no treatment couch correction was applied in that direction, was also compared for the two techniques. Results: The average MV‐CBCT and PI shifts were −0.9±3.9 and 0.0±2.2 mm, respectively, in the LR direction, −0.2±2.4 and −0.1±3.5 mm in the SI direction, −1.0±4.2 and −0.2±2.6 mm in the AP direction. The average 3D shifts were 5.2±3.6 mm and 2.2±4.3 mm for MV‐CBCT and PI, respectively. The proportion of 0 mm shifts for MV‐CBCT and PI was 55% and 88%, respectively, in the LR direction, 71% and 81% in the SI direction, 49% and 90% in the AP direction. Conclusion: The three‐dimensional nature of the MV‐CBCT localization method offers a greater ability to detect small shifts and helps reach a compromise between the 3 translational treatment couch shifts that can be applied and the 12 degrees of freedom corresponding to translational and rotational movements of the head and/or neck, independently. This greater accuracy of MV‐CBCT allows to reduce CTV‐to‐PTV margins, therefore potentially improving the therapeutic ratio. Research partially supported by Siemens Medical Solutions.

TU-EE-A4-03: Calculation of the Dose of the Day Using Megavoltage Cone-Beam CT

Purpose: The purpose of this work is to show that megavoltage cone‐beam CT (MVCBCT) images can provide accurate dose recalculation and be used to verify the daily dose distribution received by patients treated for head‐and‐neck (H&N) and prostate cancers.Method and Materials: Corrections for the cupping and missing data artifacts seen on MVCBCT images were developed for both H&N and pelvic imaging. MVCBCT images of six H&N and two prostate patients were acquired weekly during the course of their treatment. Several regions of interest were contoured including: the prostate and rectum and for H&N cases the spinal cord and parotids. Dose calculation was performed with the corrected MVCBCT images using the planned treatment beams and variations from treatment plan dosimetric endpoints were analyzed.Results: MVCBCT image correction and calibration for the H&N (pelvic) region shows standard deviations in dose calculations between kVCT and MVCBCT images of 1.9% (0.6%). The mean dose to the right parotid of H&N patients had an average increase of 18% during treatment. Increases of up to 52% were observed. The maximum dose to 1% of the spinal cord went up by 2% on average, although increases of up to 10% were noted. For prostate patients on one fraction an undetected setup error caused the dose received by 95% of the prostate to diminish by 3%. One patient had an average increase of 3.6% of the maximum dose received by 1% of the rectum. Conclusion: MVCBCT was used succesfully to verify the daily dose distribution for H&N and prostate patients. A substantial increase in the mean dose to the parotid glands was observed during treatment. For prostate patients the impact of setup errors on the prostate dose coverage was observed, along with the dosimetric consequences of volume changes in normal tissues.Conflict of interest: Supported by Siemens.

TU‐D‐AUD B‐08: Clinical Validation of a Novel Adaptive Approach for Patients Concurrently Treated with the Prostate and Pelvic Lymph Nodes

Purposes/Objectives: One of the challenges for concurrent irradiation of the prostate and pelvic lymph nodes with IMRT is the independent movement of the prostate and the pelvic nodes. To resolve this challenge, we proposed a novel adaptive approach using multiple adaptive IMRT plans (MAP). This study is to validate dosimetric accuracy of this adaptive approach. Method and Materials: MAP Plans for a patient included five IMRT plans with one planning prostate position and the four shifted prostate positions. Prior to each treatment, daily mega‐voltage cone beam CT (MV‐CBCT) was acquired to determine the prostate position relative to the pelvic bones, and one of the five MAP plans was selected for the treatment. Seventeen daily MV‐CBCTs were available for analysis. Seventeen verification plans were retrospectively created by applying delivery beams to daily MV‐CBCTs. For each verification plan, the daily prostate contour was created by shifting the planning contour with the measured prostate movement and the daily pelvic lymph nodal volume was transferred from the planning CT to the MV‐CBCT using rigid body image registration. Typical dosimetric endpoints were used to assess the accuracy of this approach. Results: The prostate moved 0.4–0.7 cm superior in 38% days, > 0.8 cm superior in 19%, 0.4–0.7 cm posterior in 12%, and less than 0.3 cm in all directions in 31%. Of these days, 12 (65%) of the daily dose to the 95% of the prostate was greater than 98% of the prescribed dose, 4 (24%) in the range of 93% to 97%, and 2 (11%) less than 90% of the prescription dose. Among these days, the pelvic lymph node volume received 100% of the planned dose. Conclusion: The MAP adaptive approach is a clinically feasible strategy. Verification plan calculated with daily MV‐CBCT as a dosimetric monitoring tool provides patient specific dose guidance.

