kV-CBCT Imaging Research Articles

Comparison of Bladder Volume Measurement Accuracy between Portable Ultrasound Diagnostic Equipment and CBCT for Pelvic Prostate Radiotherapy

We evaluated the measurement accuracy of the CubeScan BioCon-900 (here after BioCon-900), a portable ultrasound imaging diagnostic device capable of measuring bladder volume, to determine if it can accurately assess bladder volume before intensity-modulated radiation therapy (IMRT) for prostate cancer. Bladder volume was measured from kV-Cone Beam computed tomography (CBCT) images obtained immediately before the administration of IMRT for prostate cancer using Halcyon. The bladder volume measured from kV-CBCT images (23 patients, total number of scans: 139) immediately before the IMRT procedure was used as the reference value. The difference between the bladder volume measured by the BioCon-900 was then calculated. The bladder volume measured from kV-CBCT images was 117.5±49.4 cc. In contrast, the bladder volume obtained with BioCon-900 was 104.1±48.6 ml, resulting in an absolute mean difference of 18.4% and a correlation coefficient of 0.881. The measurements by BioCon-900 tended to be, on average, 11% smaller than the bladder volume measured by kV-CBCT imaging. kV-CBCT images demonstrated a strong positive correlation between bladder volume and bladder urine output obtained with BioCon-900. BioCon-900 can be used before kV-CBCT imaging to accurately and conveniently assess bladder volume.

Synthetic Megavoltage Cone Beam Computed Tomography Image Generation for Improved Contouring Accuracy of Cardiac Pacemakers.

In this study, we aimed to enhance the contouring accuracy of cardiac pacemakers by improving their visualization using deep learning models to predict MV CBCT images based on kV CT or CBCT images. Ten pacemakers and four thorax phantoms were included, creating a total of 35 combinations. Each combination was imaged on a Varian Halcyon (kV/MV CBCT images) and Siemens SOMATOM CT scanner (kV CT images). Two generative adversarial network (GAN)-based models, cycleGAN and conditional GAN (cGAN), were trained to generate synthetic MV (sMV) CBCT images from kV CT/CBCT images using twenty-eight datasets (80%). The pacemakers in the sMV CBCT images and original MV CBCT images were manually delineated and reviewed by three users. The Dice similarity coefficient (DSC), 95% Hausdorff distance (HD95), and mean surface distance (MSD) were used to compare contour accuracy. Visual inspection showed the improved visualization of pacemakers on sMV CBCT images compared to original kV CT/CBCT images. Moreover, cGAN demonstrated superior performance in enhancing pacemaker visualization compared to cycleGAN. The mean DSC, HD95, and MSD for contours on sMV CBCT images generated from kV CT/CBCT images were 0.91 ± 0.02/0.92 ± 0.01, 1.38 ± 0.31 mm/1.18 ± 0.20 mm, and 0.42 ± 0.07 mm/0.36 ± 0.06 mm using the cGAN model. Deep learning-based methods, specifically cycleGAN and cGAN, can effectively enhance the visualization of pacemakers in thorax kV CT/CBCT images, therefore improving the contouring precision of these devices.

Comparison of visual and semi-automated kilovoltage cone beam CT image QA analysis.

Established kilovoltage cone-beam computed tomography (kV-CBCT) image quality assurance (QA) guidelines often rely on recommendations provided by the American Association of Physicists in Medicine (AAPM) task group (TG) reports with metrics that use visual analysis. This can result in measurement variations by different users, especially in visually subjective analyzes such as low contrast resolution. Consequently, there is a growing interest in more automated means of image QA analysis that can offer increased consistency, accuracy, and convenience. This work compares visual QA to semi-automated software QA analysis to establish the performance and viability of a semi-automated method. In this study, a commercial product (RIT Radia. Radiological Imaging Technology, Colorado Springs, CO) was used to evaluate 68 months of kV-CBCT images of a Catphan® 504 phantom obtained from a Varian TrueBeam® linear accelerator. Six key metrics were examined: high contrast resolution, low contrast resolution, Hounsfield unit constancy, uniformity and noise, and spatial linearity. The results of this method were then compared to those recorded visually using Bland-Altman, and/or paired sample t-test. Comparison of all modules showed a non-random, statistically significant difference between visual and semi-automated methods except for LDPE and Teflon in the Hounsfield unit constancy analysis, which falls outside the paired sample t-test's 5% significance level. A small high contrast resolution bias indicates the two analysis methods are largely equivalent, while a large low contrast resolution bias indicates greater semi-automated target detection. Wide limits of agreement in the uniformity module suggests variability due to multiple visual observers. Spatial linearity results measured differences of less than 0.17%. Semi-automated QA analysis offered greater stability over visual analysis. Additionally, semi-automated QA results satisfied or exceeded visual QA passing criteria and allowed for fast and consistent image quality analysis.

