Set-up error validation with EPID images: Measurements vs Egs_cbct simulation.

Objective To investigate how much the patient setup accuracy for irradiation of head and neck cancer can be improved by online setup verification and offline setup verification using cone-beam computed tomography(CBCT), and the feasibility of image-guided adaptive correction procedure to reduce the PIN margin.Methods 16 patients of head-and-neck cancer treated with three-dimensional conformal radiotherapy (3D-CRT)or intensity modulated radiotherapy(IMRT)were investigated. The first online kV CBCT scan, rigid image registration, setup correction were performed before radiotherapy. The second kV CBCT scan were acquired immediately after treatment and analysis was performed as above. CBCT scans were acquired at two or three fractions weekly during the entire course of radiotherapy and CBCT online-guided correction data were recorded. The data was used to calculate the population-based CTV-PTV margins under the condition of non-correction, correction every fraction and compensation of the systematic setup error respectively. The number of initial images required to predict systematic setup error was evaluated with the permission of 0.5 mm residue error. Results Total of 320 sets of CBCT images were analyzed for 16 patients. Under the condition of non-correction, the margins required to account for total error are 5.7 mm,5.6 mm,and 7.3 mm in the left-right(X axis),cranio-eaudal(Y axis), and anterior-posterior (Z axis)directions respectively, when the tumor was corrected every fraction, the margins required to account for intrafraetion error are 1.7 mm,1.7 mm,and 2.3 mm in X, Y,and Z axis.To correct the systematic setup error,8 sets of CBCT images are adequate. After compensation for the effect of the systematic setup error, 2.7 mm,2.5 mm, and 3.6 mm PTV margins are necessary in X, Y, and Z axis respectively. Conclusions There exists some extent of setup error in head and neck 3D-CRT or IMRT.The on-line CBCT correction and the approach based on off-line adaptive correction both can be used to reduce the impact of setup error obviously, the required margins for the PTV was reduced accordingly. Key words: Head and neck cancer; Planning target volume; Adaptive radiotherapy; Image-guided radiotherapy ; Cone-beam CT

This study was carried out to assess whether the spatial resolution has an impact on the detection accuracy of proximal caries in flat panel CBCT (cone beam computerized tomography) images and if the detection accuracy can be improved by flat panel CBCT images scanned with high spatial resolution when compared to digital intraoral images. The CBCT test images of 45 non-restored human permanent teeth were respectively scanned with the ProMax 3D and the DCT Pro scanners at different resolutions. Digital images were obtained with a phosphor plate imaging system Digora Optime. Eight observers evaluated all the test images for carious lesion within the 90 proximal surfaces. With the histological examination serving as the reference standard, observer performances were evaluated by receiver operating characteristic (ROC) curves. The areas under the ROC curves were analyzed with two-way analysis of variance. No significant differences were found among the CBCT images and between CBCT and digital images when only proximal enamel caries was detected (p = 0.989). With respect to the detection of proximal dentinal caries, significant difference was found between CBCT and digital images (p < 0.001) but not among CBCT images. The spatial resolution did not have an impact on the detection accuracy of proximal caries in flat panel CBCT images. The flat panel CBCT images scanned with high spatial resolution did not improve the detection accuracy of proximal enamel caries compared to digital intraoral images. CBCT images scanned with high spatial resolutions could not be used for proximal caries detection.

The registration of two-dimensional MV electronic portal imaging device (EPID) images and digital reconstruction radiograph (DRR) images has been widely used for the setup error correction of radiotherapy, the approaches estimate the 3D transformation is not very accurate. The purpose of this paper is to verify the feasibility of a new setup error estimation method that registers 3D planning CT image and 3D image that is reconstructed based on EPID. EPID images were acquired and used to reconstruct the MVCT by Algebraic Reconstruction Technique (ART) algorithm. The reconstructed image and the planning CT were registered by maximizing the mutual information (MI) between two 3D images. The registration error is less than 3mm, which is suitable for clinical implementation. The study demonstrated that the 3D/3D registration method proposed for the setup error correction of radiotherapy is feasible.

