CBCT Protocols Research Articles

Estimation of the radiation dose for pediatric CBCT indications: a prospective study on ProMax3D.

An increasing number of CBCT units and a wide variability of radiation doses have been reported in dentistry lately. To estimate the effective, cumulative, and organ absorbed doses in children exposed to CBCT over 2 years. A prospective study was conducted in children who underwent CBCT diagnostic imaging with the ProMax3D machine. Organ and effective doses were calculated by Monte Carlo simulation using 5- and 8-year-old pediatric voxel phantoms. Extrapolation procedures were applied to estimate doses for other ages and CBCT protocols used in clinical conditions. The median effective dose was 137.9 μSv, and the median cumulative dose was 231.4 μSv. Statistically significant differences in the effective doses and cumulative doses were found for various indications of CBCT in children (P < 0.001). The median absorbed organ dose for brain and thyroid was significantly higher for the clinical condition that required large FOVs (2.5 mGy and 1.05 mGy, respectively) compared to medium (0.19 and 0.51 mGy) and small FOVs (0.07 and 0.24 mGy; P < 0.05). The radiation dose of salivary glands did not vary significantly with FOV. The results revealed the variation of CBCT doses and the influence of FOV size in pediatric exposure.

Evaluation of different low-dose multidetector CT and cone beam CT protocols in maxillary sinus imaging: part I—anin vitrostudy

This in vitro study aimed to assess radiation dose and image quality of different low-dose multidetector CT (MDCT) and CBCT imaging protocols in comparison with the standard MDCT protocol for maxillary sinus imaging. Effective dose (E) and image quality of 10 MDCT (changing effective milliampere second starting from 141.3 EmAs to 20 EmAs) and 3 CBCT protocols (changing milliampere second and voxel size) were assessed throughout scanning an anthropomorphic head and neck Alderson Rando phantom. E values were calculated using thermoluminescent dosemeters (TLDs) fixed at 6 sensitive organs (14 sites) on the Rando phantom. Image quality was assessed objectively (by calculating the standard deviation values of the radiographic density of water) and subjectively (by assessing the diagnostic image quality using a four-graded scale: 1 = very good, 2 = good, 3 = acceptable and 4 = unacceptable). Two MDCT protocols (120 kV/32 EmA and 120 kV/25 EmA) had lower radiation doses with statistically significant differences (p < 0.001) compared with that of the standard MDCT protocol (120 kV/141.3 EmA), and they preserved a good diagnostic image quality. One CBCT protocol (120 kV/20 mA) had a reasonable radiation dose and good image quality. There were no statistically significant differences between the above-mentioned lower dose MDCT and CBCT protocols (p > 0.05) with respect to the radiation dose and image quality. The low-dose MDCT and CBCT protocols are viable methods for maxillary sinus examination as evaluated using the above-mentioned phantom that yield a good diagnostic image quality using E approximately 7 and 11 times lower than that of the standard MDCT, respectively. These findings were evaluated in the in vivo part of this project.

Evaluation of the diagnostic efficacy of two cone beam computed tomography protocols in reliably detecting the location of the inferior alveolar nerve canal.

Reliable three-dimensional localization of the inferior alveolar nerve canal (IANC) is valuable for a variety of dentoalveolar procedures. Although conventional CBCT offers three-dimensional information at a reasonably low dose, it is still a significant amount of radiation. In this ex vivo study, we evaluated the ability of a 180° rotational CBCT acquisition protocol with lower number of basis projections to create a CBCT data set for reliable localization of the IANC compared with a conventional 360° rotational CBCT acquisition. 50 dry human skulls were imaged using 180° and 360° rotational CBCT protocols. Measurements of the IANC throughout its course in the mandible were carried out. Two raters evaluated the measurements and rated the scans based on their ability to visualize the IANC, and the measurements were carried out. The IANC length measurements for the 180° and 360° protocols were identical. There was no difference between evaluations by the two raters for the two protocols. Interexaminer reliability values were >90% for the two protocols. The sensitivity values for the two protocols were >95%. The specificity for both protocols was 100%. 180° CBCT acquisition protocol is able to accurately locate the IANC with high reliability and is comparable to a conventional 360° protocol.

