Lateral kV Research Articles

Evaluation of the voluntary deep inspiration breath-hold reproducibility in left-sided breast radiotherapy.

Voluntary deep inspiration breath hold (v-DIBH) reduces cardiac dose during left-sided breast irradiation. The purpose of this study is to evaluate the reproducibility and variability of breath-hold level (BHL) using breath-hold curves and lateral kV setup images together. A retrospective analysis of 30 left breast cancer patients treated using the v-DIBH technique in our department is performed. The BHL difference is measured from breath hold curves and lateral (LAT) kilo-Voltage (kV) setup images. The planning CT image and the selected treatment fraction data are collected. If the changes in BHL relate to the displacement of various bones in the kV setup, images are assessed. Furthermore, the maximum heart distance inside the treatment field is compared from LAT MV portal images. The median and mean values of the BHL are nearly identical in different fractions (good reproducibility). However, the mean BHL values between planning and all measured fractions are statistically different; 16.3 vs. 20.8 mm for the planning and measured fractions (p < 0.001), which indicates that the variability of BHL is significantly different. While reproducibility testing shows good agreement for inter-fractional breath-hold level, the variability between planning and fractions is relatively poor.

Intrafraction motion monitoring to determine PTV margins in early stage breast cancer patients receiving neoadjuvant partial breast SABR

Background and purposeTo quantify intra-fraction tumor motion using imageguidance and implanted fiducial markers to determine if a 5 mm planning-target-volume (PTV) margin is sufficient for early stage breast cancer patients receiving neoadjuvant stereotactic ablative radiotherapy (SABR). Materials and methodsA HydroMark© (Mammotome) fiducial was implanted at the time of biopsy adjacent to the tumor. Sixty-one patients with 62 tumours were treated prone using a 5 mm PTV margin. Motion was quantified using two methods (separate patient groups): 1) difference in 3D fiducial position pre- and post-treatment cone-beam CTs (CBCTs) in 18 patients receiving 21 Gy/1fraction (fx); 2) acquiring 2D triggered-kVimages to quantify 3D intra-fraction motion using a 2D-to-3D estimation method for 44 tumours receiving 21 Gy/1fx (n = 22) or 30 Gy/3fx (n = 22). For 2), motion was quantified by calculating the magnitude of intra-fraction positional deviation from the pretreatment CBCT. PTV margins were derived using van Herkian analysis. ResultsThe average ± standard deviation magnitude of motion across patients was 1.3 ± 1.15 mm Left/Right (L/R), 1.0 ± 0.9 mm Inferiorly/Superiorly (I/S), and 1.8 ± 1.5 mm Anteriorly/Posteriorly (A/P). 85/105 (81%) treatment fractions had dominant anterior motion. 6/62patients (9.7%) had mean intra-fraction motion during any fraction > 5 mm in any direction, with 4 in the anterior direction. Estimated PTV margins for single and three-fx patients in the L/R, I/S, and A/P directions were 6.0x4.1x5.9 mm and 4.5x2.9x4.3 mm, respectively. ConclusionOur results suggest that a 5 mm PTV margin is sufficient for the I/S and A/P directions if a lateral kV image is acquired immediately before treatment. For the L/R direction, either further immobilization or a larger margin is required.

Fiducial markers visibility and artefacts in prostate cancer radiotherapy multi-modality imaging

BackgroundIn this study, a novel pelvic phantom was developed and used to assess the visibility and presence of artefacts from different types of commercial fiducial markers (FMs) on multi-modality imaging relevant to prostate cancer.Methods and materialsThe phantom was designed with 3D printed hollow cubes in the centre. These cubes were filled with gel to mimic the prostate gland and two parallel PVC rods were used to mimic bones in the pelvic region. Each cube was filled with gelatine and three unique FMs were positioned with a clinically-relevant spatial distribution. The FMs investigated were; Gold Marker (GM) CIVCO, GM RiverPoint, GM Gold Anchor (GA) line and ball shape, and polymer marker (PM) from CIVCO. The phantom was scanned using several imaging modalities typically used to image prostate cancer patients; MRI, CT, CBCT, planar kV-pair, ExacTrac, 6MV, 2.5MV and integrated EPID imaging. The visibility of the markers and any observed artefacts in the phantom were compared to in-vivo scans of prostate cancer patients with FMs.ResultsAll GMs were visible in volumetric scans, however, they also had the most visible artefacts on CT and CBCT scans, with the magnitude of artefacts increasing with FM size. PM FMs had the least visible artefacts in volumetric scans but they were not visible on portal images and had poor visibility on lateral kV images. The smallest diameter GMs (GA) were the most difficult GMs to identify on lateral kV images.ConclusionThe choice between different FMs is also dependent on the adopted IGRT strategy. PM was found to be superior to investigated gold markers in the most commonly used modalities in the management of prostate cancer; CT, CBCT and MRI imaging.

