Feasibility of an MRI-only workflow for postimplant dosimetry of low-dose-rate prostate brachytherapy: Transition from phantoms to patients
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PURPOSE: The lack of positive contrast from brachytherapy seeds in conventional MR images remains a major challenge toward an MRI-only workflow for postimplant dosimetry of low-dose-rate brachytherapy. In this work, the feasibility of our recently proposed MRI-only workflow in clinically relevant scenarios is investigated and the necessary modifications in image acquisition and processing pipeline are proposed for transition to the clinic. METHODS AND MATERIALS: Four prostate phantoms with a total of 321 I-125 implanted dummy seeds and three patients with a total of 168 implanted seeds were scanned using a gradient echo sequence on 1.5 T and 3T MR scanners. Quantitative susceptibility mapping (QSM) was performed for seed visualization. Before QSM, the seed-induced distortion correction was performed followed by edge enhancement. Seed localization was performed using spatial clustering algorithms and was compared with CT. In addition, feasibility of the proposed method on detection of prostatic calcifications was studied. RESULTS: The proposed susceptibility-based algorithm generated consistent positive contrast for the seeds in phantoms and patients. All the 321 seeds in the four phantoms were correctly identified; the MR-derived seeds centroids agreed well with CT-derived positions (average error = 0.5 ± 0.3 mm). The proposed algorithm for seed visualization was found to be orientation invariant. In patient cases, all seeds were visualized and correctly localized (average error = 1.2 ± 0.9 mm); no significant differences between dose volume histogram parameters were found. Prostatic calcifications were depicted with negative contrast on QSM and spatially agreed with CT. CONCLUSIONS: The proposed MRI-based approach has great potential to replace the current CT-based practices. Additional patient studies are necessary to further optimize and validate the workflow.
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