Winnie Li

Cone-Beam Computed Tomographic Image Guidance for Lung Cancer Radiation Therapy

PURPOSE To determine the geometric accuracy of lung cancer radiotherapy using daily volumetric, cone-beam CT (CBCT) image guidance and online couch position adjustment. METHODS AND MATERIALS Initial setup accuracy using localization CBCT was analyzed in three lung cancer patient cohorts. The first (n = 19) involved patients with early-stage non-small-cell lung cancer (NSCLC) treated using stereotactic body radiotherapy (SBRT). The second (n = 48) and third groups (n = 20) involved patients with locally advanced NSCLC adjusted with manual and remote-controlled couch adjustment, respectively. For each group, the couch position was adjusted when positional discrepancies exceeded +/-3 mm in any direction, with the remote-controlled couch correcting all three directions simultaneously. Adjustment accuracy was verified with a second CBCT. Population-based setup margins were derived from systematic (Sigma) and random (sigma) positional errors for each group. RESULTS Localization imaging demonstrates that 3D positioning errors exceeding 5 mm occur in 54.5% of all delivered fractions. CBCT reduces these errors; post-correction Sigma and sigma ranged from 1.2 to 1.9 mm for Group 1, with 82% of all fractions within +/-3 mm. For Group 2, Sigma and sigma ranged between 0.8 and 1.8 mm, with 76% of all treatment fractions within +/-3 mm. For Group 3, the remote-controlled couch raised this to 84%, and Sigma and sigma were reduced to 0.4 to 1.7 mm. For each group, the postcorrection setup margins were 4 to 6 mm, 3 to 4 mm, and 2 to 3 mm, respectively. CONCLUSIONS Using IGRT, high geometric accuracy is achievable for NSCLC patients, potentially leading to reduced PTV margins, improved outcomes and empowering adaptive radiation therapy for lung cancer.

Impact of Immobilization on Intrafraction Motion for Spine Stereotactic Body Radiotherapy Using Cone Beam Computed Tomography

PURPOSE Spine stereotactic body radiotherapy (SBRT) involves tight planning margins and steep dose gradients to the surrounding organs at risk (OAR). This study aimed to assess intrafraction motion using cone beam computed tomography (CBCT) for spine SBRT patients treated using three immobilization devices. METHODS AND MATERIALS Setup accuracy using CBCT was retrospectively analyzed for 102 treated spinal metastases in 84 patients. Thoracic and lumbar spine patients were immobilized with either an evacuated cushion (EC, n = 24) or a semirigid vacuum body fixation (BF, n = 60). For cases treated at cervical/upper thoracic (thoracic [T]1-T3) vertebrae, a thermoplastic S-frame (SF) mask (n = 18) was used. Patient setup was corrected by using bony anatomy image registration and couch translations only (no rotation corrections) with shifts confirmed on verification CBCTs. Repeat imaging was performed mid- and post-treatment. Patient translational and rotational positioning data were recorded to calculate means, standard deviations (SD), and corresponding margins ± 2 SD for residual setup errors and intrafraction motion. RESULTS A total of 355 localizations, 333 verifications, and 248 mid- and 280 post-treatment CBCTs were analyzed. Residual translations and rotations after couch corrections (verification scans) were similar for all immobilization systems, with SDs of 0.6 to 0.9 mm in any direction and 0.9° to 1.6°, respectively. Margins to encompass residual setup errors after couch corrections were within 2 mm. Including intrafraction motion, as measured on post-treatment CBCTs, SDs for total setup error in the left-right, cranial-caudal, and anterior-posterior directions were 1.3, 1.2, and 1.0 mm for EC; 0.9, 0.7, and 0.9 mm for BF; and 1.3, 0.9, and 1.1 mm for SF, respectively. The calculated margins required to encompass total setup error increased to 3 mm for EC and SF and remained within 2 mm for BF. CONCLUSION Following image guidance, residual setup errors for spine SBRT were similar across three immobilization systems. The BF device resulted in the least amount of intrafraction motion, and based on this device, we justify a 2-mm margin for the planning OAR and target volume.