SU-GG-J-02: 3D Lung Tumor Motion Tracking Using MV Cone-Beam CT Acquisition with Synchronized Respiratory Monitoring

Purpose: Raw cone‐beam CT data consists of projection images acquired sequentially during gantry rotation. For lungimaging, respiratory motion is recorded in the projections, but is lost during back‐projection reconstruction. The goal of this study was to measure the 3D respiratory motion of lungtumors from the raw MV cone‐beam CT (MVCBCT) projection data. Method and Materials: A respiratory signal was captured using an external surrogate (Anzai/Siemens) during MVCBCT acquisition (MVision, Siemens) and time‐synchronized with the projection data using the beam pulse signal. Projection images were processed to enhance anatomical structures then the lungtumor was segmented automatically. The tumor center‐of‐mass (COM) in the rotating coordinate system of the flat panel detector was computed for each projection frame. The respiratory signal amplitude was used to sort projections into 10 respiratory phase bins. Projections within each phase bin were analyzed in a pair‐wise manner by trigonometric triangulation yielding multiple measures of the tumor COM. A weighted average COM was computed for each phase, with greater weight given to measures obtained from projection pairs with larger angular perspective. Results: Tests on phantoms containing point structures validated the accuracy (2 mm) of the methodology. Clinical MVCBCT of a patient with two lungstumors yielded measures of the tumor centers' motion over the respiratory cycle, which agreed well with tumor motions measured from kV‐4DCT. Conclusion: The analysis presented here gleans additional dynamic 3D information that is unique from the time‐averaged 3D structure normally reconstructed in CBCT; and it is obtained without any additional radiation dose when CBCT is used for static IGRT. This method could prove useful for near real‐time calibration/verification of the correlation between surrogate and tumor respiratory motion immediately prior to 4D treatment delivery. It may also be used for automated online motion tracking during rotational dynamic treatment delivery.

SU‐GG‐J‐165: Reconstruction of Missing Data On Kilovoltage and Megavoltage Cone‐Beam CT Images for Adaptive Radiation Therapy

Purpose: Kilovoltage (kV) or megavoltage (MV) cone‐beam CT (CBCT) technique become popular nowadays for patient setup verification and dose recalculation aiming image‐guided radiation therapy. In case of kV CBCT, field‐of‐view (FOV) is large enough to cover whole thoracic or pelvic region at detector off‐set mode while length of image set is limited and MV CBCT, vice versa. In this study, we dealt with missing data issue; compensation of missing part with kV CT images used for treatment planning. Method and Materials: First, obtained kV or MV CBCT images were registered with the planning kV CT images respectively. For kV CBCT, CT slices beyond the obtained volume were filled with planning CT data. For MV CBCT, overlapped regions in the kV CT images with the MV CBCT images after registration were substituted with the MV CBCT data. To reduce statistical discrepancies of pixel numbers between CBCT images and kV CT images and to minimize dose recalculation error, pixel number mapping and spatial filtering were employed. Varian Clinac iX and Siemens Primus linacs were used for CBCT image acquisition. Image registration and dose recalculation were implemented using Pinnacle3 ver 8.0d. Proposed algorithm were tested for chest and pelvic regions of a humanoid phantom and patients. Isodose comparison and gamma index were adopted as evaluation criteria. Results: Dose calculation result with the reconstructed CT image set agreed with calculation done with kV CT meaningfully within 2% on a phantom but error was up to 5% for clinical cases. Discrepancies were mainly due to anatomic changes and incomplete match of boundaries at the interface of two image set. Conclusion: The reconstruction algorithm showed a possibility of adaptive radiation therapy using CBCT images but image quality of CBCT data needs to be improved more.Research sponsored by MOST (Korean government).