11. Assessment of doses made during imaging procedures of the technical platform of the Lorraine Cancer Institute (ICL)

Introduction With the development of image-guided radiotherapy treatment techniques (IGRT), the problematic of the number of images and the associated cumulative dose is an important issue. The goal of this study was to quantify the dosimetric impact of three imaging systems: Clinac (Varian), Cyberknife G4 (CK) and Tomotherapy (Accuray) in realistic treatment conditions. Material and method For the Clinac, the dose was measured using thermoluminescent dosimeters (TLD) on the Alderson Rando anthropomorphic phantom (Rando) and on the CTDIw phantom used in CT. For the CK, the measurements were performed on and into the Rando with TLD and a semiconductor kV Unfors Xi sonde (Raysafe). For Tomotherapy, the Rando and the Cheese phantom were used with TLD and an ionization chamber (A1 SL, Tomoelectrometer, Standard Imaging). TLD700 (Harshaw) were read with a 2000D reader (Harshaw). Calibration was performed in a 6 MV beam (Clinac). Dose CBCT measurements were performed at the center of the phantom for three locations: skull (100 kV, 145 mAs, 365 projections), pelvis (125 kV, 680 mAs, 665 projections) and thorax (110 kV, 262 mAs, 655 projections). The dose in CK were measured at the entrance kV imaging beam and at OAR locations (chiasm, optic nerves, lung, femoral heads). Acquisition parameters are: 120 kV, 150 mAs, 100 ms. Given the expected low dose, measurements are cumulated over 100 expositions. For Tomotherapy, measurements were made on the Rando at different OAR locations, with a 3 mm slice thickness. The estimations of the dose to the OAR for 1 and 2 mm slice thicknesses were obtained using the ratio of measurements made at the Cheese phantom center for 1, 2 and 3 mm slice thicknesses. The dose estimations over the whole treatment were based on 30 acquisitions for CBCT and MVCT and 300 (150 pairs) for CyberKnife. Results The measured doses to OAR for one acquisition and the estimated dose to OAR throughout the whole treatment are presented in the table below. Discussion and conclusion The optimization of CBCT protocols on Varian systems allows daily IGRT without exceeding a delivered dose of 1 Gy for the whole treatment, threshold from which no action is taken in the ICL service. For the CK, the skin dose provided by the tracking images is less than 0.15 Gy for the whole treatment. In Tomotherapy, the dose provided by the MVCT with a3 mm slice thickness remains below 0.40 Gy. In comparison with the 3 mm acquisition, using a 2 mm and 1 mm slice thicknesses increase the dose to OAR by a factor of 1.41 and 2.96 respectively and the delivered dose in IGRT can exceed 1 Gy for a 1 mm slice thickness.

Image quality assessment of clinically-applied CBCT protocols using a QAT phantom.

This study has two main purposes: (1) to evaluate the effect of various clinically applied protocols [changing field of view (FOV), voxel size and tube voltage (kVp)] on CS 9300 CBCT (CareStream SM 749, Rochester, NY) image quality using its quality assurance test (QAT) phantom and (2) to evaluate the efficacy of a newly-designed prototype plugin on ImageJ (National Institutes of Health, Bethesda, MD) which was introduced to measure modulation transfer function (MTF) using a modified slanted-edge method. Using a specifically constructed QAT phantom, five image quality parameters were assessed-image uniformity, signal-to-noise ratio, contrast, position checking and the MTF-for ten variables, clinically applied protocols on a CS 9300 PREMIUM three-dimensional CBCT device. With FOV and voxel size fixed, high-dose protocols resulted in less image noise and more uniformity. On the other hand, less image uniformity was achieved with less kVp. Moreover, high-dose protocols resulted in a high contrast-to-noise ratio (CNR). There were significant differences in CNR between 60 and 70 kVp as well as between 70 and 90 kVp. Our study identified no difference between MTF and either voxel size or FOV. The image quality of the CS 9300 CBCT varied with variability in exposure protocols and different voxel sizes. The negative association between greater spatial resolution (smaller voxel size) and the MTF shows that the slanted edge method is not ideal for MTF evaluation of CBCT and is therefore not recommended for using it for this purpose.