ACR Accreditation of a Novel Linac-Based kV-CBCT System on a High-Speed Ring-Gantry

An advanced kV-CBCT imaging system mounted on a high-speed ring-gantry Linac has recently been introduced, offering improved imaging hardware and reconstruction software to produce high quality CBCT images usable for treatment planning. Imaging performance of the system was assessed using the American College of Radiology (ACR) accreditation designed for diagnostic CT systems, additionally comparing its results to pre-existing diagnostic CT and Linac-based CBCT systems. All imaging protocols on the novel imaging system were scanned with the ACR head phantom and evaluated using ACR recommended testing and passing criteria. ACR image quality parameters include contrast-to-noise ratio (CNR), spatial resolution, HU accuracy, image scaling, and HU uniformity. CNR ≥1.0 for adult head and body, ≥0.7 for pediatric head, and ≥0.4 for pediatric body imaging protocols pass ACR criteria. Spatial resolution ≥6 line-pairs/cm for head and ≥5 line-pairs/cm for body protocols pass ACR criteria. HU accuracy passing criteria includes acrylic (110 to 135 HU), air (-1005 to -970 HU), bone (850 to 970 HU), polyethylene (-107 to -84 HU), and water (-7 to 7 HU). Image scaling measurements with errors ≤5% and HU uniformity maximum differences ≤5 HU pass ACR criteria. For machine cross-comparisons, adult Head and Pelvis imaging protocols were acquired on an existing diagnostic CT and 2 Linac-based CBCT systems (1 traditional C-arm and 1 ring-gantry) and analyzed with the same ACR methodology and passing criteria, with passing rates compared. On the novel imaging system, all standard patient size imaging protocols using 125 kVp (Head, H&N, Thorax, Thorax Slow, Breast, Abdomen, Pelvis, Pediatric Head, and Pediatric Abdomen) passed all ACR criteria; while 2 larger patient focused imaging protocols using 140 kVp (Abdomen Large and Pelvis Large) produced minor deviations on HU uniformity (maximum differences of 5 - 7 HU) but passed all other ACR criteria. The novel system matched passing rates of a diagnostic CT simulator, and outperformed pre-existing Linac based CBCT imaging systems. This newly developed advanced imaging system produces high quality images, meeting diagnostic CT ACR recommendations and far surpassing the CBCT image quality currently available on pre-existing Linacs.

A generalizable new figure of merit for dose optimization in dual energy cone beam CT scanning protocols

Objective. This study proposes and evaluates a new figure of merit (FOMn) for dose optimization of Dual-energy cone-beam CT (DE-CBCT) scanning protocols based on size-dependent modeling of radiation dose and multi-scale image quality. Approach. FOMn was defined using Z-score normalization and was proportional to the dose efficiency providing better multi-scale image quality, including comprehensive contrast-to-noise ratio (CCNR) and electron density (CED) for CatPhan604 inserts of various materials. Acrylic annuluses were combined with CatPhan604 to create four phantom sizes (diameters of the long axis are 200 mm, 270 mm, 350 mm, and 380 mm, respectively). DE-CBCT was decomposed using image-domain iterative methods based on Varian kV-CBCT images acquired using 25 protocols (100 kVp and 140 kVp combined with 5 tube currents). Main results. The accuracy of CED was approximately 1% for all protocols, but degraded monotonically with the increased phantom sizes. Combinations of lower voltage + higher current and higher voltage + lower current were optimal protocols balancing CCNR and dose. The most dose-efficient protocols for CED and CCNR were inconsistent, underlining the necessity of including multi-scale image quality in the evaluation and optimization of DE-CBCT. Pediatric and adult anthropomorphic phantom tests confirmed dose-efficiency of FOMn-recommended protocols. Significance. FOMn is a comprehensive metric that collectively evaluates radiation dose and multi-scale image quality for DE-CBCT. The models and data can also serve as lookup tables, suggesting personalized dose-efficient protocols for specific clinical imaging purposes.

Assessment of dosimetric impact of interfractional 6D setup error in tongue cancer treated with IMRT and VMAT using daily kV-CBCT.