Cone‐beam CT (CBCT) has been widely used in image guided radiation therapy (IGRT) to acquire updated volumetric anatomical information before treatment fractions for accurate patient alignment purpose. However the excessive x‐ray imaging doses (a few cGy per scan) delivered to patients from serial CBCT scans raise a clinical concern in most IGRT procedures. This fact has greatly limited exploitation of IGRT's maximal potential. The imaging dose can be effectively reduced by reducing the number of x‐ray projections and/or lowering mAs levels in a CBCT scan. The image quality reconstructed from conventional FDK‐type algorithms however will be highly degraded. Recently total variation (TV) method has demonstrated its ability to reconstruct CBCT images from a few number of noisy x‐ray projections. Nonetheless such a method can hardly be applied in real clinical environments due to its long computational time (a few hours). It is highly desirable to develop a fast reconstruction scheme to obtain high quality CBCT images from undersampled and noisy projection data so as to lower the imaging dose.Utilizing GPU to speed up the computationally intensive tasks in CBCT reconstruction problems has drawn a lot of attention recently. In this talk GPU‐based CBCT reconstruction algorithms will be reviewed with an emphasis on an iterative CBCT reconstruction algorithm via TV regularization. We have recently developed a GPU‐friendly version of the forward‐backward splitting algorithm to solve the TV‐based reconstruction problem. Multi‐grid technique is also employed. It is found that 40 x‐ray projections are sufficient to reconstruct CBCT images with satisfactory quality for IGRT patient alignment purpose. Phantom studies indicate that CBCT images can be successfully reconstructed with our algorithm under as low as 0.1 mAs/projection level. Comparing with currently widely used full‐fan head‐and‐neck scanning protocol of about 360 projections with 0.4 mAs/projection it is estimated that an overall 36 times dose reduction has been achieved. Moreover the reconstruction time is about 130 sec on an NVIDIA Tesla C1060 GPU card which is estimated ∼100 times faster than similar iterative reconstruction approaches. The high computational efficiency and satisfactory image quality make the iterative low dose CBCT reconstruction approach feasible in real clinical environments.Learning Objectives:1. Understand basic concepts of GPU‐based CBCT reconstruction.2. Understand main challenges in GPU‐based iterative CBCT reconstruction approach and how an iterative CBCT reconstruction problem is solved on GPU.Conflict of Interest: The research presented here is partially supported by NVIDIA.

End of Treatment Cone-Beam CT Correlates with Post-Treatment Follow-up Imaging Response in Head and Neck Squamous Cell Carcinoma

Objective To study the more safe and accurate guidance scheme of cone beam computed tomography (CBCT) in nasopharyngeal carcinoma. Methods CBCT was regularly performed on 87 patients with nasopharyngeal carcinoma in Meizhou People's Hospital from November 2014 to August 2015. For each patient, 10 times CBCT scans were obtained pre-treatment. All the setup errors were obtained and analysed on the orientation X (left and right), Y (head and foot) and Z (vertical) axis. Results With the increase of the number of CBCT scans, there was no statistically significant difference among the mean setup errors of the 10 times scans in X (left and right) and Y (head and foot) directions (all P > 0.05). And there was no statistically significant difference among the mean setup errors of 6 times scans in Z (vertical) direction, but from the beginning of the 7th, the systematic error in Z direction was increasing, the system error of the 7th, 8th, 9th, 10th time was 0.140, 0.292, 0.461, and 0.640 mm. Moreover, the difference of the mean systematic errors among the first 7 times was statistically significant (H = 15.232, P = 0.019), and the differences of the mean systematic errors among the first 8, 9 and 10 times were statistically significant (all P < 0.05). Conclusions Non-consecutive scans of CBCT could reduce the systematic and random errors. The appropriate frequency of application of CBCT based on the change law of setup errors is an ideal CBCT guidance scheme for nasopharyngeal carcinoma. Key words: Nasopharyngeal neoplasms; Imaging guided radiation therapy; Cone beam computed tomography; Setup errors