Performance of a Novel Repositioning Head Frame for Gamma Knife Perfexion and Image-Guided Linac-Based Intracranial Stereotactic Radiotherapy

PURPOSE To evaluate the geometric positioning and immobilization performance of a vacuum bite-block repositioning head frame (RHF) system for Perfexion (PFX-SRT) and linac-based intracranial image-guided stereotactic radiotherapy (SRT). METHODS AND MATERIALS Patients with intracranial tumors received linac-based image-guided SRT using the RHF for setup and immobilization. Three hundred thirty-three fractions of radiation were delivered in 12 patients. The accuracy of the RHF was estimated for linac-based SRT with online cone-beam CT (CBCT) and for PFX-SRT with a repositioning check tool (RCT) and offline CBCT. The RCTs ability to act as a surrogate for anatomic position was estimated through comparison to CBCT image matching. Immobilization performance was evaluated daily with pre- and postdose delivery CBCT scans and RCT measurements. RESULTS The correlation coefficient between RCT- and CBCT-reported displacements was 0.59, 0.75, 0.79 (Right, Superior, and Anterior, respectively). For image-guided linac-based SRT, the mean three-dimensional (3D) setup error was 0.8 mm with interpatient (Sigma) and interfraction (sigma) variations of 0.1 and 0.4 mm, respectively. For PFX-SRT, the initial, uncorrected mean 3D positioning displacement in stereotactic coordinates was 2.0 mm, with Sigma = 1.1 mm and sigma = 0.8 mm. Considering only RCT setups <1mm (PFX action level) the mean 3D positioning displacement reduced to 1.3 mm, with Sigma = 0.9 mm and sigma = 0.4 mm. The largest contributing systematic uncertainty was in the superior-inferior direction (mean displacement = -0.5 mm; Sigma = 0.9 mm). The largest mean rotation was 0.6 degrees in pitch. The mean 3D intrafraction motion was 0.4 +/- 0.3 mm. CONCLUSION The RHF provides excellent immobilization for intracranial SRT and PFX-SRT. Some small systematic uncertainties in stereotactic positioning exist and must be considered when generating PFX-SRT treatment plans. The RCT provides reasonable surrogacy for internal anatomic displacement.

Effect of Image-Guidance Frequency on Geometric Accuracy and Setup Margins in Radiotherapy for Locally Advanced Lung Cancer

PURPOSE To assess the relative effectiveness of five image-guidance (IG) frequencies on reducing patient positioning inaccuracies and setup margins for locally advanced lung cancer patients. METHODS AND MATERIALS Daily cone-beam computed tomography data for 100 patients (4,237 scans) were analyzed. Subsequently, four less-than-daily IG protocols were simulated using these data (no IG, first 5-day IG, weekly IG, and alternate-day IG). The frequency and magnitude of residual setup error were determined. The less-than-daily IG protocols were compared against the daily IG, the assumed reference standard. Finally, the population-based setup margins were calculated. RESULTS With the less-than-daily IG protocols, 20-43% of fractions incurred residual setup errors ≥ 5 mm; daily IG reduced this to 6%. With the exception of the first 5-day IG, reductions in systematic error (∑) occurred as the imaging frequency increased and only daily IG provided notable random error (σ) reductions (∑ = 1.5-2.2 mm, σ = 2.5-3.7 mm; ∑ = 1.8-2.6 mm, σ = 2.5-3.7 mm; and ∑ = 0.7-1.0 mm, σ = 1.7-2.0 mm for no IG, first 5-day IG, and daily IG, respectively. An overall significant difference in the mean setup error was present between the first 5-day IG and daily IG (p < .0001). The derived setup margins were 5-9 mm for less-than-daily IG and were 3-4 mm with daily IG. CONCLUSION Daily cone-beam computed tomography substantially reduced the setup error and could permit setup margin reduction and lead to a reduction in normal tissue toxicity for patients undergoing conventionally fractionated lung radiotherapy. Using first 5-day cone-beam computed tomography was suboptimal for lung patients, given the inability to reduce the random error and the potential for the systematic error to increase throughout the treatment course.