This study aimed to evaluate the dosimetric influence of 6-dimensional (6D) interfractional setup error in tongue cancer treated with intensity-modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) using daily kilovoltage cone-beam computed tomography (kV-CBCT). This retrospective study included 20 tongue cancer patients treated with IMRT (10), VMAT (10), and daily kV-CBCT image guidance. Interfraction 6D setup errors along the lateral, longitudinal, vertical, pitch, roll, and yaw axes were evaluated for 600 CBCTs. Structures in the planning CT were deformed to the CBCT using deformable registration. For each fraction, a reference CBCT structure set with no rotation error was created. The treatment plan was recalculated on the CBCTs with the rotation error (RError), translation error (TError), and translation plus rotation error (T+RError). For targets and organs at risk (OARs), the dosimetric impacts of RError, TError, and T+RError were evaluated without and with moderate correction of setup errors. The maximum dose variation ΔD (%) for D98% in clinical target volumes (CTV): CTV-60, CTV-54, planning target volumes (PTV): PTV-60, and PTV-54 was -1.2%, -1.9%, -12.0%, and -12.3%, respectively, in the T+RError without setup error correction. The maximum ΔD (%) for D98% in CTV-60, CTV-54, PTV-60, and PTV-54 was -1.0%, -1.7%, -9.2%, and -9.5%, respectively, in the T+RError with moderate setup error correction. The dosimetric impact of interfractional 6D setup errors was statistically significant (p < 0.05) for D98% in CTV-60, CTV-54, PTV-60, and PTV-54. The uncorrected interfractional 6D setup errors could significantly impact the delivered dose to targets and OARs in tongue cancer. That emphasized the importance of daily 6D setup error correction in IMRT and VMAT.

The geometric and dosimetric accuracy of kilovoltage cone beam computed tomography images for adaptive treatment: a systematic review.

To provide an overview and meta-analysis of different techniques adopted to accomplish kVCBCT for dose calculation and automated segmentation. A systematic review and meta-analysis were performed on eligible studies demonstrating kVCBCT-based dose calculation and automated contouring of different tumor features. Meta-analysis of the performance was accomplished on the reported γ analysis and dice similarity coefficient (DSC) score of both collected results as three subgroups (head and neck, chest, and abdomen). After the literature scrutinization (n = 1008), 52 papers were recognized for the systematic review. Nine studies of dosimtric studies and eleven studies of geometric analysis were suitable for inclusion in meta-analysis. Using kVCBCT for treatment replanning depends on a method used. Deformable Image Registration (DIR) methods yielded small dosimetric error (≤2%), γ pass rate (≥90%) and DSC (≥0.8). Hounsfield Unit (HU) override and calibration curve-based methods also achieved satisfactory yielded small dosimetric error (≤2%) and γ pass rate ((≥90%), but they are prone to error due to their sensitivity to a vendor-specific variation in kVCBCT image quality. Large cohorts of patients ought to be undertaken to validate methods achieving low levels of dosimetric and geometric errors. Quality guidelines should be established when reporting on kVCBCT, which include agreed metrics for reporting on the quality of corrected kVCBCT and defines protocols of new site-specific standardized imaging used when obtaining kVCBCT images for adaptive radiotherapy. This review gives useful knowledge about methods making kVCBCT feasible for kVCBCT-based adaptive radiotherapy, simplifying patient pathway and reducing concomitant imaging dose to the patient.

Interfractional Liver Positional Motion Under Exhaled Breath Holding Based on Cone Beam Computed Tomography.

We measured interfractional liver positional motion in liver stereotactic body radiotherapy (SBRT) with exhaled breath holding (BH) based on kilovoltage (kV) cone-beam computed tomography (CBCT) images. We collected 528 pre-treatment kV-CBCT images from 132 patients who underwent liver SBRT under exhaled BH using the Abches system, a non-electronic contact-based respiratory monitoring device, and analyzed them to investigate interfractional liver positional motion. Planning computed tomography (CT) scans were obtained using the Abches system when the patients were under exhaled BH. Translational 3-degree-of-freedom (DOF) soft-tissue-based image registration was performed using the kV-CBCT images under exhaled BH after 6-DOF vertebral bone image registration. Interfractional liver positional motions in the left-right (LR), anteroposterior (AP), and craniocaudal (CC) directions were defined based on the differences in the position of the liver relative to the vertebral bones. For all fractions, the absolute mean±standard deviation for the interfractional liver positional motion in the LR, AP, and CC directions was 0.7±1.0 mm, 1.0±1.5 mm, and 2.8±3.1 mm, respectively. The liver interfractional systematic/random positional motions in the LR, AP, and CC directions were 0.9/1.2 mm, 1.4/1.8 mm, and 2.9/3.9 mm, respectively. For all fractions, 100.0%, 98.3%, and 86.9% of the interfractional liver positional motions in the LR, AP, and CC directions, respectively, were less than 5 mm. CBCT-guided online correction should be used to correct interfractional liver positions errors present in liver SBRT with exhaled BH.