Abstract No. 419 - Evaluation of the prostatic artery anatomy and its variants with cone beam CT scan

Learning Objectives: Cone beam CT scan (CBCT) pictorial review of the Prostatic artery (PA) anatomic variants.Discussion of optimal CBCT protocols in terms of location of the microcatheter, rate and volume of contrast injection as well as reconstruction algorithm during Prostatic artery embolization (PAE) Background: Thorough knowledge of PA anatomy and its variants is important in performing PAE successfully. Significant variability of the antero-lateral PA (AT) and posterolateral PA (PT) is observed in terms of origin, course and anastomosis with other important arteries. Identification and catheterization of these arteries is important during PAE to prevent nontarget embolization leading to ischemic damage to critical pelvic organs. Optimal quality of CBCT is important in identifying these arteries, their origins and atherosclerotic changes guiding the superselective catheterization of these arteries. Clinical Findings/Procedure Details: AT primarily supplies the central gland and benign prostatic hyperplasia nodules and most important artery to embolize. It frequently arises from the superior vesical artery. PT supplies peripheral portion of the gland and has frequent anastomosis with anal and rectal arteries. AT and PT may arise from the single artery (branch of internal iliac artery IIA) in more than 50% of cases. CBCT can identify these arteries and their anastomosis with other critical arteries in great detail. Size and location of the microcatheter, rate and volume of the contrast injection as well as reconstruction algorithm determines the CBCT quality. We have been successfully doing CBCT with microcatheter nonselectively (arterial phase) and superselective at AT or PT (parenchymal phase) without any periprocedure morbidity. Conclusion and/or Teaching Points: PAE can be performed safely and effectively with thorough knowledge of PA anatomy and its anatomic variants as well as optimizing CBCT protocols.

Effective dose comparison between stitched and single FOV in CBCT protocols for complete dental arcade

Abstract Objective The objective of this study was to assess and compare protocols with a single field of view and multiple stitched field of view with a similar clinical purpose by means of effective dose value. Materials and methods Measurements of absorbed dose were performed with thermoluminescent dosemeters inserted in the position of organs/tissues of a female anthropomorphic phantom and from these values the effective dose was calculated, utilizing weighting factor tissue-ICRP 103 (2007). Results The results obtained in this study for effective dose are within the range of 43.1 µSv and 111.5 µSv for equipment using protocols with single FOV and in the range of 44.5 µSv and 236.2 µSv for equipments that using protocols with stitched field of view. Conclusions In terms of the value of effective dose, stitched FOV protocols do not have any advantage over the single field of view protocols. This results suggest the necessity for knowledge of the exposure parameters and effective dose values associated with each image protocol.

Dose calculation accuracy of different image value to density tables for cone-beam CT planning in head & neck and pelvic localizations

PurposeWe aimed to identify the most accurate combination of phantom and protocol for image value to density table (IVDT) on volume-modulated arc therapy (VMAT) dose calculation based on kV-Cone-beam CT imaging, for head and neck (H&N) and pelvic localizations. MethodsThree phantoms (Catphan®600, CIRS®062M (inner phantom for head and outer phantom for body), and TomoTherapy® “Cheese” phantom) were used to create IVDT curves of CBCT systems with two different CBCT protocols (Standard-dose Head and Standard Pelvis). Hounsfield Unit (HU) time stability and repeatability for a single On-Board-Imager (OBI) and compatibility of two distinct devices were assessed with Catphan®600. Images from the anthropomorphic phantom CIRS ATOM® for both CT and CBCT modalities were used for VMAT dose calculation from different IVDT curves. Dosimetric indices from CT and CBCT imaging were compared. ResultsIVDT curves from CBCT images were highly different depending on phantom used (up to 1000 HU for high densities) and protocol applied (up to 200 HU for high densities). HU time stability was verified over seven weeks. A maximum difference of 3% on the dose calculation indices studied was found between CT and CBCT VMAT dose calculation across the two localizations using appropriate IVDT curves. One IVDT curve per localization can be established with a bi-monthly verification of IVDT-CBCT. ConclusionsThe IVDT-CBCTCIRS-Head phantom with the Standard-dose Head protocol was the most accurate combination for dose calculation on H&N CBCT images. For pelvic localizations, the IVDT-CBCTCheese established with the Standard Pelvis protocol provided the best accuracy.

