kV-CBCT Scans Research Articles

A kV–MV approach to CBCT metal artifact reduction using multi-layer MV-CBCT

Objective. To demonstrate that complete cone beam CT (CBCT) scans from both MV-energy and kV-energy LINAC sources can reduce metal artifacts in radiotherapy guidance, while maintaining standard-of-care x-ray doses levels. Approach. MV-CBCT and kV-CBCT scans are acquired at half normal dose. The impact of lowered dose on MV-CBCT data quality is mitigated by the use of a 4-layer MV-imager prototype and reduced LINAC energy settings (2.5 MV) to improve photon capture. Additionally, the MV-CBCT is used to determine the 3D position and pose of metal implants, which in turn is used to guide model-based poly-energetic correction and interleaving of the kV-CBCT and MV-CBCT data. Certain edge-preserving regularization steps incorporated into the model-based correction algorithm further reduce MV data noise. Main results. The method was tested in digital phantoms and a real pelvis phantom with large 2.5″ spherical inserts, emulating hip replacements of different materials. The proposed method demonstrated an appealing compromise between the high contrast of kV-CBCT and low artifact content of MV-CBCT. Contrast-to-noise improved 3-fold compared to MV-CBCT with a clinical 1-layer architecture at matched dose (37 mGy) and edge blur levels. Visual delineation of the bladder and prostate improved noteably over kV- or MV-CBCT alone. Significance. The proposed method demonstrates that a full MV-CBCT scan can be combined with kV-CBCT to reduce metal artifacts without resorting to complicated beam collimation strategies to limit the MV-CBCT dose contribution. Additionally, significant improvements in CNR can be achieved as compared to metal artifact reduction through current clinical MV-CBCT practices.

Study on the application of 3D printing head film fixation technology in cranial radiotherapy.

Objective: To investigate the use of 3D printing technology to customize individualized precise radiotherapy head masks for cranial radiotherapy patients. Through the comparison with thermoplastic head film, evaluate the effect of this material on deep dose attenuation and body surface dose, and evaluate its positioning accuracy and repeatability for clinical application. Methods: Thirty patients with head and neck radiotherapy were divided into the control group and the experimental group. The control group used the traditional thermoplastic head film fixation technique for body position fixation, and the experimental group used the 3D printing head film fixation technique. The patient setup was verified by kV-CBCT scanning to obtain the translational setup error and rotational setup error in the X, Y, and Z directions. Results: At a depth of 5 cm, both materials have a radiation attenuation rate of <1%. At the surface location, the body surface dose of control group increased by approximately 27%. With a 3D printing head film, the body surface dose increased by approximately 18%. The positioning of two groups of patients was verified by the kV-CBCT, and a total of 232 data sets were obtained. The average translation positioning errors in the X, Y, and Z direction of control group and experimental group were 1.29 mm, 1.42 mm, 1.38 mm and 1.16 mm, 1.24 mm, 1.16 mm, respectively. The average rotation positioning error in the X, Y, and Z direction of control group and experimental group were 1.29°, 1.02°, 1.01° and 1.08°, 0.96°, 1.00°, respectively. The translational setup errors in the Y and Z directions and rotational setup errors in the X direction significantly differed between the control and experimental groups (all p<0.05), but no statistical significance was found in the other directions (all p>0. 05). Conclusion: Compared to the traditional thermoplastic head membranes, 3D printing head membranes has shown a reliable and reproducible interactional positioning accuracy. Of course, further investigations are needed before the new technology can be used on a regular basis.

Dosimetric and feasibility evaluation of a CBCT-based daily adaptive radiotherapy protocol for locally advanced cervical cancer.