An Investigation of Dosimetric Consequences of the Anatomic Changes at First Fraction in VMAT of Prostate Cancer Cases using Deformable Planning CT and KVCBCT Image Registration

An Investigation of Dosimetric Consequences of the Anatomic Changes at First Fraction in VMAT of Prostate Cancer Cases using Deformable Planning CT and KVCBCT Image Registration

Evaluation of the dosimetric influence of interfractional 6D setup error in hypofractionated prostate cancer treated with IMRT and VMAT using daily kV-CBCT

IntroductionProstate cancer is one of the most common malignant tumors in men and is usually treated with advanced intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT). Significant uncorrected interfractional 6-Dimensional setup errors could impact the delivered dose. The aim of this study was to assess the dosimetric impact of 6D interfractional setup errors in hypofractionated prostate cancer using daily kilovoltage cone-beam computed tomography (kV-CBCT). MethodsThis retrospective study comprised twenty prostate cancer patients treated with hypofractionated IMRT (8) and VMAT (12) with daily kV-CBCT image guidance. Interfraction 6D setup errors along lateral, longitudinal, vertical, pitch, roll, and yaw axes were evaluated for 400 CBCTs. For targets and organs at risk (OARs), the dosimetric impact of rotational error (RError), translational error (TError), and translational plus rotational error (T+RError) were evaluated on kV-CBCT images. ResultsThe single fraction maximum TError ranged from 12–20 mm, and the RError ranged from 2.80–3.00. The maximum mean absolute dose variation ΔD in D98% (dose to 98% volume) of CTV-55 and PTV-55 was -0.66±0.82 and -5.94±3.8 Gy, respectively, in the T+RError. The maximum ΔD (%) for D98% and D0.035cc in CTV-55 was -4.29% and 2.49%, respectively, while in PTV-55 it was -24.9% and 2.36%. The mean dose reduction for D98% in CTV-55 and D98% and D95% in PTV-55 was statistically significant (p<0.05) for TError and T+RError. The mean dose variation for Dmean and D50% in the rectum was statistically significant (p<0.05) for TError and T+RError. ConclusionThe uncorrected interfractional 6D setup error results in significant target underdosing and OAR overdosing in prostate cancer. This emphasizes the need to correct interfractional 6D setup errors daily in IMRT and VMAT.

A prospective study to assess and quantify the setup errors with cone-beam computed tomography in head-and-neck cancer image-guided radiotherapy treatment.

This study was done to quantify the translational setup errors with cone-beam computed tomography (CBCT) in the image-guided radiation therapy (IGRT) treatment of head-and-neck cancer (HNC) patients. The objective was to quantify the setup errors by CBCT. One hundred patients of HNC were enrolled from March 2020 to March 2021 for IGRT treatment. Pretreatment kV-CBCT images were obtained at the first 3 days of irradiations, and setup error corrections were done in the mediolateral (ML), superior-inferior (SI), and anterior-posterior (AP) directions. Subsequently, a weekly kV-CBCT was repeated for whole duration of radiotherapy for the next 6-7 weeks. Adequacy of planning target volume (PTV) margins was assessed by van Herk's formula. Total 630 CBCT scans of 100 patients were analyzed. Setup errors greater than 3 mm and 5 mm were seen in 11.4% and 0.31% of the patients, respectively. Systematic errors and random errors before correction in ML, SI, and AP directions were 0.10 cm, 0.11 cm, and 0.12 cm and 0.24 cm, 0.20 cm, and 0.21 cm, respectively. Systematic errors and random errors after correction in ML, SI, and AP directions were 0.06 cm, 0.07 cm, and 0.07 cm and 0.13 cm, 0.10 cm, and 0.12 cm, respectively. CBCT at the first 3 fractions and then weekly during radiotherapy is effective to detect the setup errors. An isotropic PTV margin of 5 mm over clinical target volume is safe to account for setup errors, however, in the case of close organ at risk, or with IGRT, a PTV margin of 3 mm can be considered.

Tumor regression during radiotherapy as a predictor of response in locally advanced nonsmall cell carcinoma.

