Ablative therapy is a minimally invasive approach for the treatment of prostate cancer that serves as an alternative to radical prostatectomy or radiation therapy.¹ Prostate ablation, intended to help preserve organ function, is gaining in clinical application and can be performed via an array of technologies, including cryoablation, high-intensity focused ultrasound, laser ablation, and radiofrequency ablation. Although historically performed using a total or hemi-gland approach, ablation is increasingly being applied in a focal manner, solely targeting a dominant lesion that is typically defined by MRI. Strategies for the follow-up of focal ablation of prostate cancer combine serial post-procedural PSA, multiparametric MRI examinations, and biopsy, although the optimal follow-up regimen remains controversial.²

One challenge in assessing the success of treatment of focal therapy is that T2-weighted (T2W) and diffusion-weighted imaging (DWI) sequences are typically used to localize the lesion and guide the ablation, although the post-ablation treatment cavity is best visualized, or potentially only visualized on post-contrast images. Consequently, radiologists may attempt to mentally compare pre-contrast T2W/DWI and dynamic-contrast enhanced (DCE) images to assess whether the post-ablation treatment cavity encompasses the entire pre-treatment tumor volume. The purpose of this study was to evaluate a novel co-registration and 3D visualization method for comparing focal lesions on pre-treatment MRI with the ablation cavity on post-treatment DCE as a measure of treatment efficacy of focal ablative therapy for prostate cancer.

Four men (ages 54-66 years) with organ-confined prostate cancer underwent multi-parametric MRI before focal radiofrequency ablation, with additional MRI 1 week post-ablation. All examinations included a 3D turbo spin echo T2W imaging sequence (SPACE) with a spatial resolution of 0.6 x 0.6 x 1.0 mm. The ablation cavities were initially identified on the post-treatment DCE sequence. Co-registration of the pre and post-ablation MR images was performed (Mimics, Materialise) using a landmark approach³ (Figure 1), allowing both the pre-treatment lesion and the post-treatment ablation cavity to be viewed simultaneously (Figure 2). Image segmentation was performed, in which the dominant tumor, prostatic capsule, and ablation cavity were segmented as different objects (Figure 3). All patients underwent biopsy six months after treatment, including targeted sampling of the treatment site. Biopsy results were compared with the 3D co-registered pre- and post-treatment image sets. All initial lesions and ablation cavities were well visualized on the co-registered MR images. Figure 4 shows simultaneous visualization of pre-treatment tumors and post-treatment ablation cavities. The ablation cavity fully encompassed the lesion in two cases (biopsy: Gleason score 3+3, benign), had almost complete overage of the lesion in one case (biopsy: benign), and only partial coverage of the lesion in one case (biopsy; Gleason score 4+3). This study describes the use of a novel co-registration technique to allow simultaneous overlapping visualization of the pre-ablation prostate lesion (defined on T2WI) and the post-treatment ablation cavity (defined on DCE). Differences in gray scale intensities between MR sequences, as well as variable bowel and bladder filling, lead to inherent limitations in co-registering the prostate on different sequences. Such challenges are exacerbated when co-registering pre- and post-treatment scans due to changes in prostate shape and volume resulting from the ablation procedure. Our preliminary findings suggest that this co-registration method may be used to help assess treatment efficacy. Larger studies are required to validate the initial observations and show the role of the co-registration technique in a clinical setting.