SU‐GG‐J‐50: Comparing IGRT Shifts with MVision and CT On Rails Systems Using Anthropomorphic Phantom

Purpose: To evaluate the IGRT repositioning shifts determined by MV Cone Beam CT and by CT on rails using an anthropomorphic phantom. Method and Materials: Two IGRT systems installed in the same Linac room were studied. The CT based IGRT has been used in our Siemens Primus since 2004. Recently, a MV conebeam CT (Siemens MVision) was implemented with the objective to reduce the therapist time to obtain the IGRT shifts. The shifts and the corresponding total time were evaluated using an anthropomorphic PIXY® phantom. CTs of the phantom were obtained for head‐and‐neck, pelvis and lung and treatment planning was generated in XIO‐CMS for each site. The set of images, structures and plans was sent to both IGRT stations. Bbs were placed on the phantom surface at the isocenter axis, as a reference for the daily CT images. For each plan the MVCBCT was obtained three times to determine its reproducibility for 5 and 8 MU protocols. The registration for MVCBCT was done automatically and verified with manual registration. The table was then flipped to the CT position and the images acquired. In this case the registration is done manually and the isocenter shift is obtained through the bbs position. The procedures ware repeated for all sites. The total time to obtain the shifts and the image quality were also registered. Results and Discussion: The reproducibility of the MVCBCT for different protocols was within 2 mm. The maximum difference for MVCBCT and CT was 4 mm. The MVCBCT has poor image for soft tissue, and the fusion registration is based on mutual information. The total time to obtain the shifts is about 3 min for the new system and 7 to 8 min for the CT based system. This study excludes any deviation due to patient rotation and internal changes.

SU‐GG‐I‐24: A Quantitative Assessment of the Effects of a Simple Dead Detector Pixel Correction Method for Megavoltage Cone Beam CT Images

Purpose: Determining the extremes such that correction methods for missing or truncated projection data begin to fail could aid in the design of more efficient imaging systems in the future. We quantitatively assessed the effectiveness of a dead detector pixel correction method for megavoltage cone beam computed tomography (MVCBCT). Method and Materials: Ninety‐six MVCBCT images of an image quality phantom were reconstructed from projections that were acquired from four MVision™‐equipped Siemens Oncor™ linacs with 1024×1024 amorphous silicon electronic portal imaging devices (EPIDs). Both 8 monitor unit (MU) and 60 MU head‐and‐neck scanning protocols were used for the acquisitions. Dead pixel maps were simulated for the cases of randomly distributed singular dead pixels and 13‐dead‐pixel clusters, each with six dead pixel percentages: 2%, 25%, 75%, 80%, 90%, and 95%. Transmission images were corrected in orthogonal directions using a 1‐D line‐by‐line linear interpolation algorithm. Results: Image quality for the baseline case (< 0.5% dead pixels) and the case of 75% coverage by 13‐dead‐pixel clusters was similar. Image quality began to degrade severely after the EPID was covered to 80% by 13‐dead‐pixel clusters and 90% for singular dead pixels. Higher acquisition MU decreased the standard deviation of the difference images (corrected minus baseline), but had no effect on the means of the difference images. Conclusion: With a simple correction method, MVCBCT images are relatively insensitive to randomly distributed dead detector pixels until 80–90% of the EPID is covered with dead detector pixels. Image degradation for the same percentage of dead detectors was more pronounced when the EPID was covered with clusters of 13 dead pixels than with singular dead pixels, and was only weakly dependent on the MU used during the image acquisition.

SU‐GG‐I‐63: Calculations of Organ Doses From Cone‐Beam CT IGRT Procedures Using a Suite of Patient Phantoms

Purpose: Widespread uses of Computed Tomography (CT) and image‐guided radiation treatment (IGRT) can significantly increase the cumulative exposure to health tissues. AAPM TG‐75 was formed to compile data on imaging exposures and to identify ways to optimize the imaging procedure. To address the lack of organ doses and effective doses assessment in the IGRT imaging procedure, this paper describes a study to use a Monte Carlo based method to simulate kV and MV cone beam CT (CBCT) scanners and then calculate organ doses and effective doses. Method and Materials: This paper utilizes a whole‐body adult male phantom called the VIP‐Man, and Adult Male and Female that are adjusted to match with 50th percentile ICRP average population. The MCNPX code is used to simulate CT scanners and finally calculate doses. Several IGRT procedures are considered to demonstrate how this method can assess and compare various protocols. Results: For the VIP‐Man phantom, the effective dose for kV CBCT (assuming total exposure of 1350 mAs) is about 9.5 mSv for the two anatomical sites while the effective dose for MV CBCT (assuming a total of 6 MU) ranges from 5.10 mSv for the H&N case to 8.39 mSv for the prostate case. For different procedures, the effective dose data can be adjusted accordingly. Conclusion: With realistic patient phantoms and imaging scenario, organ dosees can be obtained by Monte Carlo simulation, and consequently effective doses have been calculated. The results show that showed that the effective dose is a useful quantity for the comparison of the performance and patient safety between kV CBCT and MV CBCT. These information can aid the assessment of radiation risk to the patient from imaging procedure. This study demonstrates the feasibility of determining detailed organ doses for various IGRT related cone‐beam CT procedures.

