Brachytherapy, or radioactive seed implantation, is a standard option for the curative treatment of prostate cancer. Brachytherapy involves implanting around 100 radioactive seeds directly into the prostate. Its popularity has increased due to its effectiveness, convenience, relatively low incidence of erectile dysfunction, and minimal invasiveness. 1 Radiation released from the seeds penetrates the surrounding prostate tissue at a distance of ~15 mm, with most of the radiation concentrated within the prostate. Outcomes after brachytherapy can be excellent but depend greatly on the quality of the implant: 8-year prostate-specific antigen (PSA) relapse-free survival rates of 93% for high-quality implants vs. 76% for low-quality ones have been reported. 2
The inability of standard imaging options (ultrasound, computed tomography [CT], fluoroscopy) to clearly visualize soft tissue complicates accurate assessment of the radiation dose delivered by implanted seeds. Prostate edge detection is very poor on CT images, which can result in a high degree of inter-observer variability in prostate contouring and hence in implant quality evaluation. A study estimating prostate gland volumes and subsequent quantifiers of implant adequacy highlighted the significant inter-observer differences when using post-implant CT scans. These variations led to significant differences in Dose Volume Histograms (DVH) according to the individual reviewers. 3
The efficacy of MRI for evaluating soft tissues after seed implantation has been well described. 4 The internal and external urinary sphincters in particular cannot be adequately identified with CT imaging, but they are well visualized with MRI. However, the use of MRI in prostate brachytherapy has been limited by the inability to accurately identify the seeds. All implanted seeds have the radioactive element encased within a paramagnetic titanium shell, which appears under MRI as a signal void that cannot be easily distinguished from needle tracks or blood vessels. Moreover, seeds in vascularized regions or periprostatic tissue can also be difficult to identify. In brachytherapy, where mm-level precision is necessary to limit excessive radiation dose (“hotspots”) within normal tissue and to prevent poor radiation coverage (“cold spots”) of the prostate 5, it is unlikely that adequate seed separation can be achieved with MRI detection of seeds alone.
The development of a positive-signal MRI marker to aid in seed detection and localization could be an advance for prostate brachytherapy, with the potential to improve the quality of implants. Clinicians could have more confidence in the quality of treatment delivered, and supplemental seeds could be implanted immediately if needed. In addition, effective management strategies could be immediately employed if regions are known to have inadvertently receive an excess dose. The superior image quality of MRI could improve the accuracy of prostate contouring, allow detailed appreciation of adjacent critical organs and reveal the true relationship of the implanted seeds to the prostate and critical dose-limiting structures.