Evaluate a cone-beam computed tomography (CBCT)-based daily adaptive platform in cervical cancer for multiple endpoints: (1) physics contouring accuracy of daily CTVs, (2) CTV coverage with adapted plans and reduced PTV margins versus non-adapted plans with standard-of-care (SOC) margins, (3) dosimetric improvements to CTV and organs-at-risk (OARs), and (4) on-couch time. Using a Varian Ethos™ emulator and KV-CBCT scans, we simulated the doses 15 retrospective cervical cancer patients would have received with/without online adaptation for five fractions. We compared contours and doses from SOC plans (5-15mm CTV-to-PTV margins) to adapted plans (3mm margins). Auto-segmented CTVs and OARs were reviewed and edited by trained physicists. Physics-edited targets were evaluated by an oncologist. Time spent reviewing and editing auto-segmented structures was recorded. Metrics from the CTV (D99%), bowel (V45Gy, V40Gy), bladder (D50%), and rectum (D50%) were compared. The physician approved the physics-edited CTVs for 55/75 fractions; 16/75 required reductions, and 4/75 required CTV expansions. CTVs were encapsulated by unadapted, SOC PTVs for 56/75 (72%) fractions-representative of current clinical practice. CTVs were completely covered by adapted 3mm PTVs for 71/75 (94.6%) fractions. CTV D99% values for adapted plans were comparable to non-adapted SOC plans (average difference of -0.9%), while all OAR metrics improved with adaptation. Specifically, bowel V45Gy and V40Gy decreased on average by 87.6 and 109.4 cc, while bladder and rectum D50% decreased by 37.7% and 35.8%, respectively. The time required for contouring and calculating an adaptive plan for 65/75 fractions was less than 20 min (range: 1-29 min). Improved dose metrics with daily adaption could translate to reduced toxicity while maintaining tumor control. Training physicists to perform contouring edits could minimize the time physicians are required at adaptive sessions improving clinical efficiency. All emulated adaptive sessions were completed within 30 min however extra time will be required for patient setup, image acquisition, and treatment delivery.

Assessment and implication of rectal filling on vaginal motion in postoperative carcinoma endometrium patients during image guided radiotherapy.

While delivering radiotherapy it is utmost important to minimize target motion to decrease margins in postoperative gynaecological patients. Hence certain bladder and rectum filling protocols are followed by each institute. During cone beam computed tomography (CBCT) verification, we observed that this motion was more affected by rectal filling. To verify, we retrospectively analysed the vaginal movement and its relation with bladder and rectum filling. We evaluated CBCTs of 15 patients of carcinoma endometrium. Bladder and rectum both were contoured offline on each scan. To assess the motion of vagina, two reference points were selected. Posterior movement of bladder and anterior movement of rectum were noted on these points on each scan. Total 150 scans (135 KV-CBCT scans and 15 planning computed tomography) of 15 patients were studied. Stepwise regression analysis reported that bladder wall changes has a nonsignificant relationship with bladder volume among all the individuals. The significant rectal wall changes both at Point X and Y were observed in six patients and only at Point X in three patients. Rest of the patients showed no significant relationship in their CBCT scans. Thus 60% patients showed significant relation between rectal volume and rectal wall changes. Hence we suggest to advice our patients more regarding volume of rectum to decrease vaginal motion. Bladder volume is needed to decrease the dose to small intestine. However prospective data with large number of patients in the study is required to confirm these findings.

Prior-image-based Reconstruction for On-board CBCT Imaging: Application in Image-guided Radiotherapy for Head and Neck Cancer

On-board cone-beam CT (CBCT) imaging has been widely used to reduce set-up errors in head and neck cancer treated with IMRT/VMAT. However, its frequent use has raised concerns for excessive imaging dose and increased risk of developing secondary malignancies. This work investigates the feasibility of significantly reducing CBCT radiation dose by taking advantage of high-quality planning CT and a prior-image-based reconstruction approach. In image-guided radiotherapy for head and neck cancer, one standard protocol (Full-fan, 200 deg rotation, 100 kVp, 0.3 mAs/projection, 500 projections, 150 mAs in total) is employed for kV-CBCT scans. Since the high-quality planning CT scan is a rich source of patient-specific anatomical knowledge, it offers an opportunity to integrate the planning CT into on-board CBCT reconstruction for radiation dose reduction. Therefore, we studied a prior-image-based reconstruction approach for on-board CBCT imaging, which only requires partial projections and thus substantially reduces radiation dose. The prior-image-based reconstruction takes into consideration differences in attenuation coefficients (via histogram matching) and patient positioning (via affine registration) between the planning CT and on-board CBCT scans. Additionally, proper prior information control is applied to ensure the prior-image-based reconstruction does not introduce false features when there are anatomical changes between scans. An anthropomorphic head phantom and two head and neck patient datasets were used to validate the efficacy of the proposed approach. We extracted certain number of projections from the 500 projections to emulate different radiation dose levels. As we decrease the number of projections to as low as 100 (30 mAs equivalent, CTDIvol = 0.63 mGy), the standard FDK reconstructed images are degraded with severe noise and streak artifacts, and the conventional model-based iterative reconstruction generates images with inferior image quality, which could jeopardize the accuracy of patient set-up. In contrast, our proposed prior-image-based reconstruction can yield comparable image quality as the ground truth images reconstructed with the full projections, in terms of noise, resolution, and contrast-to-noise ratio. This indicates an 80% radiation dose reduction for on-board CBCT imaging by the proposed reconstruction scheme. The proposed prior-image-based reconstruction framework provides an opportunity to substantially reduce the radiation dose for on-board CBCT, without compromising image quality. This work offers a solution to reduce cumulative imaging dose as the use of CBCT for image-guided radiotherapy increases.