SU‐E‐J‐152: Evaluation of TrueBeam OBI V. 1.5 CBCT Performance in An Adaptive RT Environment

Purpose:To evaluate the image quality and imaging dose of the Varian TrueBeam OBIv.1.5 CBCT system in a clinical adaptive radiation therapy environment, simulated by changing phantom thickness.Methods:Various OBI CBCT protocols(Head, Pelvis, Thorax, Spotlight) were used to acquire images of Catphan504 phantom(nominal phantom thickness and 10 cm additional phantom thickness). The images were analyzed for low contrast detectability(CNR), uniformity(UI), and HU sensitivity. These results were compared to the same image sets for planning CT(pCT)(GE LightSpeed 16‐ slice). Imaging dose measurements were performed with Gafchromic XRQA2 film for various OBI protocols (Pelvis, Thorax, Spotlight) in a pelvic‐sized phantom(nominal thickness and 4cm additional thickness). Dose measurements were acquired in the interior and at the surface of the phantom.Results:The nominal CNR[additional thickness CNR] for OBI was—Pelvis:1.45[0.81],Thorax:0.86[0.48], Spotlight:0.67[0.39],Head:0.28 [0.10]. The nominal CNR[additional thickness CNR] for pCT was— Pelvis:0.87[0.41],Head:0.60[0.22]. The nominal UI[additional thickness UI] for OBI was—Pelvis:11.5[24.1],Thorax:17.0[20.6], Spotlight:23.2[23.2], Head:15.6[59.9]. The nominal UI[additional thickness UI] for pCT was— Pelvis:9.2[8.6],Head:2.1[2.9]. The HU difference(averaged over all material inserts) between nominal and additional thickness scans for OBI: 8.26HU(Pelvis), 33.39HU(Thorax), 178.98HU(Head), 108.20HU (Spotlight); for pCT: 16.00HU(Pelvis), 19.85HU(Head). Uncertainties in electron density were calculated based on HU values with varying phantom thickness. Average electron‐density deviations (ρ(water)=1)for GE‐Pelvis, GE‐Head, OBI‐Pelvis, OBI‐Thorax, OBI‐Spotlight, and OBI‐Head were: 0.0182, 0.0180, 0.0058, 0.0478, 0.2750, and 0.3115, respectively. The average phantom interior dose was(OBI‐nominal):2.35cGy(Pelvis), 0.60cGy(Thorax), 1.87cGy(Spotlight); OBI‐increased thickness: 1.77cGy(Pelvis), 0.43cGy(Thorax), 1.53cGy (Spotlight). Average surface dose(OBI‐nominal): 2.29cGy(Pelvis), 0.56cGy(Thorax), 1.79cGy (Spotlight); OBI‐increased thickness: 1.94cGy(Pelvis), 0.48cGy(Thorax), 1.47cGy (Spotlight).Conclusion:The OBI‐Pelvis protocol offered comparable CNR and HU constancy to pCT for each geometry; other protocols, particularly Spotlight and Head, exhibited lower HU constancy and CNR. The uniformity of pCT was superior to OBI for all protocols. CNR and UI were degraded for both systems/scan types with increased thickness. The OBI interior dose decreased by approximately 30% with additional thickness.This work was funded, in part, under a grant with the Pennsylvania Department of Health. The Department of Health specifically declaims responsibility for any analyses, interpretations, or conclusions.