To compare the predicted response with observed response to treatment by measuring gross tumor volume-primary (GTVp) using onboard kilovoltage (kV) cone-beam computed tomography (CBCT), to analyze the serial tumor volumes during radiotherapy (RT) with serial tumor volumes during follow-up, and to identify the variables associated with survival outcomes. Between June 2017 and December 2019, 23 patients of histologically proven locally advanced nonsmall cell lung cancer (LA-NSCLC) received definitive chemoradiation. Serial kV-CBCT images X-ray volume imaging (XVI) were generated weekly for image guidance and were used to generate serial GTVp. Posttreatment follow-up images were used to generate follow-up GTVp. Relative volume regression (VR) during RT and relative response assessment (RA) during follow-up were defined from Avg Vol, of planning CT. The predicted progression model was generated from VR and analyzed against observed progression events. Regression-response model was generated to analyze VR against RA. The median XVI vol1, XVI vol2, and XVI vol3 were 78.123, 56.571, and 48.513 cc during the 2nd, 4th, and 6th weeks of RT, respectively. The median VR0 was 11.777% in the 2nd-week, VR1 was 20.959% in the 4th week, and 33.661% in the 6th week. The predicted responders and progression using the VR were similar to the observed response during the follow-up. The prediction of both RA0 and RA1 obtained from VR2 was statistically significant. Predication of RA0 from VR1 tended towards significant (P=0.084). VR2 was statistically significant in predicting RA2 (P = 0.04). The median progression-free survival (PFS) was not reached and the median overall survival (OS) was 24.2 months (95% confidence interval, 20.3-28.2 months). There was no statistically significant difference in PFS and OS between Avg Vol ≤ 99.5 cc and > 99.5 cc or other clinical parameters. Tumor regression during RT is a potential predictor of response in LA-NSCLC. kV-CBCT is a strong tool in assessing tumor regression during RT.

Evaluation of optimal kilovoltage-cone beam technique on image quality, registration accuracy, time of imaging and relative dose for head radiotherapy: A phantom study

Background: Image-guided radiation therapy (IGRT) has improved geometry accuracy of patient positioning in radiotherapy. Nowadays, a kilo-voltage cone-beam computed tomography (kV-CBCT) is widely applied in IGRT to confirm daily patient positioning. However, one problem with IGRT is the increased dose to normal tissue outside the target area. Reduction in the dose of kV-CBCT verification imaging leads to deterioration in image quality and registration results. Objectives: The objective of this study was to evaluate an optimal kV cone-beam technique on image quality, registration accuracy, relative dose, and imaging time using different combinations of angular range, angular separation, and mA (in total eight protocols) in the head phantom. Materials and methods: Catphan® 503 phantom was used to evaluate image quality and a PIXY Anthropomorphic Training/Teaching Phantom was used to verify image registration accuracy. The kV-CBCT imaging was performed using an X-ray volumetric imaging (XVI) system mounted on an Elekta Versa HD linear accelerator. The absorbed dose of kV-CBCT imaging was determined using the head phantom with an ionization chamber. The eight protocols were analyzed for image quality, image registration, relative dose change, and imaging delivery time. Results: Image quality parameter results showed that maximum contrast to noise ratio (CNR) increased by approximately 65% at 100 kV, 20 mA, θ=360°, and Δθ=0.54°, while uniformity compared with 100 kV, 10 mA, θ=200°, Δθ=0.54° (default protocol) varied within 7%. The spatial resolution showed no change (0.167 cm), with geometric distortions of less than 0.2 mm. Image registration errors were within 0.2 cm, with the highest magnitude of error seen in the vertical direction. Imaging dose can be reduced using 100 kV, 10 mA, θ=200°, Δθ=1.09° about 50% and 100 kV, 10 mA, θ=360°, Δθ=1.09° about 15% with similar CNR of default protocol. Imaging time decreased by approximately 2-folds, while Δθ increased by 2-folds. Conclusion: The suggested protocols suitable for optimal kV-CBCT image quality, accuracy of image registration, decreased imaging time, and reduced image dose in the head region were 1) 10 mA, θ=200°, Δθ=1.09° 2) 10 mA, θ=360°, Δθ=1.09°, and 3) 20 mA, θ=360°, Δθ=1.09°. These protocols decreased imaging dose about 50%, 15%, and 2%, respectively. The developing kV-CBCT technique should be counterbalanced by careful consideration of imaging dose, image quality, imaging time, and accuracy of image registration.