Registration accuracy with the low dose kilovoltage cone-beam CT: A phantom study.

Objective:The aim of this study was to investigate low-dose kilovoltage cone-beam CT (kV-CBCT) for image-guided radiotherapy, with a particular focus on the accuracy of image registration with low-dose protocols.Methods:Imaging doses were measured with a NOMEX semiconductor detector positioned at the front of head, thorax, and pelvis human body phantoms while kV-CBCT scans were acquired at different tube currents. Aspects of image quality (spatial resolution, noise, uniformity, contrast, geometric distortion, and Hounsfield unit sensitivity) and image registration accuracy using bone and soft tissue were evaluated.Results:With preset and the lowest tube currents, the imaging doses were 0.16 and 0.08 mGy, 5.29 and 2.80 mGy, and 18.23 and 2.69 mGy for head, thorax, and pelvis, respectively. Noise was the only quality aspect directly dependent on tube current, being increased by 1.5 times with a tube current half that of the preset in head and thorax, and by 2.2 times with a tube current 1/8 of the preset in the pelvis. Accurate auto-bone matching was performed within 1 mm at the lowest tube current. The auto-soft tissue matching could not be performed with the lowest tube current; however, manual-soft tissue matching could still be performed within 2 mm or less.Conclusion:Noise was the only image quality aspect dependent on the imaging dose. Auto-bone and manual-soft tissue matching could still be performed at the lowest imaging dose.Advances in knowledge:When optimizing kV-CBCT imaging dose, the impact on bone and soft tissue image registration accuracy should be evaluated.

Study on the significance of consistency of preoperative and postoperative bladder filling in patients with cervical cancer for target area and organ at risk in IMRT

Objective Bladder filling in patients with cervical cancer before and after operation during intensity-modulated radiotherapy (IMRT) was controlled by using bladder volume measurement instrument (BVI 9400). The displacement errors of the target area and the changes of the irradiation dose and the morphology of the filling bladder were compared. The significance of consistency of preoperative and postoperative bladder filling in IMRT for cervical cancer was investigated. Methods Forty-five patients with cervical cancer treated with radiotherapy in our hospital in 2018 were recruited and divided into A, B and C groups (n=15). In group A, patients did not undergo surgery, patients in group B received radical resection of cervical cancer (bladder volume measurement (BVI 9400) was utilized to monitor the bladder urine volume in both A and B groups before treatment). In group C, patients who self-controlled urination without monitoring bladder urine volume before treatment were randomly selected. All patients in three groups underwent KV-CBCT scan before treatment for online registration analysis. The displacement errors in the x-(left and right), y-(head and foot), z-axis directions (ventral and dorsal), PTV coverage volume percentage(V100) and percentage difference (ΔV100) of the prescription dose were calculated. Results The average displacement errors in the x-, y-, z-axis directions were as follows: 1.67 mm, 1.55 mm and 1.67 mm in group A, 1.43 mm, 1.58 mm and 1.84 mm in group B, and 2.27 mm, 2.30 mm and 2.08 mm in group C, respectively. In group A, the V100 was calculated as 96.96% and ΔV100 was 1.17%. In group B, V100 was 95.9% and ΔV100 was 1.29%. In group C, V100 was 94.02% and ΔV100 was 2.17%. Conclusions Controlling the consistency of bladder filling can reduce the effect of inconsistency of bladder filling upon the displacement errors to certain extent, which can guarantee the accuracy of the target location and the irradiation dose of the target, protect the bladder, small intestine and rectum, and alleviate the radiation-induced response of cervical cancer patients. Key words: Bladder filling status; Planning target volume; Cervical neoplasm/intensity-modulated radiotherapy

Characterizing a novel scintillating glass for application to megavoltage cone-beam computed tomography.