Setup Reproducibility for Thoracic and Upper Gastrointestinal Radiation Therapy: Influence of Immobilization Method and On-Line Cone-Beam CT Guidance

We report the setup reproducibility of thoracic and upper gastrointestinal (UGI) radiotherapy (RT) patients for 2 immobilization methods evaluated through cone-beam computed tomography (CBCT) image guidance, and present planning target volume (PTV) margin calculations made on the basis of these observations. Daily CBCT images from 65 patients immobilized in a chestboard (CB) or evacuated cushion (EC) were registered to the planning CT using automatic bony anatomy registration. The standardized region-of-interest for matching was focused around vertebral bodies adjacent to tumor location. Discrepancies >3 mm between the CBCT and CT datasets were corrected before initiation of RT and verified with a second CBCT to assess residual error (usually taken after 90 s of the initial CBCT). Positional data were analyzed to evaluate the magnitude and frequencies of setup errors before and after correction. The setup distributions were slightly different for the CB (797 scans) and EC (757 scans) methods, and the probability of adjustment at a 3-mm action threshold was not significantly different (p = 0.47). Setup displacements >10 mm in any direction were observed in 10% of CB fractions and 16% of EC fractions (p = 0.0008). Residual error distributions after CBCT guidance were equivalent regardless of immobilization method. Using a published formula, the PTV margins for the CB were L/R, 3.3 mm; S/I, 3.5 mm; and A/P, 4.6 mm), and for EC they were L/R, 3.7 mm; S/I, 3.3 mm; and A/P, 4.6 mm. In the absence of image guidance, the CB slightly outperformed the EC in precision. CBCT allows reduction to a single immobilization system that can be chosen for efficiency, logistics, and cost. Image guidance allows for increased geometric precision and accuracy and supports a corresponding reduction in PTV margin.

Accuracy of automatic couch corrections with on-line volumetric imaging.

The purpose of this study was to characterize automatic remote couch adjustment and to assess the accuracy of automatic couch corrections following localization with cone‐beam CT (CBCT). Automatic couch movement was evaluated through passive reflector markers placed on a phantom, tracked with an optical tracking system (OTS). Repeated couch movements in the lateral, cranial/caudal, and vertical directions were monitored through the OTS to assess velocity and response time. In conjunction with CBCT, remote table movement for patient displacements following initial setup was available on four linear accelerators (Elekta Synergy). After the initial CBCT scan assessment, patients with isocenter displacements that exceeded clinical protocol tolerances were corrected using remote automatic couch movement. A verification CBCT scan was acquired after any remote movements. These verification CBCT datasets were assessed for the following time periods: one month post clinical installation, and six months later to monitor remote couch correction stability. Residual error analysis was evaluated using the verification scans. The mean ± standard deviations (μ±σ) of couch movement based on phantom measurements with the OTS were 0.16±0.48mm,0.32±0.30mm,0.11±0.12mm in the L/R, A/P, and S/I couch directions, respectively. The fastest maximum velocity was observed in the inferior direction at 10.5 mm/s, and the slowest maximum velocity in the left direction at 3.6 mm/s. From 1134 verification CBCT registrations for 207 patients, the residual error for each translational direction from each month evaluated are reported. The μ was less than 0.3 mm in all directions, and σ was in the order of 1 mm. At a 3 mm threshold, 21 of the 1134 fractions (2%) exceeded tolerance, attributed to patient intrafraction movement. Remote automatic couch movement is reliable and effective for adjusting patient position with a precision of approximately 1 mm. Patient residual error observed in this study demonstrates that displacement is minimal after remote couch adjustment. PACS number: 87.55.Qr, 87.56.bd, 87.57.Q

Geometric Performance and Efficiency of an Optical Tracking System for Daily Pre-treatment Positioning in Pelvic Radiotherapy Patients