Development of a practical clinical application of NIPAM kV-CBCT dosimetry

We report our progress towards developing a clinical application of NIPAM kV-CBCT dosimetry. The goal is to develop a practical kV-MV isocenter verification test for which the measurement and analysis can be carried out quickly (within an hour), and that eliminates the need for separate readout (other than on board kV-CBCT) or extra analysis steps such as image registration. Isocenter verification is performed using a NIPAM 3D gel dosimeter which is irradiated with a small field to ~16Gy at eight unique couch/gantry angles. Pre- and post-irradiation kV-CBCT images are acquired and dose is manifest as the intensity difference between pre- and post-CBCTs due to radiation induced changes in density. Code was developed to detect the geometry of each beam in the kV-CBCT and quantify relevant parameters. We applied this technique to verify the isocenter for MLCs as well as for SRS cones. The measured radius to encompass all beams for 4mm, 6mm, 7.5mm, 12.5mm, and 15mm cones was 0.55±0.11mm. The efficiency, robustness to setup errors, and unique ability to visualize spatial uncertainties in the kV-CBCT coordinate system make the NIPAM kV-CBCT test a practical and unique tool for kV-MV isocenter verification.

PO-1986 Early toxicity with stereotactic body radiation therapy for prostate cancer

Purpose or Objective: Stereotactic body radiotherapy (SBRT) has proven to be a new option for localized prostate cancer. SBRT specific indications has been transformed due to SARS-2-COVID19 scenario. This communication shows our data about acute toxicity in patients treated with SBRT during the last sixteen months. Materials and Methods: This retrospective and observational study was performed on prostate cancer patients admitted into our radiotherapy department for SBRT between October 2019 until January 2021. SBRT treatment at our institution has been standardized as an IGRT technique using VMAT radiation therapy due to deliver total doses of 36.25 or 40 Gy in 5 fractions. The following variables were recorded: age, past medical history, previous treatment with anticoagulant or antiplatelet drugs, Performance Status (PS), ACE-27 score, IPSS score, Gleason score, Prostate Volume, Tumor Stage, PSA levels, Risk group following NCCN, Hormonotherapy, Radiation Area, RT Doses as well as different toxicity types depending on Genitourinary (GU) or Gastrointestinal (GI) location. Toxicity review was recorded (after RT, six month and 12 months) according to Common Terminology Criteria for Adverse Events (CTCAE). For the descriptive analysis, analytical variables were represented using absolute and relative frequencies. All statistical analysis were conducted using software from SPSS (version 25.0, Chicago, IL). The hypothesis of our study is SBRT technique will reduce radiotherapy sessions with a low rate of toxicity. Results: A total of 47 patients were included, with a median age of 71 years, performance status 0-1, including high risk prostate cancer (25 patients with Gleason score 6, 17 with score 7, 4 with score 8 and 1 with score 9), all of them without seminal vesicles affected. Related to NCCN risk group, 57.4 % of the patient had very low risk, low risk and favorable intermediate risk and 42.6% were unfavorable intermediate, high and very high risk. Androgen deprivation was associated in 40.4% of the cases with a median of treatment in six months (RI:6-24). Image Guided Radiotherapy was performed by matching of fiducial markers on kvCBCT images that were acquired prior to treatment, for positioning, and during treatment, for intrafraction movement control in 74.5% of the patients. The remaining 25.5% were treated by means of a gating system guided by continuous transperineal ultrasound scanning (Clarity ®system). Acute toxicity grade 1 and 2 after RT was greater at GU (51.1%/21,3%) than GI (12.8%/4.3%) location. During subsequent follow-up no patient developed acute GU or GI toxicity greater than or equal to grade 2. At 6 and 12 months evaluation GU toxicity was greater than GI, 31.7% Vs. 17.1% and 12%Vs.4%, respectively. Conclusion: Evaluation of toxicity during the early phase SBRT has proven to be a well tolerated treatment for prostate cancer in our series.

MVCT versus kV-CBCT for targets subject to respiratory motion: A phantom study.