The purpose of this study was to evaluate the performance of a prototype electric portal imaging device (EPID) with a high detective quantum efficiency (DQE) scintillator, LKH-5. Specifically, image quality in context of both planar and megavoltage (MV) cone-beam computed tomography (CBCT) is analyzed. Planar image quality in terms of modulation transfer function (MTF), noise power spectrum (NPS), and DQE are measured and compared to an existing EPID (AS-1200) using the 6MV beamline for a Varian TrueBeam linac. Imager performance is contextualized for three-dimensional (3D), MV-CBCT performance by measuring imager lag and analyzing the expected degradation of the DQE as a function of dose. Finally, comparisons between reconstructed images of the Catphan phantom in terms of qualitative quality and signal-difference-to-noise ratio (SDNR) are made for 6MV images using both conventional and LKH-5 EPIDs as well as for the kilovoltage (kV) on-board imager (OBI). Analysis of the NPS reveals linearity at all measured doses using the prototype LKH-5 detector. While the first zero of the MTF is much lower for the LKH-5 detector than the conventional EPID (0.6 cycles/mm vs 1.6 cycles/mm), the normalized NPS (NNPS) multiplied by total quanta (qNNPS) of the LKH-5 detector is roughly a factor of seven to eight times lower, yielding a DQE(0) of approximately 8%. First, second, and third frame lag were measured at approximately 23%, 5%, and 1%, respectively, although no noticeable image artifacts were apparent in reconstructed volumes. Analysis of low-dose performance reveals that DQE(0) remains at 80% of its maximum value at a dose as low as 7.5×10-6 MU. For a 400 projection technique, this represents a total scan dose of 0.0030MU, suggesting that if imaging doses are increased to a value typical of kV-CBCT scans (~2.7cGy), the LKH-5 detector will retain quantum noise limited performance. Finally, comparing Catphan scans, the prototype detector exhibits much lower image noise than the conventional EPID, resulting in improved small object representation. Furthermore, SDNR of H2 O and polystyrene cylinders improved from -1.95 and 2.94 to -15 and 18.7, respectively. Imaging performance of the prototype LKH-5 detector was measured and analyzed for both planar and 3D contexts. Improving noise transfer of the detector results in concurrent improvement of DQE(0). For 3D imaging, temporal characteristics were adequate for artifact-free performance and at relevant doses, the detector retained quantum noise limited performance. Although quantitative MTF measurements suggest poorer resolution, small object representation of the prototype imager is qualitatively improved over the conventional detector due to the measured reduction in noise.

Clinical study of the time of repeated computed tomography and replanning for patients with nasopharyngeal carcinoma.

PurposeTo study the necessity of repeat computed tomography (CT) scan and replanning and know a more accurate time using weekly kilovoltage cone beam computed tomography (kV-CBCT) scans for patients with nasopharyngeal carcinoma (NPC) during radiotherapy.Methods and MaterialsThirteen NPC patients treated with IMRT were enrolled into this prospective study. Weekly pretreatment kV-CBCT scans were performed on the 1st, 6th, 11st, 16th, 21st and 26th radiation time, respectively. Target delineations were contoured on all fractionated CBCT images, including the gross tumor volume of the primary nasopharyngeal tumor (GTVnx) and parotid glands. The volumes of GTVnx and parotid glands were calculated automatically using the Pinnacle3 8.0 system. Compared to the original GTVnx, the percentage of shrinking volume (ΔP) ≥ 50% was considered significantly.ResultsAs the radiation proceeding, the GTVnx had a trend of shrinkage. Of all 13 patients, 11 cases (84.6%) had the volume shrinking ≥ 50% before the 21st radiation and 12 cases (92.3%) before the 26th radiation. And the parotid volume decreased significantly in the first four-week radiation, 6.45 ± 3.16cm3 (range, 3.06-13.9cm3) for the left parotid gland and 5.78 ± 2.39cm3 (range, 2.70-11.2cm3) for the right. Furthermore, only a little displacement occurred to bilateral parotid glands.ConclusionThe replanning for NPC patients with IMRT is necessary, and the time between the 21st to 25th radiations is appropriate.