WE‐E‐AUD C‐02: Impact of Image Quality On CT Guided Prostate Radiotherapy

Purpose: Different CT modalities with varying image quality are being used to correct for interfractional variations in patient setup and anatomy changes, thereby reducing CTV‐to‐PTV margins, for prostate radiotherapy (RT). We explore how CT image quality affects patient repositioning and CTV‐to‐PTV margin. Method and Materials: Three CT‐based IGRT modalities routinely used in our institute for prostate RT are considered in this study: MV fan beam CT (Tomotherapy), MV cone beam CT (MVision, Siemens) and kV fan beam CT (CTVision, Siemens). Daily shifts are determined by manual registration to achieve the best soft tissue agreement. Effect of image quality on patient repositioning was determined by statistical analysis of daily shifts for 65 prostate cancer patients (34 Tomotherapy, 21 CTVision,10 MVision) treated in our clinics. The impact of soft tissue contrast on organ interface identification was evaluated by analyzing contours drawn by 7 users on the scans from each imaging modality. In addition, variability of soft tissue registration between 10 users was evaluated based on the registration of representative scan for each CT modality with its corresponding planning scan. CTV‐to‐PTV margin was defined as 1.96σ. Results: Inferior image quality with MV CT based IGRT leads to increased variations in daily shifts (3, 4, 5 mm for CTVision, Tomotherapy and MVision) and in prostate delineation (6, 3, 10 mm for CTVision, Tomotherapy and MVision). Superior image quality with the kV CT results in reduced variation between 10 users in soft tissue registration. Uniform margin introduced to account for the uncertainty in the identification of prostate edge are determined to be 2, 6 and 5 mm for CTVision, Tomotherapy and MVision. Conclusion: Image quality adversely affects the reproducibility of the manual registration for IGRT and necessitates a margin of 2 mm for kV CT and 6 mm for MV CT to ensure adequate coverage.

SU‐GG‐J‐72: Development of Megavoltage Cone Beam Image Guided Radiation Therapy (IGRT) for Mini‐Multileaf Collimator (mMLC) Based IMRT and Radiosurgery Applications

Purpose: The purpose of this work was to investigate the technical feasibility of using the widely deployed Megavoltage Cone Beam CT (MVCBCT) for 3D IGRT applications with mMLC based treatment delivery. Method and Materials: In most clinical applications of MVCBCT IGRT, the largest possible field of view (FOV) is sought for evaluating adjacent organs‐at‐risk. However, an add‐on mMLC with maximum field size of 10cm×12cm is preferred for precise small‐field hypofractionated treatments. mMLC based treatments utilizing 2.5mm leaf widths are usually performed for SRS, SRT and SBRT applications, where the higher accuracy of 3D IGRT localization is also warranted. Current clinical systems do not allow restricting the field size of CBCT acquisition in the X direction. Performing imaging with the mMLC mounted and the field size opened to ∼27cm results in major truncation artifacts rendering the images unusable, as well as the irradiation of the mMLC electronics during image acquisition, which is undesirable. The current study was conducted with MVision™ and Moduleaf™ mMLC (Siemens Medical Systems), using anthropomorphic head and neck, and chest phantoms. A custom cerrobend aperture block was fabricated to shield the mMLC from radiation damage during imaging with the X Jaw open. The clinically used reconstruction algorithm was set to only reconstruct a smaller field of view corresponding to the clinically useful restricted treatment field size, although the data acquisition field size was set to the required 27.4 cm in the X direction. Results: With the changes described above, we were able to accomplish high quality images with the mMLC. The image quality was better than images obtained for the full field acquisition in the clinical mode for the same phantoms. Conclusion: We believe this to be the first study demonstrating high quality 3D IGRT through a small mMLC collimator designed for SRT.