The use of kilovoltage cone‐beam computed tomography (kV‐CBCT) or megavoltage computed tomography (MVCT) for image guidance prior to lung stereotactic body radiation therapy (SBRT) is common clinical practice. We demonstrate that under equivalent respiratory conditions, image guidance using both kV‐CBCT and MVCT may result in the inadequate estimation of the range of target motion under free‐breathing (FB) conditions when standard low‐density window and levels are used. Two spherical targets within a respiratory motion phantom were imaged using both long‐exhale (LE) and sinusoidal respiratory traces. MVCT and kV‐CBCT images were acquired and evaluated for peak‐to‐peak amplitudes of 10 or 20 mm in the cranial‐caudal direction, and with 2, 4 or 5 s periods. All images were visually inspected for artifacts and conformity to the ITV for each amplitude, period, trace‐type, and target size. All LE respiratory traces required a lower threshold HU window for MVCT and kV‐CBCT compared to sinusoidal traces to obtain 100% volume conformity compared with the theoretical ITV (ITVT). Excess volume was less than 2% for all kV‐CBCT contours regardless of trace‐type, breathing period, or amplitude, while the maximum excess volume for MVCT was 48%. Adjusting window and level to maximize conformity with the ITVT is necessary to reduce registration uncertainty to less than 5 mm. To fully capture target motion with either MVCT or kV‐CBCT, substantial changes in HU levels up to −600 HU are required which may not be feasible clinically depending on the target's location and surrounding tissue contrast. This registration method, utilizing a substantially decreased window and level compared to standard low‐density settings, was retrospectively compared to the automated registration algorithm for five lung SBRT patients exposed to pre‐treatment kV‐CBCT image guidance. Differences in registrations in the super‐inferior (SI) direction greater than the commonly used ITV to PTV margin of 5 mm were encountered for several cases. In conclusion, pre‐treatment image guidance for lung SBRT targets using MVCT or kV‐CBCT is unlikely to capture the full extent of target motion as defined by the ITVT and additional caution is warranted to avoid registration errors for small targets and patients with LE respiratory traces.

Evaluation of patient organ doses from kilovoltage cone-beam CT imaging in radiation therapy.

Currently, CBCT system is an indispensable component of radiation therapy units. Because of that, it is important in treatment planning and diagnosis. CBCT is also an crucial tool for patient positioning and verification in image-guided radiation therapy (IGRT). Therefore, it is critical to investigate the patient organ doses arising from CBCT imaging. The purpose of this study is to evaluate patient organ doses and effective dose to patients from three different protocols of Elekta Synergy XVI system for kV CBCT imaging examinations in image guided radiation therapy. Organ dose measurements were done with thermoluminescent dosimeters in Alderson RA NDO male phantom for head & neck (H&N), chest and pelvis protocols of the Elekta Synergy XVI kV CBCT system. From the measured organ dose, effective dose to patients were calculated according to the International Commission on Radiological Protection 103 report recommendations. For H&N, chest and pelvis scans, the organ doses were in the range of 0.03-3.43 mGy, 6.04-22.94 mGy and 2.5-25.28 mGy, respectively. The calculated effective doses were 0.25 mSv, 5.56 mSv and 4.72 mSv, respectively. The obtained results were consistent with the most published studies in the literature. Although the doses to patient organs from the kV CBCT system were relatively low when compared with the prescribed treatment dose, the amount of delivered dose should be monitored and recorded carefully in order to avoid secondary cancer risk, especially in pediatric examinations.

P53.05 Evaluation of IGRT-Induced Imaging Doses and Secondary Cancer risk for SBRT Early Lung Cancer Patients in Silico Study

The imaging dose produced by Image guided radiotherapy (IGRT) will increase the secondary cancer risk.This study performed dosimetry studies and secondary cancer risk assessments on using electronic portal imaging device (EPID) and cone beam computed tomography (CBCT) as image guided tools for the early lung cancer patients treated with SBRT. This study retrospectively analyzed the treatment plans of sixty-one early lung cancer patients treated with SBRT. The imaging doses from MV-EPID and kV-CBCT of the Edge accelerator were added to treatment plans. The MV-EPID imaging dose (6MV Photon beam) was calculated in Pinnacle TPS, and the kV-CBCT imaging dose was simulated and calculated by modeling of the kV energy beam in TPS using Pinnacle automatic modeling program. Three types of plans, namely PlanEPID, PlanCBCT and Planorigin, were generated with incorporating doses of EPID, CBCT and no imaging, respectively, for analysis. The effects of imaging doses on dose-volume-histogram (DVH) criteria and plan quality were analyzed, and the excess absolute risk (EAR) of secondary cancer for ipsilateral lung was evaluated. The regions that received less than 50cGy were significantly impacted by the imaging doses, while the isodose lines greater than 1000cGy were barely changed. The DVH values of ipsilateral lung increased the most in PlanEPID, followed by PlanCBCT. Compared to Planorigin on the average, the estimated EAR of ipsilateral lung in PlanEPID increased by 3.43%, while the corresponding EAR increase in PlanCBCT was much smaller (about 0.4%). Considering only the contribution of the imaging dose, the EAR values for the ipsilateral lung due to the MV-EPID dose in 5 years,10 years and 15 years were 1.49 cases, 2.09 cases and 2.88 cases per 104PY respectively, and those due to the kV-CBCT dose were about 9 times lower, correspondingly. The imaging doses produced by MV-EPID and kV-CBCT had little effects on the target dose coverage. The secondary cancer risk caused by MV-EPID dose is more than 8.5 times that of kV-CBCT.