EP-1738: Streamlined clinical CBCT protocols: Reduced dose and variation and maintained image quality

EP-1738: Streamlined clinical CBCT protocols: Reduced dose and variation and maintained image quality

TU‐G‐103‐07: Quantifying the Dose Distribution of OBI CBCT Using Monte Carlo Simulations with a Measurement‐Based Source Model

Purpose: To test the feasibility of using a measurement‐based source model in Monte Carlo simulations to calculate in‐phantom 3D dose distributions, with the application to kV‐CBCT dose quantification on a Varian OBI platform. Methods: A 3‐part nested CTDI phantom was used in this investigation. It contains 7 tracks in the x‐y plane that can be probed. The central reference point of the phantom was aligned at machine isocenter, and using standard head and pelvis CBCT protocols, point dose measurements were made at various 3D locations with a 0.6cc thimble ionization chamber by moving the phantom along the z‐direction and changing the ionization chamber position among the tracks under repetitive scans. We have generated a measurement‐based source model for the OBI system and incorporated it with the Monte Carlo software MCNPX to model each CBCT protocol under examination. The in‐phantom CBCT dose (mGy per particle) was simulated at all 3D measurement locations and converted to absolute dose (mGy) by normalizing with the ratio between measured and Monte Carlo simulated dose at the reference center. The calculated results were compared with the physical measurement and their discrepancies were assessed quantitatively. Results: Agreement was observed between Monte Carlo simulation results and in‐phantom measurements for both the OBI CBCT head and pelvis protocols. For the head protocol, the average agreement was 3.40% with a maximum and minimum disagreement of 5.72% and 1.61%, respectively. For the pelvis protocol, the average agreement was 3.18% with a maximum and minimum disagreement of 4.29% and 1.87%, respectively. Conclusion: We have demonstrated the accuracy of a measurement‐based OBI source model to reproduce an array of measured point doses in phantom when using the CBCT mode of operation. These results serve as the basis for using this source model to investigate OBI dosimetry with anthropomorphic phantoms and patient specific models. (a) Dr. McNitt‐Gray: Institutional research agreement, Siemens AG;Recipient research support Siemens AG; Consultant, Flaherty Sensabaugh Bonasso PLLC; Consultant, Fulbright and Jaworski, LLC; (b) Dr. Dan Ruan: Recipient research support AACR; Recipient research support TRDRP; (c) K McMillan: Research partially supported by a grant from Siemens AG; Research partially supported by a grant from AACR; Research partially supported by a grant from TRDRP

Phantom based KV-CBCT dose plan calculations for re-planning decision: Comparison with CT-based dose plans

Objective Kilovoltage cone-beam computed tomography scans (kVCBCT) are helpful for patients positioning and viewing anatomical changes. This study investigates, on an anthropomorphic phantom, with no movement and anatomical changes, the possibility and accuracy of using CBCT-based dosimetry for re-planning decision. Materials and method Rando phantom's brain, Head and Neck (H&N), thorax and pelvis were scanned. Targets and main organ at risks (OAR) were contoured. IMRT was used for H&N and VMAT for brain, lung and pelvis. Dose distributions were calculated using Eclipse 10.0 AAA Algorithm (Varian Medical System). CBCT scans were acquired per region on a Novalis Tx. After registration with the reference CT, structures were copied and dosimetric plans were recalculated on the CBCT. The dose volume histograms (HVH) for targets and OAR and 2D dose maps were compared. Furthermore the Hounsfield units to electron density (HU to ED) curves for the different CBCT protocols were obtained using a Catphan® 504 phantom. To end examples of patients with anatomical changes are presented. Results PTV and OAR average mean dose differences for the brain, H&N, lungs and pelvic region were −1.1 ± 0.7%, −0.6 ± 0.4%, −0.9 ± 1.0% and 0.1 ± 0.5% respectively. Passing rate superior than 98% was achieved (gamma index criteria of 2%/1 mm) for absolute 2D CBCT and CT-based dose distributions. CBCT HU to ED curves were within ±30 HU compared with the reference HU of the materials contained inside phantom, amply inside requested CT tolerance values. Conclusion Decision to re-plan VMAT or IMRT treatments seems feasible using CBCT data. Future investigations on the HU to ED curve and curve stability will be done with the help of a CBCT electron density phantom (CIRS).