Objectives: Accurate setup margin (SM) estimation is critical in radiotherapy to ensure target coverage while sparing organs at risk. Traditional methods like van Herk’s formalism assume that setup errors follow a normal or Gaussian distribution, which may not hold for small or skewed datasets and may result in under- or over-estimated SMs.This study evaluates an unconventional method based on the non-Gaussian percentile approach against the van Herk formalism for SM estimation across multiple tumor sites using electronic portal imaging device (EPID) setup data. Material and Methods: Eighty patients (20 per site: brain, head and neck, thorax, pelvis) treated with external beam radiotherapy were analyzed retrospectively. Setup errors were measured via EPID-based imaging. Conventional SMs were calculated using van Herk’s method (2.5Σ + 0.7σ), while the unconventional method derived SMs directly from the setup-error histogram, i.e., the 5th–95th percentiles (90% range of setup errors, RSE) of per-patient mean daily shifts.Normality was assessed using Shapiro-Wilk(S-W) tests, skewness/kurtosis analysis, and Q-Q plots. Methods were compared using the Wilcoxon signed-rank test (p&lt;0.05) and effect sizes (Cohen’s d)reported without claiming clinical significance. Results: Unconventional SMs were consistently larger than conventional SMs (overall medians: 4.21 vs 2.71 mm; p&lt;0.05), with the largest differences in thorax (Δ up to 3.63 mm) and pelvis (Δ up to 2.17 mm). Several datasets were non-normal by S–W and/or skewness/kurtosis. As a practical benchmark, margin differences ≥2 mm were considered clinically meaningful for interpretation, given typical planning target volume (PTV) rounding and published cone-beam computed tomography (CBCT)-era margins. Conclusion: In non-Gaussian or limited-data scenarios, the percentile-based method yields more conservative SMs than van Herk. Given EPID’s inability to capture organ motion/rotation, validation with CBCT or 4D imaging is recommended before clinical implementation.

Cone-beam computed tomography (CBCT) is widely used for pre-treatment verification and patient setup in image-guided radiation therapy (IGRT). CBCT imaging is employed daily and several times per patient, resulting in potentially high cumulative imaging doses to healthy tissues that surround exposed target organs. Computed tomography dose index (CTDI) is the parameter used by CBCT equipment as indication of the radiation output to patients. This study aimed to increase the knowledge on the relation between CBCT organ doses and weighted CTDI (CTDIW) for a thorax scanning protocol. A CBCT system was modelled using the Monte Carlo (MC) radiation transport program MCNPX2.7.0. Simulation results were validated against half-value layer (HVL), axial beam profile, patient skin dose (PSD) and CTDI measurements. For organ dose calculations, a male voxel phantom ("Golem") was implemented with the CBCT scanner computational model. After a successful MC model validation with measurements, a systematic comparison was performed between organ doses (and their distribution) and CTDI dosimetry concepts [CTDIW and cumulative dose quantities f100(150) and [Formula: see text]]. The results obtained show that CBCT organ doses vary between 1.2 ± 0.1mGy and 3.3 ± 0.2mGy for organs located within the primary beam. It was also verified that CTDIW allows prediction of absorbed doses to tissues at distances of about 5cm from the isocentre of the CBCT system, whereas f100(150) allows prediction of organ doses at distances of about 10cm from the isocentre, independently from its location. This study demonstrates that these dosimetric concepts are suitable methods that easily allow a good approximation of the additional CBCT imaging doses during a typical lung cancer IGRT treatment.

This study aims to help broaden the use of electronic portal imaging devices (EPIDs) for pre-treatment patient positioning verification, from photon-beam radiotherapy to photon- and electron-beam radiotherapy, by proposing and testing a method for acquiring clinically-useful EPID images of patient anatomy using electron beams, with a view to enabling and encouraging further research in this area. EPID images used in this study were acquired using all available beams from a linac configured to deliver electron beams with nominal energies of 6, 9, 12, 16 and 20 MeV, as well as photon beams with nominal energies of 6 and 10 MV. A widely-available heterogeneous, approximately-humanoid, thorax phantom was used, to provide an indication of the contrast and noise produced when imaging different types of tissue with comparatively realistic thicknesses. The acquired images were automatically calibrated, corrected for the effects of variations in the sensitivity of individual photodiodes, using a flood field image. For electron beam imaging, flood field EPID calibration images were acquired with and without the placement of blocks of water-equivalent plastic (with thicknesses approximately equal to the practical range of electrons in the plastic) placed upstream of the EPID, to filter out the primary electron beam, leaving only the bremsstrahlung photon signal. While the electron beam images acquired using a standard (unfiltered) flood field calibration were observed to be noisy and difficult to interpret, the electron beam images acquired using the filtered flood field calibration showed tissues and bony anatomy with levels of contrast and noise that were similar to the contrast and noise levels seen in the clinically acceptable photon beam EPID images. The best electron beam imaging results (highest contrast, signal-to-noise and contrast-to-noise ratios) were achieved when the images were acquired using the higher energy electron beams (16 and 20 MeV) when the EPID was calibrated using an intermediate (12 MeV) electron beam energy. These results demonstrate the feasibility of acquiring clinically-useful EPID images of patient anatomy using electron beams and suggest important avenues for future investigation, thus enabling and encouraging further research in this area. There is manifest potential for the EPID imaging method proposed in this work to lead to the clinical use of electron beam imaging for geometric verification of electron treatments in the future.