Evaluation of IGRT-Induced Imaging Doses and Secondary Cancer Risk for SBRT Early Lung Cancer Patients In Silico Study.

Objectives:This study performed dosimetry studies and secondary cancer risk assessments on using electronic portal imaging device (EPID) and cone beam computed tomography (CBCT) as image guided tools for the early lung cancer patients treated with SBRT.Methods:The imaging doses from MV-EPID and kV-CBCT of the Edge accelerator were retrospectively added to sixty-one SBRT treatment plans of early lung cancer patients. The MV-EPID imaging dose (6MV Photon beam) was calculated in Pinnacle TPS, and the kV-CBCT imaging dose was simulated and calculated by modeling of the kV energy beam in TPS using Pinnacle automatic modeling program. Three types of plans, namely PlanEPID, PlanCBCT and Planorigin, were generated with incorporating doses of EPID, CBCT and no imaging, respectively, for analysis. The effects of imaging doses on dose-volume-histogram (DVH) and plan quality were analyzed, and the excess absolute risk (EAR) of secondary cancer for ipsilateral lung was evaluated.Results:The regions that received less than 50 cGy were significantly impacted by the imaging doses, while the isodose lines greater than 1000 cGy were barely changed. The DVH values of ipsilateral lung increased the most in PlanEPID, followed by PlanCBCT. Compared to Planorigin on the average, the estimated EAR of ipsilateral lung in PlanEPID increased by 3.43%, while the corresponding EAR increase in PlanCBCT was much smaller (about 0.4%). Considering only the contribution of the imaging dose, the EAR values for the ipsilateral lung due to the MV-EPID dose in 5 years,10 years and 15 years were 1.49 cases, 2.09 cases and 2.88 cases per 104PY respectively, and those due to the kV-CBCT dose were about 9 times lower, correspondingly.Conclusions:The imaging doses produced by MV-EPID and kV-CBCT had little effects on the target dose coverage. The secondary cancer risk caused by MV-EPID dose is more than 8.5 times that of kV-CBCT.

Spatial and dosimetric evaluation of residual distortions of prostate and seminal vesicle bed after image-guided definitive and postoperative radiotherapy of prostate cancer with endorectal balloon.

PurposeTo quantify daily residual deviations from the planned geometry after image‐guided prostate radiotherapy with endorectal balloon and to evaluate their effect on the delivered dose distribution.MethodsDaily kV‐CBCT imaging was used for online setup‐correction in six degrees of freedom (6‐dof) for 24 patients receiving definitive (12 RTdef patients) or postoperative (12 RTpostop patients) radiotherapy with endorectal balloon (overall 739 CBCTs). Residual deviations were evaluated using several spatial and dosimetric variables, including: (a) posterior Hausdorff distance HDpost (=maximum distance between planned and daily CTV contour), (b) point Pworst with largest HDpost over all fractions, (c) equivalent uniform dose using a cell survival model (EUDSF) and the generalized EUD concept (gEUDa with parameter a = −7 and a = −20). EUD values were determined for planned (EUDSFplan), daily (EUDSFind), and delivered dose distributions (EUDSFaccum) for plans with 6 mm (=clinical plans) and 2 mm CTV‐to‐PTV margin. Time series analyses of interfractional spatial and dosimetric deviations were conducted.ResultsLarge HDpost values ≥ 12.5 mm (≥15 mm) were observed in 20/739 (5/739) fractions distributed across 7 (3) patients. Points Pworst were predominantly located at the posterior CTV boundary in the seminal vesicle region (16/24 patients, 6/7 patients with HDpost ≥ 12.5 mm). Time series analyses revealed a stationary white noise characteristic of HDpost and relative dose at Pworst. The EUDSF difference between planned and accumulated dose distributions was < 5.4% for all 6‐mm plans. Evaluating 2‐mm plans, EUDSF deteriorated by < 10% (<5%) in 75% (58.5%) of the patients. EUDSFaccum was well described by the median value of the EUDSFind distribution. PTV margin calculation at Pworst yielded 8.8 mm.ConclusionsAccumulated dose distributions in prostate radiotherapy with endorectal balloon are forgiving of considerable residual distortions after 6‐dof patient setup if they are observed in a minority of fractions and the median value of EUDSFind determined per fraction stays within 95% of prescribed dose. Common PTV margin calculations are overly conservative because after online correction of translational and rotational errors only residual deformations need to be included. These results provide guidelines regarding online navigation, margin optimization, and treatment adaptation strategies.