IMAGING DOSE OF HUMAN ORGANS FROM kV-CBCT IN IMAGE-GUIDED RADIATION THERAPY.

This study investigates dose distribution due to kV cone-beam computed tomography (CBCT) for the patients undergoing CBCT-based image-guided radiation therapy (IGRT). The kV-CBCT provides an efficient image-guidance tool for acquiring the latest volumetric image of a patient's anatomy, and has been being routinely used in clinics for an accurate treatment setup. Imaging radiation doses resulting from six different acquisition protocols of the on-board imager (OBI) were calculated using a Geant4 Application for Tomographic Emission (GATE) Monte Carlo simulation toolkit, and the absorbed doses by various organs were analyzed for the adult and pediatric numerical XCAT phantoms in this study. The calculated organ doses range from 0.1 to 24.1 mGy in the adult phantom, and from 0.1 to 36.8 mGy in the pediatric one. The imaging organ doses to the pediatric phantom turn out to be consistently higher than those to the adult phantom. It is believed that our results would provide reliable data to the clinicians for their making better decisions on CBCT scanning options and would also provide a platform for developing a new kV-CBCT scanning protocol in conjunction with a low-dose capability.

Comparison of difference registration landmarks for image-guided radiotherapy for lung cancer

Objective To investigate the impact of anatomical landmarks on registration in image-guided radiotherapy (IGRT) for central and peripheral lung cancer. Methods Twenty-five patients with central or peripheral lung cancer for IGRT were enrolled in this study. Kilo-voltage cone-beam CT (kV-CBCT) scanning was acquired before irradiotion. Tumor coverage on CBCT was assessed using gross tumor volume (GTV), clinical target volume (CTV), and planning target volume (PTV) contours according to tumor alignment, carina registration, and spine registration, respectively. The grading analysiswas based on visual tumor assessment as follows: grade 0, tumor within GTV; grade 1, tumor outside GTV but inside CTV; grade 2, tumor outside CTV but inside PTV; and grade 3, tumor outside PTV. Results Totally 177 sets of kV-CBCT of 25 patients was collected. According to the registration landmarks of the tumor, carina and spine for central lung cancer, the percentages were 57.55%, 53.77% and 16.04% in grade 0, 39.62%, 45.28% and 58.49% in grade 1, and 1.89%, 0.94% and 25.47% in grade 2, respectively. For peripheral lung cancer, the percentages were 47.89%, 14.08% and 2.82% in grade 0, 43.66%, 29.58% and 45.07% in grade 1, and 8.45%, 40.85% and 35.21% in grade 2, respectively. Conclusions For central lung cancer, the tumor was recommended as the best registration landmark, and the carina was recommended as well, while the spine was not recommended. For peripheral lung cancer, the tumor was recommended as the best registration landmark, while the spine and the carina were not recommended. Key words: Lung neoplasms, central; Lung neoplasms, peripheral; Image-guided intensity-modulated radiotherapy; Registration landmarks; Grade

Weekly kilovoltage cone-beam computed tomography for detection of dose discrepancies during (chemo)radiotherapy for head and neck cancer

ABSTRACTBackground. Use of highly conformal radiotherapy in patients with head and neck carcinoma may lead to under-/overdosage of gross target volume (GTV) and organs at risk (OAR) due to changes in patients’ anatomy. A method to achieve more effective radiation treatment combined with less toxicity is dose-guided radiotherapy (DGRT). The aim of this study was to evaluate discrepancies between planned and actually delivered radiation dose in head and neck patients and to identify predictive factors.Methods. In this retrospective analysis, 20 patients with cT2-4 N0-3 M0 carcinoma originating from oropharynx, oral cavity, larynx and hypopharynx (Cohort 1), and seven patients with cT1-4 N0-3 M0 nasopharyngeal carcinoma (Cohort 2) treated with primary (chemo)radiotherapy and undergoing weekly kV-CBCT scans were included. Radiation dose was recalculated on 184 kV-CBCT images, which was quantified by D95% (GTV), Dmean (parotid and submandibular glands) and D2% (spinal cord). Predictive factors investigated for changes in these dose metrics were: gender, age, cT/N-stage, tumor grade, HPV-status, systemic therapy, body mass index at start of treatment, weight loss and volume change over the duration of the radiotherapy.Results. There was no significant difference between the planned and delivered dose for GTV and OARs of Week 1 to subsequent weeks for Cohort 1. In Cohort 2, actually delivered Dmean to parotid glands was significant higher than planned dose (1.1 Gy, p = 0.002). No clinically relevant correlations between dose changes and predictive factors were found.Conclusion. Weekly dose calculations do not seem to improve dose delivery for patients with tumors of the oral cavity, oropharynx, larynx and hypopharynx. In patients with nasopharyngeal carcinoma, however, mid-treatment imaging may facilitate DGRT.

