Challenges in MR-only seed localization for postimplant dosimetry in permanent prostate brachytherapy

PURPOSE

An MR-only postimplant dosimetry workflow for low dose rate (LDR) brachytherapy could reduce patient burden, improve accuracy, and improve cost efficiency. However, localization of brachytherapy seeds on MRI scans remains a major challenge for this type of workflow. In this study, we propose and validate an MR-only seed localization method and identify remaining challenges.

METHODS AND MATERIALS

The localization method was based on template matching of simulations of complex-valued imaging artifacts around metal brachytherapy seeds. The method was applied to MRI scans of 25 prostate cancer patients who underwent LDR brachytherapy and for whom postimplant dosimetry was performed after 4 weeks. The seed locations found with the MR-only method were validated against the seed locations found on CT. The circumstances in which detection errors were made were classified to gain an insight in the nature of the errors.

RESULTS

A total of 1490 of 1557 (96%) seeds were correctly detected, while 67 false-positive errors were made. The correctly detected seed locations had a high spatial accuracy with an average error of 0.8 mm compared with CT. A majority of the false positives occurred near other seeds. Most false negatives were found in either stranded configurations without spacers or near other seeds.

CONCLUSIONS

The low detection error rate and high localization accuracy obtained by the complex-valued template matching approach are promising for future clinical application of MR-only dosimetry. The most important remaining challenge is robustness with regard to configurations of multiple seeds in close vicinity, such as in strands of seeds without spacers. This issue could potentially be resolved by simulating specific configurations of multiple seeds or by constraining the treatment planning to avoid these configurations, which could make the proposed method competitive with CT-based seed localization.

Automated target tracking in kilovoltage images using dynamic templates of fiducial marker clusters

PURPOSE

Implanted fiducial markers are often used in radiotherapy to facilitate accurate visualization and localization of tumors. Typically, such markers are used to aid daily patient positioning and to verify the target's position during treatment. These markers can also provide a wealth of information regarding tumor motion, yet determining their accurate position in thousands of images is often prohibitive. This work introduces a novel, automated method for identifying fiducial markers in planar x-ray imaging.

METHODS

In brief, the method was performed as follows. First, using processed CBCT projection images, an automated routine of reconstruction, forward-projection, tracking, and stabilization generated static templates of the marker cluster at arbitrary viewing angles. Breathing data were then incorporated into the same routine, resulting in dynamic templates dependent on both viewing angle and breathing motion. Finally, marker clusters were tracked using normalized cross correlations between templates (either static or dynamic) and CBCT projection images. To quantify the accuracy of the technique, a phantom study was performed and markers were manually tracked by two users to compare the automated technique against human measurements. Then, 75 pretreatment CBCT scans of 15 pancreatic cancer patients were analyzed to test the automated technique under real-life conditions, including several challenging scenarios for tracking fiducial markers (e.g., extraneous metallic objects, field-of-view limitations, and marker migration).

RESULTS

In phantom and patient studies, for both static and dynamic templates, the method automatically tracked visible marker clusters in 100% of projection images. For scans in which a phantom exhibited 0D, 1D, and 3D motion, the automated technique showed median errors of 39 μm, 53 μm, and 93 μm, respectively. Human precision was worse in comparison; median interobserver differences for single markers and for the averaged coordinates of four markers were 183 μm and 120 μm, respectively. In patient scans, the method was robust against a number of confounding factors. Automated tracking was performed accurately despite the presence of radio-opaque, nonmarker objects (e.g., metallic stents, surgical clips) in five patients. This success was attributed to the distinct appearance of clusters as a whole compared to individual markers. Dynamic templates produced higher cross-correlation scores than static templates in patients whose fiducial marker clusters exhibited considerable deformation or rotation during the breathing cycle. For other patients, no significant difference was seen between dynamic and static templates. Additionally, transient differences in the cross-correlation score identified instances where markers disappeared from view.

CONCLUSIONS

A novel, automated method for producing dynamic templates of fiducial marker clusters has been developed. Production of these templates automatically provides measurements of tumor motion that occurred during the CBCT scan that was used to produce them. Additionally, using these templates with intrafractional images could potentially allow for more robust real-time target tracking in radiotherapy.

MRI-BASED PROSTATE BRACHYTHERAPY SEED LOCALIZATION

A magnetic resonance imaging (MRI) pulse sequence and a corresponding image processing algorithm to localize prostate brachytherapy seeds during or after therapy are presented. Inversion-Recovery with ON-resonant water suppression (IRON) is an MRI methodology that generates positive contrast in regions of magnetic field susceptibility, as created by prostate brachytherapy seeds. Phantoms comprising of several materials found in brachytherapy seeds were created to assess the usability of the IRON pulse sequence for imaging seeds. Resulting images show that seed materials are clearly visible with high contrast using IRON, agreeing with theoretical predictions. A seed localization algorithm to process IRON images demonstrates the potential of this imaging technique for seed localization and dosimetry.

A magnetic resonance-based seed localization method for I-125 prostate implants

This study was performed to develop and evaluate a semi-automatic seed localization algorithm from magnetic resonance (MR) images for interstitial prostate brachytherapy. The computerized tomography (CT) and MR images (3 mm-slice thickness) of six patients who had received real-time MR imaging-guided interstitial prostate brachytherapy were obtained. An automatic seed localization method was performed on CT images to obtain seed coordinates, and an algorithm for seed localization from MR images of the prostate was developed and tested. The resultant seed distributions from MR images were then compared to CT-derived distribution by matching the same seeds and calculating percent volume receiving 100% of the prescribed dose and the extent of the volume in 3-dimensions. The semiautomatic seed localization method made it possible to extract more than 90% of the seeds with either less than 8% of noises or 3% of missing seeds. The mean volume difference obtained from CT and MR receiving 100% of the prescribed dose was less than 3%. The maximum extent of the volume receiving the prescribed dose were 0.3, 0.6, and 0.2 cm in x, y, and z directions, respectively. These results indicate that the algorithm is very useful in identifying seeds from MR image for post-implant dosimety.