Software-controlled, highly automated intrafraction prostate motion correction with intrafraction stereographic targeting: System description and clinical results.

PURPOSE: A new system for software-controlled, highly automated correction of intrafraction prostate motion," intrafraction stereographic targeting" (iSGT), is described and evaluated. METHODS: At our institute, daily prostate positioning is routinely performed at the start of treatment beam using stereographic targeting (SGT). iSGT was implemented by extension of the SGT software to facilitate fast and accurate intrafraction motion corrections with minimal user interaction. iSGT entails megavoltage (MV) image acquisitions with the first segment of selected IMRT beams, automatic registration of implanted markers, followed by remote couch repositioning to correct for intrafraction motion above a predefined threshold, prior to delivery of the remaining segments. For a group of 120 patients, iSGT with corrections for two nearly lateral beams was evaluated in terms of workload and impact on effective intrafraction displacements in the sagittal plane. RESULTS: SDs of systematic (Σ) and random (σ) displacements relative to the planning CT measured directly after initial SGT setup correction were <0.5 and <0.8 mm, respectively. Without iSGT corrections, effective Σ and σ for the 11-min treatments would increase to Σ(eff) < 1.1 mm and σ(eff) < 1.2 mm. With the iSGT procedure with an action level of 4 mm, effective positioning errors were reduced to Σ(eff) < 0.8 mm and σ(eff) < 1.0 mm, with 23.1% of all fractions requiring a correction. Computer simulations demonstrated that with an action level of 2 mm, the errors would have been reduced to Σ(eff) < 0.6 mm and σ(eff) < 0.7 mm, requiring corrections in 82.4% of the fractions. Because iSGT is highly automated, the extra time added by iSGT is <30 s if a correction is required. CONCLUSIONS: Without increasing imaging dose, iSGT successfully reduces intrafraction prostate motion with minimal workload and increase in fraction time. An action level of 2 mm is recommended.

Day-to-day reproducibility of prostate intrafraction motion assessed by multiple kV and MV imaging of implanted markers during treatment.

PURPOSE: 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. METHODS AND MATERIALS: 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. RESULTS: 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). CONCLUSION: 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.

Margin evaluation in the presence of deformation, rotation, and translation in prostate and entire seminal vesicle irradiation with daily marker-based setup corrections.

PURPOSE: To develop a method for margin evaluation accounting for all measured displacements during treatment of prostate cancer. METHODS AND MATERIALS: For 21 patients treated with stereographic targeting marker-based online translation corrections, dose distributions with varying margins and gradients were created. Sets of possible cumulative delivered dose distributions were simulated by moving voxels and accumulating dose per voxel. Voxel motion was simulated consistent with measured distributions of systematic and random displacements due to stereographic targeting inaccuracies, deformation, rotation, and intrafraction motion. The method of simulation maintained measured correlation of voxel motions due to organ deformation. RESULTS: For the clinical target volume including prostate and seminal vesicles (SV), the probability that some part receives <95% of the prescribed dose, the changes in minimum dose, and volume receiving 95% of prescription dose compared with planning were 80.5% ± 19.2%, 9.0 ± 6.8 Gy, and 3.0% ± 3.7%, respectively, for the smallest studied margins (3 mm prostate, 5 mm SV) and steepest dose gradients. Corresponding values for largest margins (5 mm prostate, 8 mm SV) with a clinical intensity-modulated radiotherapy dose distribution were 46.5% ± 34.7%, 6.7 ± 5.8 Gy, and 1.6% ± 2.3%. For prostate-only clinical target volume, the values were 51.8% ± 17.7%, 3.3 ± 1.6 Gy, and 0.6% ± 0.5% with the smallest margins and 5.2% ± 7.4%, 1.8 ± 0.9 Gy, and 0.1% ± 0.1% for the largest margins. Addition of three-dimensional rotation corrections only improved these values slightly. All rectal planning constraints were met in the actual reconstructed doses for all studied margins. CONCLUSION: We developed a system for margin validation in the presence of deformations. In our population, a 5-mm margin provided sufficient dosimetric coverage for the prostate. In contrast, an 8-mm SV margin was still insufficient owing to deformations. Addition of three-dimensional rotation corrections was of minor influence.

Deformation of prostate and seminal vesicles relative to intraprostatic fiducial markers.

PURPOSE: To quantify the residual geometric uncertainties after on-line corrections with intraprostatic fiducial markers, this study analyzed the deformation of the prostate and, in particular, the seminal vesicles relative to such markers. PATIENTS AND METHODS: A planning computed tomography (CT) scan and three repeat CT scans were obtained for 21 prostate cancer patients who had had three to four cylindrical gold markers placed. The prostate and whole seminal vesicles (clinical target volume [CTV]) were delineated on each scan at a slice thickness of 1.5 mm. Rigid body transformations (translation and rotation) mapping the markers onto the planning scan positions were obtained. The translation only (T(only)) or both translation and rotation were applied to the delineated CTVs. Next, the residue CTV surface displacements were determined using nonrigid registration of the delineated contours. For translation and rotation of the CTV, the residues represented deformation; for T(only), the residues stemmed from deformation and rotation. T(only) represented the residues for most currently applied on-line protocols. The patient and population statistics of the CTV surface displacements were calculated. The intraobserver delineation variation was similarly quantified using repeat delineations for all patients and corrected for. RESULTS: The largest CTV deformations were observed at the anterior and posterior side of the seminal vesicles (population average standard deviation

Stereographic targeting in prostate radiotherapy: speed and precision by daily automatic positioning corrections using kilovoltage/megavoltage image pairs.

PURPOSE: A fully automated, fast, on-line prostate repositioning scheme using implanted markers, kilovoltage/megavoltage imaging, and remote couch movements has been developed and clinically applied. The initial clinical results of this stereographic targeting (SGT) method, as well as phantom evaluations, are presented. METHODS AND MATERIALS: Using the SGT method, portal megavoltage images are acquired with the first two to six monitor units of a treatment beam, immediately followed by acquisition of an orthogonal kilovoltage image without gantry motion. The image pair is automatically analyzed to obtain the marker positions and three-dimensional prostate displacement and rotation. Remote control couch shifts are applied to correct for the displacement. The SGT performance was measured using both phantom images and images from 10 prostate cancer patients treated using SGT. RESULTS: With phantom measurements, the accuracy of SGT was 0.5, 0.2, and 0.3 mm (standard deviation [SD]) for the left-right, craniocaudal, and anteroposterior directions, respectively, for translations and 0.5 degrees (SD) for the rotations around all axes. Clinically, the success rate for automatic marker detection was 99.5%, and the accuracy was 0.3, 0.5 and 0.8 mm (SD) in the left-right, craniocaudal, and anteroposterior axes. The SDs of the systematic center-of-mass positioning errors (Sigma) were reduced from 4.0 mm to <0.5 mm for all axes. The corresponding SD of the random (sigma) errors was reduced from 3.0 to <0.8 mm. These small residual errors were achieved with a treatment time extension of <1 min. CONCLUSION: Stereographic targeting yields systematic and random prostate positioning errors of <1 mm with <1 min of added treatment time.