SU‐D‐207‐05: Real‐Time Intrafractional Motion Tracking During VMAT Delivery Using a Conventional Elekta CBCT System

Purpose:Real‐time kV projection streaming capability has become recently available for Elekta XVI version 5.0. This study aims to investigate the feasibility and accuracy of real‐time fiducial marker tracking during CBCT acquisition with or without simultaneous VMAT delivery using a conventional Elekta linear accelerator.Methods:A client computer was connected to an on‐board kV imaging system computer, and receives and processes projection images immediately after image acquisition. In‐house marker tracking software based on FFT normalized cross‐correlation was developed and installed in the client computer. Three gold fiducial markers with 3 mm length were implanted in a pelvis‐shaped phantom with 36 cm width. The phantom was placed on a programmable motion platform oscillating in anterior‐posterior and superior‐inferior directions simultaneously. The marker motion was tracked in real‐time for (1) a kV‐only CBCT scan with treatment beam off and (2) a kV CBCT scan during a 6‐MV VMAT delivery. The exposure parameters per projection were 120 kVp and 1.6 mAs. Tracking accuracy was assessed by comparing superior‐inferior positions between the programmed and tracked trajectories.Results:The projection images were successfully transferred to the client computer at a frequency of about 5 Hz. In the kV‐only scan, highly accurate marker tracking was achieved over the entire range of cone‐beam projection angles (detection rate / tracking error were 100.0% / 0.6±0.5 mm). In the kV‐VMAT scan, MV‐scatter degraded image quality, particularly for lateral projections passing through the thickest part of the phantom (kV source angle ranging 70°‐110° and 250°‐290°), resulting in a reduced detection rate (90.5%). If the lateral projections are excluded, tracking performance was comparable to the kV‐only case (detection rate / tracking error were 100.0% / 0.8±0.5 mm).Conclusion:Our phantom study demonstrated a promising Result for real‐time motion tracking using a conventional Elekta linear accelerator. MV‐scatter suppression is needed to improve tracking accuracy during MV delivery.This research is funded by Motion Management Research Grant from Elekta.

TU-AB-303-02: A Novel Surrogate to Identify Anatomical Changes During Radiotherapy of Head and Neck Cancer Patients

Purpose: To develop a novel method to monitor external anatomical changes in head and neck cancer patients in order to help guide adaptive radiotherapy decisions. Methods: The method, developed in MATLAB, reveals internal anatomical changes based on variations observed in external anatomy. Weekly kV-CBCT scans from 11 Head and neck patients were retrospectively analyzed. The pre-processing step first corrects each CBCT for artifacts and removes pixels from the immobilization mask to produce an accurate external contour of the patient's skin. After registering the CBCTs to the initial planning CT, the external contours from each CBCT (CBCTn) are transferred to the first week — reference — CBCT₁. Contour radii, defined as the distances between an external contour and the central pixel of each CBCT slice, are calculated for each scan at angular increments of 1 degree. The changes in external anatomy are then quantified by the difference in radial distance between the external contours of CBCT1 and CBCTn. The radial difference is finally displayed on a 2D intensity map (angle vs radial distance difference) in order to highlight regions of interests with significant changes. Results: The 2D radial difference maps provided qualitative and quantitative information, such as the location and the magnitude of external contour divergences and the rate at which these deviations occur. With this method, anatomical changes due to tumor volume shrinkage and patient weight loss were clearly identified and could be correlated with the under-dosage of targets or over-dosage of OARs. Conclusion: This novel method provides an efficient tool to visualize 3D external anatomical modification on a single 2D map. It quickly pinpoints the location of differences in anatomy during the course of radiotherapy, which can help determine if a treatment plan needs to be adapted.