SU‐HH‐AUD C‐01: Empirical Investigation of Flat‐Panel Imagers Incorporating High‐Efficiency, Segmented BGO and CsI:Tl Detectors for Radiotherapy Imaging

Purpose: Megavoltage active matrix flat‐panel imagers (AMFPIs) are routinely used for portal imaging in external beam radiotherapy. However, conventional MV AMFPIs are inefficient, utilizing only ∼2% of the incident radiation, which leads to a detective quantum efficiency (DQE) of only ∼1%. Recent theoretical studies have shown that incorporation of thick, segmented scintillating detectors in MV AMFPIs can significantly increase DQE performance, leading to improved image quality at low dose and the possibility for dose‐efficient, cone‐beam computed tomography (CBCT). Method and Materials: Building upon the findings of our earlier studies, four prototype AMFPIs have recently been constructed, each consisting of a segmented BGO (11.3 mm thick) or CsI:Tl (11.4, 25.6 and 40.0 mm thick) detector coupled to an indirect detection flat‐panel array. Each detector consists of a matrix of 120 × 60 scintillator elements at 1.016 mm pitch, covered by a black or mirror top reflector. X‐ray sensitivity, MTF, NPS, DQE and phantom images were obtained for each prototype using a 6 MV photon beam at extremely low doses (e.g., 1 beam pulse, equivalent to 0.028 MU). Results: The BGO prototype shows better MTF (∼20% at the Nyquist frequency) than the CsI:Tl prototype at similar thickness. The measured DQE(0) at 1 beam pulse for the BGO and the three CsI:Tl prototypes were ∼19%, 12%, 19% and 23%, respectively. Moreover, the BGO prototype offers higher DQE values compared to the CsI:Tl prototypes over most of the spatial frequency range. Images of a contrast detail phantom using the BGO prototype at 2 beam pulses are comparable to that obtained from a conventional AMFPI with 18 times more dose. Conclusion: Prototype AMFPIs employing thick, segmented BGO and CsI:Tl detectors offer significant improvement in image quality at extremely low doses, leading to the possibility of soft tissue visualization using MV CBCT at clinically practical doses.

TH‐B‐342‐01: X‐Ray Imaging Dose to Patients in the Era of Image‐Guided Radiation Therapy

Image‐guided radiation therapy (IGRT) makes it possible to locate the treatment target before each radiation dose is delivered while the patient is in the treatment position. It not only minimizes the volume of normal tissue exposed to radiation during treatment but also ensures adequate dose to the treatment target. With the advance of technology, IGRT has emerged as the new paradigm in radiotherapy. The increased clinical availability of new emerging imaging procedures, such as cone beam CT, has resulted in a dramatic growth in its usage and therefore, IGRT procedures may also entail new risk to radiosensitive organs. Unlike diagnostic imaging in radiology, image‐guided procedures are performed much more frequently or daily. Knowledge of radiation dose to patients and accurate dosimetry of radiation dose to organs from each imaging procedure is becoming increasingly important for clinicians as information to consider in making informed decisions regarding the increased dose to radiosensitive organs. This lecture will provide an overview of the commonly used image‐guided rocedures and discuss recent advances in the accurate dosimetry for these imaging procedures, especially for emerging cone beam CT scans. The x‐ray imaging procedures in radiotherapy include megavoltage electronic portal imaging (MV EPI), kilovoltage digital radiography (kV DR), megavoltage cone‐beam CT (MV‐CBCT) and kilovoltage cone‐beam CT (kV‐CBCT).Educational Objectives:1. Understand the commonly used image‐guided procedures in radiation therapy and differences between diagnostic x‐ray imaging and image‐guidance imaging.2. Obtain a prospective view about radiation doses to organs from each image‐guided procedure, especially cone beam CT.3. Learn the large variations between dose to bone (or bone marrow) and dose to soft tissues for x‐rays at kilovoltage energy range.

WE-A-AUD A-02: Imaging for Planning Verification: In Room Imaging: Pre-Treatment Verification

It was not too long ago when CT based treatment planning was reserved for the few patients that was critical to have volumetric imaging, and treatment verification was limited to planar films of mediocre quality from a megavoltage imaging source. Today, we use multi-modality based planning (CT, MRI, PET etc) and we employ sophisticated imaging tools and techniques to verify the correct delivery of the treatment. Such imaging capabilities include electronic portal imaging devices coupled with megavoltage (MV) or kilovoltage (kV) x-ray sources, MV and kV cone beam CT, ultrasound guidance, optical systems, MV computed tomography, CT on rails and others. The goal of any in-room imaging device is to verify that the patient setup is accurate and in accordance to the treatment plan. While some modalities allow for pre-treatment verification, others can also provide on-treatment verification feedback. The choice of the modality to use depends largely on user preference but also on the investment the clinic is willing to make to purchase the necessary equipment. Nonetheless, image based treatment verification has become a necessary and valuable aspect of our clinical practice and is enabling us to become a lot more aggressive with the planning and delivery of radiation treatments. Image Guided Radiation Therapy (IGRT) is undoubtedly the future of radiation therapy. In this presentation we will discuss some of the most popular methods of imaging for treatment verification techniques. Clinical examples will also be presented to demonstrate the implementation, use, and clinical experience with these modalities. Educational Objectives: 1. Review of existing image based treatment verification modalities. 2. Discuss pros and cons of each modality and appropriateness of use.