Effect of patient size on image quality in radiotherapy kV planar verification imaging: a phantom study.

IntroductionThis study aimed to determine a maximal pelvic separation and waist circumference in pelvic patients to guide radiation therapists in acquiring kilovoltage (kV) planar images of acceptable quality for treatment verification.MethodsA pelvic anthropomorphic phantom modified with different bolus thicknesses was imaged at various default kV exposure settings. Radiation therapists rated image quality and acceptance/rejection of these images for treatment verification.ResultsSixteen radiation therapists participated in the study. Image quality was inversely proportional to phantom size. AP and lateral kV images were acceptable for treatment verification up to a waist circumference of 143 cm.ConclusionsExposure settings for kV image verification of large patients should be individualised to avoid unnecessary patient radiation dose through repeated imaging.

Improving the reproducibility of voluntary deep inspiration breath hold technique during adjuvant left-sided breast cancer radiotherapy

Background: Adjuvant radiotherapy (RT) of left-sided breast cancer (LSBC) with voluntary deep inspiration breath hold (vDIBH) technique reduces the cardiac dose. In this study, the effect of marker block position and the efficacy of breath hold level (BHL) correction based on lateral kV setup images are evaluated to improve the daily reproducibility. Material and methods: A total of 148 consecutive LSBC patients treated with vDIBH RT were included in this study. The real-time position management (RPM) marker block was placed on the abdominal wall in 63 patients (group A) and on the sternum in 85 patients (group S). Acquired 900 (group A) + 1040 (group S) orthogonal image pairs were retrospectively analyzed. The actual BHL was determined from the lateral kV images. The height of the BHL gating window in RPM was corrected if errors of the actual BHL exceeded 4 mm. Setup margins were calculated for the chest wall and for bony surrogates of the lymph node regions. Results: The sternal marker block reduced the random residual errors in the actual BHL (p < 0.05). The BHL correction was required for 26/63 patients in group A and for 26/85 patients in group S. Correction of the BHL window significantly reduced both the systematic and the random residual error in both groups. In patients with lymph node irradiation, the effect of both marker placement and BHL window correction was significant in the superior-inferior direction. Correction of the BHL reduced the mean cardiac dose by 0.5 Gy (p < 0.01) in group A and 0.6 Gy (p < 0.05) in group S. Conclusions: Reproducibility of the BHL can be improved by placing the marker block on the sternum and correcting the height of the BHL window based on lateral kV setup images. Acquisition of lateral kV images in the first 3 fractions and once a week during RT is recommended.

SU‐E‐T‐633: Preparation and Planning of a VMAT Multi ‐ Arc Radiation Therapy Technique for Full Scalp Treatment

Purpose:The target volume for angiosarcoma of the scalp encompasses the entire scalp. Full scalp radiotherapy (FSRT) requires careful design of required bolus, immobilization and marking of the field before the patient CT is acquired. A VMAT multi‐arc technique was designed to deliver FSRT for a patient with angiosarcoma of the scalp to a dose of 6000cGy in 25 fractions.Methods:A custom bolus helmet was fabricated from a 0.5 cm thick sheet of aquaplast material, which was molded to the patient's head. With the bolus helmet in place the patient was then positioned supine on a H&amp;N immobilization board. A custom vaclock bag positioned on a standard headrest and a thermoplastic mask were used to immobilize the patient. Additional bolus to cover the remaining treatment area was attached to the mask. We acquired two CT scans of the patient's head, one in treatment position and an additional scan without the immobilization mask with wires marking the treatment area that the oncologist had delineated on the patient's skin. The second scan was registered to the first and used to define the treatment CTV. A four‐arc VMAT treatment planned using Varian‐Eclipse was optimized to cover the skin with a PTV margin while sparing the brain and limiting the dose to the optic apparatus and lacrimal glands. Daily treatment setup was verified using anterior and lateral kV on‐board‐imaging. To verify the treated dose, TLDs were positioned on the patient's scalp during one fraction.Results:With full dose coverage to the PTV, the mean dose to the brain was less than 24 Gy. The dose measured by the TLDs (mean difference 1%, standard deviation 4%)showed excellent agreement with the treatment planning calculation.Conclusion:FSRT delivered with a bolus helmet and a VMAT multi‐arc technique can be accurately delivered with high dose uniformity and conformality.