The purpose of this study was to characterize the accuracy of a novel in-house optical tracking system (OTS), and to determine its efficiency for daily pre-treatment positioning of pelvic radiotherapy patients compared to conventional optical distance indicator (ODI) methodology. The OTS is comprised of a passive infrared stereoscopic camera, and custom control software for use in assisting radiotherapy patient setup. Initially, the system was calibrated and tested for stability inside a radiation therapy treatment room. Subsequently, under an ethics approved protocol, the clinical efficiency of the OTS was compared to conventional ODI setup methodology through 17 pelvic radiotherapy patients. Differences between orthogonal source-to-skin distance (SSD) readings and overall set-up time resultant from both systems were compared. The precision of the OTS was 0.01 ± 0.01 mm, 0.02 ± 0.02 mm, and −0.01 ± 0.06 mm in the left/right (L/R), anterior/posterior (A/P), and cranial/caudal (C/C) directions, respectively. Discrepancies measured between the linac radiographic center in the treatment room and the calibrated origin of the camera (OTS) by two independent observers was submillimeter. Analysis of 146 fractions from 17 patients showed a high correlation between the SSD readings of the OTS and ODI setup methodologies (r = 0.99). The average time for pre-treatment positioning using the OTS couch shift calculation was 2.60 ± 0.69 minutes, and for conventional ODI setup, 3.62 ± 0.82 minutes; the difference of 1.02 minutes was statistically significant (p < 0.001). In conclusion, the OTS is a precise and robust tool for use as an independent check of treatment room patient positioning. The system is indicated as geometrically equivalent to current methods of daily pre-treatment patient positioning with potential for gains in efficiency by decreasing setup times in the treatment room.

How long does it take? An analysis of volumetric image assessment time.

This work measured time taken to integrate image-guidance with CBCTs in routine clinical practice for patients treated between 2007 and 2010 across eight linear accelerators. 117,301 CBCTs from 4592 patients across thirteen disease sites were included. The mean image assessment decision time was 79s. Decision time was correlated with setup displacement magnitude.

SU-FF-J-44: Accuracy of Automatic Couch Corrections with On-Line Volumetric Imaging

Purpose: To assess the accuracy of patient positioning using on‐line image‐guidance and automatic couch correction. Method and Materials: During device commissioning, automated movements of the robotic couch were assessed using a Rando phantom and a Polaris optical navigation system giving sub‐millimeter accuracy at approximately 18 Hz. Clinically, after assessment of a volumetric scan, a couch correction is applied if the displacement falls outside a pre‐defined action‐level of 3mm in any translational plane or greater than 5° of rotation. If the setup was outside of tolerance, the remote automatic couch was used to adjust patient position. Verification scans were acquired after the shift and the residual error analysis was evaluated retrospectively. The data from 34 patients over a one month period is included in this study, representing a total of 135 CBCT volumetric images.Results: The Rando phantoms residual error over 10 trials for a 10mm movement in the L/R, S/I, and A/P directions was 0.16±0.48mm, 0.32±0.30mm, and 0.11±0.12mm. The mean residual error for 34 patients in the L/R, S/I and A/P directions was 0.00±0.11cm, 0.01±0.10cm, and −0.03±0.08cm respectively. The rotational setup errors about the L/R, S/I and A/P axes were 0.67±1.33°, −0.47±1.30°, and −0.32±1.31°. The maximum measurement in the translational L/R, S/I and A/P directions was 0.28cm, 0.29cm, and 0.22cm, while the minimum values were −0.30cm, −0.29cm, and −0.27cm. The maximum rotational residual was 4.3°, 3.0°, and 3.1° about the L/R, S/I and A/P axes, while the minimum values were −2.2°, −3.9°, and −3.8°. Rotational errors are still apparent, as the remote couch does not correct for this. Conclusions: The remote automatic couch movement is reliable and accurate for adjusting the patient position. This gives the confidence that we can treat the intended target on a daily basis, quickly and efficiently. Research sponsored by Elekta.

Image Guided Radiation Therapy: Unlocking the Future Through Knowledge Translation

Image Guided Radiation Therapy: Unlocking the Future Through Knowledge Translation Caitlin Gillan, MRT(T), BSc, MEd, FCAMRT,*,y Meredith Giuliani, MBBS, MEd, FRCPC,*,y Nicole Harnett, MRT(T), ACT, BSc, MEd,*,y Winnie Li, MRT(T), MSc,*,y Laura A. Dawson, MD, FRCPC,*,y Mary Gospodarowicz, MD, FRCPC,*,y and David Jaffray, PhD*,y *Princess Margaret Cancer Centre; and yDepartment of Radiation Oncology, University of Toronto, Toronto, Ontario, Canada