TU‐SAMS‐AUD B‐01: KV and MV Cone Beam CT Imaging for Daily Localization: Commissioning, QA, Clinical Use, and Limitations

Three‐dimensional conformal radiotherapy (3D‐CRT), intensity‐modulated radiotherapy (IMRT), and body stereotactic radiotherapy (BSRT) allow for the generation of highly conformal dose distributions for patients with tumor volumes wrapped around or adjacent to critical structures. As a consequence of steep dose gradients between the target and organs‐at‐risk, precise localization of the target volume and surrounding normal tissue is essential. However, variations in patient setup and organ motion are limiting factors in the accurate delivery of radiation treatment with a high degree of precision. Recent online image‐guided RT techniques using kilo‐Voltage cone‐beam CT (kV‐CBCT) and Mega‐Voltage CBCT (MV‐CBCT) are now offering the opportunity to overcome these limitations and greatly improve treatment localization accuracy. These IGRT techniques using the latest imaging technologies represent an improvement over traditional techniques such as 2D portal imaging. It allows for the use of volumetric online imaging to account for organ motion and setup variation by providing multiple anatomical views of the patient during the full course of RT treatment. The selection of imaging modality and system to use in support of a treatment protocol is often a compromise between image quality, efficiency, availability, and dose.Educational Objectives:1. To review kV‐CBCT and MV‐CBCT imaging systems for daily localization including:a. Commissioning, image quality, dose, registration process, and acquisition modes.b. Clinical integration.c. QA, stability over time, and downtime.d. Standard clinical applications.e. Novel clinical applications.f. Technology evolution and future directions.

Siemens Multileaf Collimator Characterization and Quality Assurance Approaches for Intensity-Modulated Radiotherapy

Application of the multileaf collimator (MLC) has evolved from replacing blocks to create treatment fields to creating photon fluence modulation for intensity-modulated radiotherapy (IMRT). Multileaf collimator system performance requirements are far more stringent for such applications and will require increased performance for future applications, such as motion tracking. This article reviews Siemens MLC systems, including a technical description and dosimetric characteristics of 56-, 82-, and 160-leaf designs. Routine quality assurance of MLC for IMRT necessitates frequent and critical assessment of MLC leaf position calibration errors that can present in many different ways (e.g., accuracy, reproducibility, longevity, hysteresis, and collimator/gantry angle dependencies). Several techniques for measuring these errors are presented, along with qualitative and quantitative techniques for analyzing results. In particular, increased accuracy of leaf position measurement at variable gantry angles is enabled by spatial transformations to electronic portal imaging device position quantified by calibration protocols introduced with megavoltage cone beam. Measured values of X-ray transmission (intra-leaf, inter-leaf, and through abutting leaf pairs) and penumbra (leaf end, leaf tongue, leaf groove) are presented with an evaluation of their characterization by a treatment-planning system. The dosimetric impact of planning system model inadequacies is demonstrated for collimator scatter, dose profile values within 30 mm of the field edge, and the resultant effect demonstrated on clinical cases. Finally, a description of automated quality assurance delivery, analysis, and calibration protocols applicable for the specific vendor's system is provided.

Correction of megavoltage cone‐beam CT images for dose calculation in the head and neck region

Megavoltage cone-beam computed tomography (MVCBCT) imaging systems are now available for image-guided radiation therapy delivery and verification. In order to use the three-dimensional anatomical information for dose calculation, the MVCBCT image must provide accurate electron density. This work proposes a new method that has been developed to correct for the cupping and missing data artifacts seen on MVCBCT images of the head and neck region. It uses a conventional kilovoltage CT (kVCT) image as a reference for electron density and rigid registration with a MVCBCT image to obtain correction factors. Dose calculations performed on MVCBCT images corrected with the proposed method agree with calculations done on kVCT images within +/- 1% on phantoms. With patients images the agreement is within +/- 13% above the shoulders and +/- 5% below the shoulder line. This level of dose calculation accuracy allows the use of MVCBCT images for dose verification purposes.