Analysis and Reduction of Daily kV Planar Imaging Dose in IGRT Patients

Analysis and Reduction of Daily kV Planar Imaging Dose in IGRT Patients

SU-E-J-39: Minimizing IGRT Imaging Exposures: KV Radiograph Vs. KV-CBCT Vs. MV Portal Images.

To compare the IGRT doses from MV, kV and CBCT images. kV imaging systems integrated into Varian Trilogy and TrueBeam accelerators were modeled using BEAMnrc/DOSXYZnrc Monte Carlo codes and the dose to calibration phantoms for a variety of kV beams(kVp, bow-tie filters, etc.) were calculated. The doses to the same phantoms and kV beams were then measured experimentally using calibrated ion-chambers.The "calibrated" Monte Carlo kV beams were used to calculate dose to CT images of patients. Organ doses were analyzed using DVHs. The doses to the prostate are 0.015 and 2.2cGy using AP kV and MV images; are 0.06 and 2.3cGy using lateral kV and MV images; and, are 1.7 cGy using CBCT images. For head and neck images, the doses to the eye are 0.08 and 0.001 cGy using AP and PA kV images; are 2.3 and 1.8cGy using AP and PA MV images; are 0.001 and 2.4cGy for lateral kV and MV images; and, are 0.2 cGy for CBCT images. For kV radiographs, organ doses can be further reduced, by over 30%, by using bow-tie filters. CBCT doses to the prostate are 1.6 and 0.9cGy for OBI and TrueBeam pelvis scans; a >40% dose reduction for the same image quality. For OBI CBCT head scans the doses to the eye and brain stem are 0.2 and 2.8cGy, respectively. Due to the low penetration of kV beams,selecting beam angles so that the sensitive organs are near the beam exit, and/or using bow-tie filters, can substantially reduce organ doses when using kV radiographs. For daily positioning of pediatric brain patient with a set of orthogonal kV images, a CBCT scan, or a set of orthogonal MV images, the doses to the eyes are 0.1, 0.2, and 4.7 cGy, respectively.

Fiducial markers in prostate for kV imaging: quantification of visibility and optimization of imaging conditions

The purpose of this work is to investigate possible smaller, less-dense fiducial markers implantable into the prostate for target localization and patient repositioning verification in an on-board kV–kV imaging system on a proton gantry. The experiments used a pelvic phantom and a variety of commercially available fiducial markers: CIVCO carbon marker of ϕ; 1 × 3 mm, gold seed markers of ϕ; 0.8 × 3 mm and ϕ; 1.2 × 3 mm, and IBA Visicoil helical gold linear markers in diameters of 0.35, 0.50, 0.75 and 1.15 mm. Two orthogonal on-board kV imagers were arranged for digital radiographic imaging of the phantom through the lateral and anterior–posterior directions. The contrast-to-noise ratio (CNR) for a given marker was calculated and used as a quantitative measure of its visibility. The patient entrance skin exposure (ESE) was measured and parameterized for kVp, mAs and source-to-surface distance. The ratio of CNR to ESE was first introduced to characterize the efficiency for imaging a marker using a given x-ray technique in order to optimize the marker's visibility and simultaneously minimize the x-ray imaging dose. If CNR > 2, which corresponds to a significance p < 0.05, is required for acceptable visibility, the carbon marker and the smallest Visicoil marker are not suitable for imaging through dense bone but the others are capable of being employed in the clinic. It is predicted that other markers in development should have a greater thickness than equivalent of 0.14 mm thick gold in order to produce the acceptable visibility in the lateral kV imaging. The linear Visicoil marker of ϕ; 0.50 × 5 mm is most suitable for kV imaging in the prostate for proton therapy as it induces the least proton dose perturbation amongst the acceptable markers. An optimal range of 120–130 kVp and 40–80 mAs is determined using the maximal CNR/ESE and CNR > 2 for laterally imaging this marker in the prostate.