SU‐E‐T‐714: Study On Set‐Up Accuracy of Three Different Immobilization Methods in Radiotherapy for Thoracic Cancer

Purpose:Using VARIAN Clinac ix cone beam CT to evaluate the different immobilization methods in thoracic cancer radiotherapy positioning accuracy.Methods:Ninety thoracic cancer patients were selected and were divided into 3 group randomly according to immobilization method, with Body fixed film, vacuum mat, Arm bracket joint vacuum mat. All of the patients in the first three times before radiotherapy with OBI system KV CBCT scanning,plan to get the volume of the image was treated with CT image comparison,two image space position deviation. After treatment of CBCT position verification once a week.Results:Body fixed way setup errors X, Y, Z axis deviation mean respectively (2.038±1.416)mm, (2.267±1.568)mm, (1.412±1.408)mm. vacuum mat fixed way respectively (2.395±1.576)mm, 2.721±1.764)mm,(2.419±1.516)mm. Arm bracket joint vacuum mat fixed way respectively (2.325±1.419)mm,(1.837±0.814)mm, (1.674±1.213)mm. Body fixed group Compared with the vacuum mat group P&lt;0.05. Arm bracket joint vacuum mat fixed group Compared with the vacuum mat group P&lt;0.05. Arm bracket joint vacuum mat fixed group Compared with the Body fixed group P&lt;0.05.Conclusion:Thoracic tumor radiotherapy using Arm bracket joint vacuum mat fixed patients can significantly reduce the position error,improve the positioning accuracy,precise radiotherapy to provide quality assurance

Posterior kV-CBCT scanning of the head and neck region minimizes doses to critical organs with sustained image quality

We evaluated the absorbed dose to critical organs, as well as the image quality, at different partial angles in kV-CBCT (Cone Beam Computed Tomography) scanning of the head and neck region. CBCT images of phantom from a 200° rotation were performed by using three different scanning paths, anterior, posterior, and right lateral with Catphan504 and RANDO phantoms. Critical organ dose was measured using TLD 100H in the RANDO phantom. The image quality of those phantoms was evaluated, using HU uniformity, HU linearity, contrast-to-noise ratio, low contrast visibility and spatial resolution with the Catphan504 dataset; and 5-point grading scales for the RANDO phantom dataset by five radiation oncologists. The image qualities from Catphan504 and RANDO phantom of every scanning path were comparable, with no statistically significant difference (p ≥ 0.05). However, there was a significant difference in the critical organ dose in all paths (p < 0.05), depending on the critical organ location and the scanning direction. Scanning directions show no effects on the image quality. Differences in absorbed dose to critical organs should were evaluated. The posterior scanning path for the CBCT was deemed preferable due because of considerably lower doses to several critical organs of the head and neck region.

Assessment of Volumetric and Dosimetric Variations for Image Guided Stereotactic Body Radiation Therapy Treatment

Assessment of Volumetric and Dosimetric Variations for Image Guided Stereotactic Body Radiation Therapy Treatment

An alternative approach to account for patient organ doses from imaging guidance procedures

PurposeTo investigate the feasibility of an alternative method of accounting for additional organ doses resulting from image guidance procedures during patient treatment planning through tabulated values based on scan protocol and scan site. Methods and materialsPatient-specific imaging dose to 30 patients resulting from Varian OBI kV–CBCT scans using the Standard Head (17 patients), Low-dose Thorax (8 patients), and Pelvic (5 patients) scan protocols were retrospectively calculated using Monte Carlo methods. Dose dependence on scan location and patient geometry was explored. Patient organ doses were analyzed by using dose–volume histograms and expressed by the mean, minimum dose delivered to 50% of the organ volume, D50. The reported doses are dose-to-medium instead of dose-to-water. ResultsThe organ doses from all patient-specific calculations show predictable and limited ranges across patients. For brain isocenters using Standard Head Scans: Bone: 0.7–1.1cGy, Brain: 0.2–0.3cGy, Brainstem: 0.2–0.3cGy, Skin: 0.3–0.4cGy, Eye: 0.03–0.3cGy. For head and neck patients using the Standard Head Scan: Bone: 0.3–0.6cGy, Parotids: 0.3–0.4cGy, Spinal Cord: 0.15–0.25cGy, Thyroid: 0.1–0.25cGy, Skin: 0.2–0.3cGy, Trachea–Esophagus: 0.1–0.2cGy. For chest using Thorax Scans: Bone: 1.1–1.8cGy, Soft tissue organs (Bowel, Lung, Heart, Kidney, Esophagus, and Spinal Cord): 0.3–0.6cGy. For abdominal site using Pelvic Scans: Bone: 3.2–4.2cGy. Soft tissue organs (Bladder, Bowel, Rectum, Prostate, and Skin) D50s fell between 1.2 and 2.2cGy. Femoral Heads: 2.5–3.4cGy. ConclusionsIt is adequate to estimate and account for organ dose by using tabulated values based on scan procedure and site because organ doses from imaging procedures are only modestly dependent upon scan location and body size. Considering the dose variation and magnitude of dose from each scan protocol in comparison to therapeutic doses, this approach provides a simple alternative to account for additional imaging guidance doses during patient treatment planning. Clinicians can use these tabulated values to make informed decisions in selecting the appropriate imaging procedures and imaging frequency during radiotherapy treatment.

