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

Abstract

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 complexvalued 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 CTbased seed localization.

Improving fiducial and prostate capsule visualization for radiotherapy planning usingMRI

“…There have been a number of studies investigating dedicated MRI sequences for fiducial detection . Both balanced steady‐state free precession sequences and sequences based on spoiled gradient‐echoes have been employed in 2D and 3D acquisitions, relying on T2*‐related signal loss to create a detectable signal void in the vicinity of the fiducials. The averaging of consecutive echoes in multi‐echo recalled sequences, such as the one used here is an attractive mechanism to increase the signal‐to‐noise ratio.…”

“…Studies so far for similar sequences have focused on accuracy of fiducial detection . In this article we examine the T2*W sequence and investigate whether it is possible to use this sequence alone in prostate studies, considering geometric integrity, the ability to locate marker positions and the ability to provide enough contrast for prostate volume outlining.…”

Deformable registration of x ray and MRI for postimplant dosimetry in low dose rate prostate brachytherapy

“…The first is MRI‐based seed localization. Metallic seeds do not generate signal intensity using conventional pulse sequences, so appear as signal voids leading to some nonseed features identified as seeds (false positives) and some missing seeds (false negatives) . Seeds designed to generate positive contrast have been demonstrated, but are not typically integrated with isotopes used for therapy .…”

“…Metallic seeds do not generate signal intensity using conventional pulse sequences, so appear as signal voids leading to some nonseed features identified as seeds (false positives) and some missing seeds (false negatives). 14,15 Seeds designed to generate positive contrast have been demonstrated, but are not typically integrated with isotopes used for therapy. 16 Balanced steady-state free precession (bSSFP) pulse sequences can improve MRI quality for seed identification 17 and susceptibility-based pulse sequences have been developed to provide positive contrast with conventional seeds.…”

Postimplant Dosimetry of Permanent Prostate Brachytherapy: Comparison of MRI-Only and CT-MRI Fusion-Based Workflows

“…Zijlstra et al have studied the feasibility of a template-matching algorithm that uses the simulated magnetic field distortions around the seeds as the template to match with those in patients; although the proposed method has acceptable performance in detection of spaced seeds (average error of 0.8 AE 0.4 mm), one-third of nonspaced or clumped seeds were not correctly identified. 10 Martin et al have compared the MRI-only LDR postimplant dosimetry (using stranded seeds with cobaltdichloride-N-acetyl cysteine [C4] MR-markers as spacers) to the standard MR-CT fusion-based approach. 11,12 They have demonstrated that MRI-only dosimetry using C4 MR markers is feasible and accurate; however, there are some limitations associated with C4 application such as higher cost compared with conventional seeds, the necessity of endorectal coil for marker visualization, and nonapplicability to plans with nonspaced or loose seeds.…”