Day-to-Day Reproducibility of Prostate Intrafraction Motion Assessed by Multiple kV and MV Imaging of Implanted Markers During Treatment

When one is performing online setup correction for prostate positioning displacements prior to daily dose delivery, intrafraction motion can become a limiting factor to prostate targeting accuracy. The aim of this study was to quantify and characterize prostate intrafraction motion assessed by multiple kilovoltage (kV) and megavoltage (MV) imaging of implanted markers during treatment in a large patient group. Intrafraction motion in the sagittal plane was studied by retrospective analysis of displacements of implanted gold markers on (nearly) lateral kV and MV images obtained at various time points during the treatment fractions (mean, 27 per patient) in 108 consecutive patients. The effective prostate motion in a fraction was defined as the time-weighted mean displacement. Prostate displacements in the sagittal plane increased during the fraction (mean, 0.2 ± 0.2 mm/min). Forty percent of patients had a systematic (i.e., appearing in all fractions) effective displacement in the sagittal plane greater than 2 mm. Observed effective population mean-of-means (μeff) +/- systematic (Σeff) intrafraction motion (μ(eff) ± Σ(eff)) was 0.9 ± 1.1 mm and 0.6 ± 1.0 mm for the anterior-posterior and superior inferior directions, respectively. Corresponding random motion (σ(eff)) was 1.2 mm and 1.1 mm. Mean effective prostate motion in the first 5 fractions was predictive for mean effective displacement in the remaining fractions (p < 0.001). For a large subgroup of patients, the systematic component of intrafraction prostate motion was substantial. Intrafraction motion correction prior to each beam delivery or offline corrections could likely be beneficial for the subgroup of patients with significant motion. The systematic component is well predicted by measurements in the initial fractions.

Reduction of Daily Imaging Dose Associated with Prostate Planar kV IGRT

IGRT is the means by which the high degree of accuracy and precision required for modern radiotherapy techniques is achieved. Several recent articles in the popular press have patients concerned that too much radiation exposure, such as from daily kilo-voltage (kV) images or therapeutic doses, puts them at risk for serious adverse effects. In our clinic, daily kV images are used for patient setup verification and repositioning. The aim of this study is to determine if the kV IGRT dose may be reduced by manipulating image parameters while producing high quality images. In our clinic, image parameters (kVp, mA, and time) are based on predefined standardized protocols. To estimate the dose delivered near the surface for a kV IGRT protocol, an ion chamber was placed 0.5 cm beneath the surface of a phantom made from solid water material. Using these standard parameters, anterior and lateral kV images were taken, and the measurements from the ion chamber were converted to absorbed dose. Image parameters were then chosen to reduce the image dose by decreasing the mAs and varying the kVp. Ion chamber measurements were subsequently obtained and compared with the nominal values. Using the various imaging parameters, anterior and lateral kV images were acquired on a pelvic RANDO phantom. To quantitatively compare the pelvic images taken with different parameters, a program was written to calculate a metric known as the contrast-to-noise ratio (CNR) for each image. CNR represents the difference between a high contrast object (such as bone or fiducial markers) and the background, divided by the standard deviation (noise) of each of these regions of interest. Thus, CNR provides a quantitative evaluation of image quality. CNR values were then compared based on the imaging parameters, as well as the absorbed doses that were required to obtain each image. The absorbed dose per anterior image using standardized parameters was 0.18 cGy. The associated CNR was 2.24. By decreasing the mAs, the anterior image dose could be reduced by 39% (0.11 cGy/image) with only a 2% reduction in the CNR (2.19). For the lateral images, the absorbed dose was 1.51 cGy/image. The associated CNR was 4.24. Reducing the mAs by 49% decreased the dose near the surface to 0.77 cGy/image, while maintaining a CNR that decreased by only 1% (4.21). By altering the imaging parameters, the absorbed dose from anterior and lateral kV imaging may be greatly reduced without sacrificing image quality. Future studies will involve applying these altered parameters clinically.

SU‐GG‐J‐95: Reproducibility of a Vaginal Dilator Used for Female Anal Cancer Patients