Effective dose range for dental cone beam computed tomography scanners

To estimate the absorbed organ dose and effective dose for a wide range of cone beam computed tomography scanners, using different exposure protocols and geometries. Two Alderson Radiation Therapy anthropomorphic phantoms were loaded with LiF detectors (TLD-100 and TLD-100 H) which were evenly distributed throughout the head and neck, covering all radiosensitive organs. Measurements were performed on 14 CBCT devices: 3D Accuitomo 170, Galileos Comfort, i-CAT Next Generation, Iluma Elite, Kodak 9000 3D, Kodak 9500, NewTom VG, NewTom VGi, Pax-Uni3D, Picasso Trio, ProMax 3D, Scanora 3D, SkyView, Veraviewepocs 3D. Effective dose was calculated using the ICRP 103 (2007) tissue weighting factors. Effective dose ranged between 19 and 368 μSv. The largest contributions to the effective dose were from the remainder tissues (37%), salivary glands (24%), and thyroid gland (21%). For all organs, there was a wide range of measured values apparent, due to differences in exposure factors, diameter and height of the primary beam, and positioning of the beam relative to the radiosensitive organs. The effective dose for different CBCT devices showed a 20-fold range. The results show that a distinction is needed between small-, medium-, and large-field CBCT scanners and protocols, as they are applied to different indication groups, the dose received being strongly related to field size. Furthermore, the dose should always be considered relative to technical and diagnostic image quality, seeing that image quality requirements also differ for patient groups. The results from the current study indicate that the optimisation of dose should be performed by an appropriate selection of exposure parameters and field size, depending on the diagnostic requirements.

Kilovoltage cone‐beam CT: Comparative dose and image quality evaluations in partial and full‐angle scan protocols

To assess imaging dose of partial and full-angle kilovoltage CBCT scan protocols and to evaluate image quality for each protocol. The authors obtained the CT dose index (CTDI) of the kilovoltage CBCT protocols in an on-board imager by ion chamber (IC) measurements and Monte Carlo (MC) simulations. A total of six new CBCT scan protocols were evaluated: Standard-dose head (100 kVp, 151 mA s, partial-angle), low-dose head (100 kVp, 75 mA s, partial-angle), high-quality head (100 kVp, 754 mA s, partial-angle), pelvis (125 kVp, 706 mA s, full-angle), pelvis spotlight (125 kVp, 752 mA s, partial-angle), and low-dose thorax (110 kVp, 271 mA s, full-angle). Using the point dose method, various CTDI values were calculated by (1) the conventional weighted CTDI (CTDIw) calculation and (2) Bakalyar's method (CTDIwb). The MC simulations were performed to obtain the CTDIw and CTDIwb, as well as from (3) central slice averaging (CTDI(2D)) and (4) volume averaging (CTDI(3D)) techniques. The CTDI values of the new protocols were compared to those of the old protocols (full-angle CBCT protocols). Image quality of the new protocols was evaluated following the CBCT image quality assurance (QA) protocol [S. Yoo et al., "A quality assurance program for the on-board image, "Med. Phys. 33(11), 4431-4447 (2006)] testing Hounsfield unit (HU) linearity, spatial linearity/resolution, contrast resolution, and HU uniformity. The CTDI, were found as 6.0, 3.2, 29.0, 25.4, 23.8, and 7.7 mGy for the new protocols, respectively. The CTDI, and CTDIwb differed within +3% between IC measurements and MC simulations. Method (2) results were within +/- 12% of method (1). In MC simulations, the CTDIw and CTDIwb were comparable to the CTDI(2D) and CTDI(3D) with the differences ranging from -4.3% to 20.6%. The CTDI(3D) were smallest among all the CTDI values. CTDIw of the new protocols were found as approximately 14 times lower for standard head scan and 1.8 times lower for standard body scan than the old protocols, respectively. In the image quality QA tests, all the protocols except low-dose head and low-dose thorax protocols were within the tolerance in the HU verification test. The HU value for the two protocols was always higher than the nominal value. All the protocols passed the spatial linearity/resolution and HU uniformity tests. In the contrast resolution test, only high-quality head and pelvis scan protocols were within the tolerance. In addition, crescent effect was found in the partial-angle scan protocols. The authors found that CTDIw of the new CBCT protocols has been significantly reduced compared to the old protocols with acceptable image quality. The CTDIw values in the point dose method were close to the volume averaging method within 9%-21% for all the CBCT scan protocols. The Bakalyar's method produced more accurate dose estimation within 14%. The HU inaccuracy from low-dose head and low-dose thorax protocols can render incorrect dose results in the treatment planning system. When high soft-tissue contrast data are desired, high-quality head or pelvis scan protocol is recommended depending on the imaging area. The point dose method can be applicable to estimate CBCT dose with reasonable accuracy in the clinical environment.