SU-E-J-271: A Feasibility Study to Utilize XR-QA2 Radiochromic Films as An Alternative Tool for Measuring Beam Parameters of Kilovoltage CBCT Systems

Purpose: The beam parameters of the x-ray beams used in image guidance are difficult to measure in water due to the low exposure. This study is to investigate if XR-QA2 radiochromic films can be used in obtaining beam parameters, such as beam profiles and outputs for kV-CBCT systems. Methods: The kV-CBCT beams were from a Varian OBI system. The films used were XR-QA2-GAFCHROMICTM films placed in the middle of two Plastic -Water slab phantoms at isocenter. The scanner used to analyze the films was an Epson Expression 1680 flat-bed scanner. Pre and 24 h-post-irradiated scans of films were obtained by using the reflective mode. The corresponding pixel-by-pixel dose in the films was obtained using a dose calibration equation as a function of net reflectance from the literature. Beam profiles of different scan protocols with and without bow-tie filters were measured. Monte Carlo simulations were used to compare the accuracy of the measured dose profiles. Results: The beam profiles measured from XR-QA2 films for different clinical default kV-CBCT scan protocols were benchmarked against Monte Carlo simulated profiles which have been validated by measurements performed in water phantom. We confirmed that a minimum post-irradiated time of 24 hours is necessary to pass to scan the films to account for light scatter stability. We found that the uncertainties of the results depend on the scan orientation of the film when analyzed in different directions. Conclusion: The XR-QA2 radiochromic films are sensitive to measure the x-ray beam profiles with irradiated dose ranging 1–5 cGy. The film measurements can be performed with an easy setup compared to ion chamber measurements in a water phantom. These films provide an alternative method to obtain the beam profiles for kV-CBCT beams.

An algorithm for kilovoltage x-ray dose calculations with applications in kV-CBCT scans and 2D planar projected radiographs

A new model-based dose calculation algorithm is presented for kilovoltage x-rays and is tested for the cases of calculating the radiation dose from kilovoltage cone-beam CT (kV-CBCT) and 2D planar projected radiographs. This algorithm calculates the radiation dose to water-like media as the sum of primary and scattered dose components. The scatter dose is calculated by convolution of a newly introduced, empirically parameterized scatter dose kernel with the primary photon fluence. Several approximations are introduced to increase the scatter dose calculation efficiency: (1) the photon energy spectrum is approximated as monoenergetic; (2) density inhomogeneities are accounted for by implementing a global distance scaling factor in the scatter kernel; (3) kernel tilting is ignored. These approximations allow for efficient calculation of the scatter dose convolution with the fast Fourier transform. Monte Carlo simulations were used to obtain the model parameters. The accuracy of using this model-based algorithm was validated by comparing with the Monte Carlo method for calculating dose distributions for real patients resulting from radiotherapy image guidance procedures including volumetric kV-CBCT scans and 2D planar projected radiographs. For all patients studied, mean dose-to-water errors for kV-CBCT are within 0.3% with a maximum standard deviation error of 4.1%. Using a medium-dependent correction method to account for the effects of photoabsorption in bone on the dose distribution, mean dose-to-medium errors for kV-CBCT are within 3.6% for bone and 2.4% for soft tissues. This algorithm offers acceptable accuracy and has the potential to extend the applicability of model-based dose calculation algorithms from megavoltage to kilovoltage photon beams.