Purpose: Dysparunia is a major source of post‐treatment morbidity among patients treated with chemoradiation for squamous cell carcinoma of the anal canal. The main goal of IMRT plans in this setting is sparing of the anterior vaginal wall, vulva, and urethra. The daily of reproducibility vaginal dilators was investigated in this study. Method and Materials: Seven patients were included in this study. Patients were placed in a frog‐legged position and immobilized with a vacloc bag. A silicone vaginal dilator measuring 2.9 cm in diameter and 11.4 cm in length (Soul Source Enterprises, Portland, OR) was inserted in each patient before simulation and each treatment fraction. Antero‐posterior (AP) and lateral kV images were used for daily set‐up on bony anatomy. BBs inserted in the dilator improved visibility on kV images. Dilator and bony anatomy positions were compared with the digitally reconstructed radiographs (DRRs) generated from the simulation CT. Results: Within the AP images, the average deviation of bony anatomy from the DRR was 0.3 ± 0.2 cm, while for the lateral images, it was 0.5 ± 0.3 cm. The 3‐dimensional position of the apex of the dilator showed strong variation, with an average deviation of 1.3 ± 0.8 cm. However, the position of the dilator axis near the tumor volume showed less deviation, 0.5 ± 0.3 cm. Variation of the dilator angle was small in the AP images (−2° ± 3°) and slightly larger in the lateral images (−3° ± 9°). Conclusion: The reproducibility of a reference point along the vaginal dilator's longitudinal axis was comparable to that of bony anatomy. While dilator depth was not as reproducible, the reproducibility of the distal dilator was excellent and could allow maximal sparing of the female genitalia in patients undergoing IMRT.

SU-FF-J-103: Intra-Fraction Prostate Motion Evaluation with On-Line Fluoroscopic Imaging

Purpose: This study attempts to quantify intra-fraction organ motion of prostate patients undergoing external beam radiotherapy. Method and Materials: Lateral kV fluoroscopic images are captured on the first 3 days of treatment and then weekly thereafter for a total of 10 sequences per patient. The fiducial markers (FM) are located in the apex, posterior aspect and base of the prostate. Each fluoro sequence is taken after 3D setup correction. Images are acquired at 5.5 fps for a duration of 30 seconds at a technique of 120kVp, 64mA, 20ms per frame. A normalized cross-correlation provides 2D localization of the three FM's in each frame. Displacement metrics were computed using the magnitude of the Euclidean distance of each FM from its initial position. Four patients are included in the study for a total of 30 fluoro sequences. The 2D vector displacements for all patient fractions in the study was computed and compared. Results: The standard deviation of the displacement distribution is (0.3, 0.9, 0.7) [mm] for the fiducial marker at the apex, posterior aspect and base of the prostate respectively. The posterior FM which is immediately adjacent to the rectum showed the largest motion. The ‘institutional mean’ (i.e. are all patients, on average, shifting in a particular direction) was quite small, with a max of 0.2 [mm] for the base FM. Conclusion: On-line kV fluoro is a useful tool for characterization of intra-fraction prostate motion supporting “evidence-based” margin design.

MO‐D‐ValB‐02: OpenGL Based 2D‐3D Registration of a CT Image Dataset With OBI Images

Introduction. A limiting factor in registration of a three‐dimensional (3D) computed tomography (CT) dataset with two‐dimensional (2D) x‐ray images has been the time‐consuming generation of digitally reconstructed radiographs (DRRs). This can be overcome using a commercial graphics card for DRR generation enabling fast, robust, and accurate automatic image‐based 2D‐3D registration. Methods and Materials. For the iterative registration process hundreds of DRRs are created using hardware rendering in OpenGL. Each DRR from a 512×512×100 CT volume is rendered in less than 0.1 seconds using an nVidia 7800GT graphics card. The registration is based on a publicly available implementation of Mattes Mutual Information (ITK, U.S. National Library of Medicine, Bethesda, MD). To improve speed, the registration is performed on a sub‐image. A two‐step registration strategy is adopted for robustness with the first pass using a larger margin around the sub‐image and a down‐sampled resolution. A thoracic phantom (Model 602, CIRS, Norfolk, VA) was imaged and setup according to our clinical protocol and then shifted from 0–1.5 cm along each of the major axes. Anterior‐Posterior and Lateral kV x‐ray images were acquired using a commercial patient imaging system (OBI, Varian Medical System, Palo Alto, CA). Results: The mean registration times were 8 and 16 seconds without and with rotations respectively. We observed a systematic 1.1 mm offset in the longitudinal direction that we believe results from 2.5 mm CT slice spacing and OBI calibration. With this removed the mean three‐dimensional distance of the registered positions from the phantom positions was 0.4 mm with the largest disagreement being 0.75 mm. The systems ability to calculate rotations was only tested numerically.Conclusions. The speed and accuracy of this system demonstrate that it could be a viable tool for reducing daily setup uncertainty by automating the analysis of setup images.Research sponsored by Phillips Medical Systems.