Dosimetric Variations Assessed with CBCT for Head and Neck Cancer Radiation Therapy

Purpose: Interests in electronic portal imaging devices (EPIDs) for radiation dosimetry increase over the past decade, particularly with recent technique advances in amorphous‐silicon EPID. For a clinical quality assurance (QA) program, EPIDs may provide a useful tool with benefits of reduced cost and added convenience, e.g., shortening setup time. This study is to investigate the clinical application of EPID in routine machine and IMRT QA and develop an EPID‐based QA system. Materials and Methods: EPID images were acquired using Varian aS1000 EPID (integrated mode; 6 MV beam; 600 MU/min dose rate). Signal intensity of EPID images were converted to dose maps using a calibrated linear signal‐dose relationship. An integrated EPID‐based QA system was designed, which include functions for machine QA and IMRT QA. For routine machine QA, recommendations by TG‐40 and TG‐45 reports were used. Specific EPID‐based QA procedures and image analysis algorithms were designed and developed for basic QA tasks, e.g., beam flatness/symmetry test and Winston‐Lutz isocenter test. For IMRT QA, a phantom‐less method was proposed, which used fluence maps reconstructed from EPID‐measured fluence as input to treatment plan system for dose calculation and comparison. Results: EPID‐measured machine QA results were comparable to those measured monthly using films in our institute. For example, EPID‐measured parameters were: flatness = 0.9–1.45%; symmetry = 0.48–1.31%; penumbra = 3.2–4 mm at SSD = 150 cm. Star‐shot images were well reconstructed for isocenter check from EPID images acquired at different gantry or collimator angles. The EPID‐based IMRT QA method was able to verify the treatment plan and capture the reasonable plan‐versus‐QA dose difference. Conclusion: The results show that EPID may be properly used as a tool for clinical QA tasks and the developed EPID‐based QA system may be used for machine and IMRT QA. (Research sponsored by Oncology Data Systems, Inc, Oklahoma City, OK)

Objective To determine the planning target volume margins of head and neck cancers treated by image guided radiotherapy (IGRT).Methods 464 sets cone beam computed tomography (CBCT) images before setup correction and 126 sets CBCT images after correction were obtained from 51 head and neck cancer patients treated by IGRT in our department.The systematic and random errors were evaluated by either online or offline correction through registering the CBCT images to the planning CT.The data was divided into 3 groups according to the online correction times.Results The isocenter shift were 0.37 mm±2.37 mm, -0.43 mm±2.30 mm and 0.47 mm±2.65 mm in right-left (RL), anterior-posterior (AP) and superior-inferior (SI) directions respectively before correction, and it reduced to 0.08 mm±0.68 mm, -0.03 mm±0.74 mm and 0.03 mm±0.80 mm when evaluated by 126 sets corrected CBCT images.The planning target volume (PTV) margin from clinical target volume (CTV) before correction were:6.41 mm,6.15 mm and 7.10 mm based on two parameter model, and it reduced to 1.78 mm,1.80 mm and 1.97 mm after correction.The PTV margins were 3.8 mm,3.8 mm,4.0 mm;4.0 mm,4.0 mm,5.0 mm and 5.4 mm,5.2 mm,6.1 mm in RL, AP and SI respectively when online-correction times were more than 15 times, 11-15 times,5-10 times.Conclusions CBCT-based on online correction reduce the PTV margin for head and neck cancers treated by IGRT and ensure more precise dose delivery and less normal tissue complications. Key words: Intensity modulated radiotherapy; Image guided radiotherapy; Setup error; Treatment margin