TH-C-201B-05: Equivalent Doses and Secondary Malignancy Risk Estimates for Gynecological Patients Undergoing CBCT and IMRT

Purpose: To assess risk of secondary malignancy associated with peripheral doses from kV CBCT for gynecological patients undergoing IMRT and to compare doses to organs peripheral to the treated volume from kV Cone Beam with scatter and leakage doses from MV IMRT. Method and Materials: In field and peripheral doses for kV CBCT gynecologic studies were simulated by Monte Carlo code BEAMnrc and DOSXYZnrc. Dose-mean lineal energy based quality factor for kV beam was calculated to account for the difference of relative biological effectiveness between kV CBCT beams and MV beams. Life time attributable risk of cancer incidence (LAR) and relative risk (RR) for bladder and colon for patients undergoing IMRT and daily CBCT were estimated based on the Monte Carlo simulated equivalent doses. Peripheral dose from image-guided radiation therapy (IGRT) using kV CBCT was evaluated relative to the scatter and leakage doses from the Linac delivering the IMRT treatment. Results: The estimated quality factor for CBCT is about 1.3 times that of MV beams. With daily imaging over a 25 fraction IMRT treatment regimen with prescribed dose of 4500 cGy RRs of cancer incidence for bladder and colon are 21.2 and 3.4 of which the contribution from CBCT is 1.83 and 1.20. In the peripheral region the average CBCT dose is of the same order of magnitude as the leakage dose. Scatter doses from the IMRT treatment are however a factor of 5 higher than the CBCT peripheral doses and leakage doses. Conclusion: Even with daily kV CBCT the relative risk caused by CBCT is much lower than the risk from IMRT scattering in both in field and peripheral regions. Understanding the relative risks associated with the doses from these procedures is important for designing appropriate CBCT protocols. Conflict of Interest: Research supported by Varian Medical Systems.

SU-GG-I-57: Dose and Image Quality Evaluation for Partial and Full-Angle Kilovoltage Cone-Beam CT Protocols

Purpose: To assess imaging dose of partial and full‐angle CBCT scan protocols, and to evaluate their image quality. Method and Materials: The CTDI of the six new CBCT protocols for Varian OBI were obtained by ion chamber (IC) measurements and Monte Carlo(MC) simulations: standard‐dose‐head, low‐dose‐head, high‐quality‐head, pelvis, pelvis‐spotlight, and low‐dose‐thorax. Various CTDI values were calculated by (1) the conventional weighted CTDI(CTDIw) calculation and (2) Bakalyar's method (CTDIwb). The MC simulations were performed to obtain the CTDIw and CTDIwb, as well as from (3) central slice averaging (CTDI2D) and (4) volume averaging (CTDI3D) techniques. Image quality of the new protocols was evaluated testing HU verification/uniformity, spatial linearity/resolution, and contrast resolution. Results: The CTDIw were found as 6.0, 3.2, 29.0, 25.4, 23.8, and 7.7 mGy for the new protocols respectively. The CTDIw and CTDIwb differed within +3% between IC and MC. Method (2) results were within ±12% of Method (1). In MC simulations, the CTDIw and CTDIwb were comparable to the CTDI2D and CTDI3D with the differences ranging from ‐4.3% to 20.6%. CTDIw of the new protocols were lower (1.8∼14 times) than the old protocols respectively. In the image quality analyses, all the protocols except low dose head and low dose thorax protocols were within the tolerance in the HU verification test. All the protocols passed the spatial linearity/resolution and HU uniformity tests. Only high quality head and pelvis scan protocols passed the contrast resolution test. Conclusion:CTDIw of the new CBCT protocols has been significantly reduced compared to the old protocols. The new scan protocols resulted in lower radiation dose with acceptable image quality. HU inaccuracy in CBCTimages from low‐dose‐head and low‐dose‐thorax protocols can render incorrect dose results. When a high soft‐tissue contrast data is desired, high quality head or pelvis scan protocol is recommended.