Objective To investigate the application of CT image and cone beam computed tomography (CBCT) image registration based on 3D Slicer software in image-guided radiotherapy for uterine cervical neoplasms. Methods Based on 3D Slicer software and Slicer RT toolkit, 10 positioning CT images and 50 CBCT images of 10 patients with uterine cervical neoplasms in Henan Provincial People's Hospital between January 2018 and October 2018 had rigid registration and b-spline deformation registration respectively. The dice similarity coefficient (DSC) and Hausdorff distance (HD) of the bladder, rectum, femoral head, spinal cord, and body of CT-CBCT images were compared by using paired t-test before and after the registration. Results Pre-registration, rigid registration and after b-spline deformation registration of CT images and CBCT images, the DSC in the bladder (0.459±0.177, 0.528±0.184, 0.542±0.187, respectively), the rectum (0.564±0.141, 0.632±0.091, 0.684±0.097, respectively), the femoral head (0.695±0.088, 0.833±0.030, 0.865±0.027, respectively), the spinal cord (0.587±0.119, 0.746±0.085, 0.834±0.032, respectively) and the body surface (0.922±0.013, 0.948±0.011, 0.959±0.009, respectively) showed an increased trend; HD in the bladder (12.8±7.2, 12.2±7.1, 11.7±7.3, respectively), the rectum (5.0±1.8, 4.4±1.2, 3.4±1.2, respectively), the femoral head (3.6±1.2, 1.8±0.5, 1.5±0.5, respectively), the spinal cord (4.0±1.0, 2.7±1.3, 1.8±0.5, respectively) and the body surface (6.3±2.1, 5.2±2.0, 4.3±2.0, respectively) showed a decreased trend. The differences of pairwise comparison in the same parts were statistically significant (all P < 0.05). Conclusions Both rigid registration and b-spline deformation registration of CT-CBCT images based on 3D Slicer software can improve the radiotherapy accuracy of uterine cervical neoplasms, and b-spline deformation registration has more significant advantages. Key words: Uterine cervical neoplasms; Radiotherapy; 3D Slicer software; Cone-beam computed tomography; Image registration; Image-guided radiation therapy

Objective To evaluate patient dose from linac-integrated kilovolt cone beam CT (CBCT) imaging in image-guided radiotherapy. Methods Air CT dose index (CTDI air ) and weighted CT dose index (CTDI w ) were measured with PTW TM30009 CT ion chamber in the air, in T40017 head phan-tom and T40016 body phantom, respectively. During the measurement, different combinations of tube volt-age, current, exposure time per frame, collimator, and gantry rotation range of ⅩⅥ CBCT were used. Dose length products (DLP) were subsequently calculated. Normalized CTDI was then compared to ACQSim sim-ulator CT. Results Both CTDI air and CTDI w depended quadratically on the voltage, while linearly on mAs. CTDI w and DLP had close relation with the collimator and the gantry rotation range. Normalized CTDI w for ⅩⅥ CBCT under routine clinical condition was lower than that of ACQSim simulator CT . Conclusions CTDI w , being convenient and reliable for measurement, can be used to assess the patient dose during CBCT imaging, Regular measurement should be performed in QA & QC program. Optimal image parameters should be chosen to reduce the scanning range and patient dose. Key words: Radiotherapy; image-guided; Tomograph; X-ray computed; cone beam; Tomograph; X-ray computed; dose index; Dose length product

246 Background: Cone Beam Computed Tomography(CBCT) is the cornerstone of image guided radiotherapy(IGRT) which is an integral part of pelvic cancers like prostate cancer. Each pelvic radiotherapy (RT) session is preceded by a planned CBCT to ensure target localisation and organ at risk avoidance. When these criteria are not met, an unplanned CBCT is performed until the CBCT is satisfactory. Possible reasons for an unsatisfactory CBCT include an under filled bladder, distended rectum or prostate gland movement. Repeated unplanned CBCT results in unnecessary excess radiation for patients. We aimed to reduce the incidence of unplanned CBCT from a baseline of 21% to 9% over 5 months. Methods: We conducted the project using both conventional quality improvement methodology and the design thinking methodology. In the diagnostic phase up to March 2017, the baseline incidence of CBCT in patients receiving pelvic RT was 21%. We hypothesized that there were reversible factors leading to a higher incidence of unplanned CBCT and sought to identify and rectify these reversible factors to reduce the incidence of unplanned CBCT. Using human centred design, we designed a new process of performing a bladder ultrasound prior to CBCT to ensure a full bladder prior to RT using the steps empathy, define, ideation, prototype, testing, sharing. Results: A total of 97 patients that underwent pelvic radiotherapy were included in this study, 40 patients were pre intervention and 61 patients received the bladder ultrasound intervention implemented from April 2017 onwards. After intervention, incidence of unplanned decreased from 21% to 5.8%. A 2 sample t test was used to compare the unplanned CBCT pre and post intervention. We found the mean pre and post intervention difference in pooled mean incidence of unplanned CBCT to be significantly different by 13.3%. The reduction in unplanned CBCT translated to saving each patient on average equivalent to 3000 chest x rays worth of unnecessary radiation. Conclusions: Design Thinking is a feasible strategy in quality improvement. We report the first design thinking project in radiation oncology quality improvement. An automated ultrasound bladder is a feasible strategy to screen for bladder filling prior to each CBCT to reduce reliance on CBCT and also patient exposure to radiation in IGRT of pelvic cancers.