Tractography-Guided Nerve-Sparing in Prostate Cancer

In Switzerland and Western countries, prostate cancer (PCa) is the most frequently diagnosed solid cancer in men and represents the second leading cause of cancer-related deaths. Similarly, in Geneva, between 1995 and 2021, PCa has been the most common cancer with 8,373 new cases reported.

For patients with prolonged life expectancy harboring high-risk and intermediate-risk disease, radical prostatectomy (RP) is one of the gold standard treatments. While RP is a safe and effective surgical procedure leading to a high rate of cancer control, the genito-urinary function can be severely impaired after surgery since the prostate is closely surrounded by neurovascular fibers that are critical for urinary continence and erectile function. In 1982, Walsh developed the nerve-sparing (NS) procedure to preserve the neurovascular bundles (NVBs) . Nevertheless, accurately identifying the NVBs still remains a challenge nowadays. Despite the advancements made in the area of precision surgery with the advent of 3D robotic-assisted RP (RARP) as well as preoperative planning (particularly with Multiparametric magnetic resonance imaging (mpMRI) providing information about tumor localization and extracapsular extension), erectile dysfunction (ED) is currently the most common post-operative sequela occurring in up to 74.7% of patients. One of the key reasons is linked to the fact that the NVBs are not clearly visible during surgery and are in contact with the prostatic capsule.

Novel intraoperative imaging modalities have emerged to solve this unmet clinical need. First, it has been suggested that assistance can be provided by intraoperative transrectal ultrasound (TRUS) eventually combined with a robotic manipulator. TRUS can identify key anatomical landmarks. However, it is rarely used due to the poor spatial resolution. Second, electrical nerve stimulation offers fast functional feedback for nerve mapping. Although this solution is deliverable, its inconsistent clinical response reduces its efficacy and usefulness. Third, fluorescent imaging allows for a real-time assessment, but its clinical use is limited due to, among others,the toxicity of dyes.

Tractography, mainly used in brain surgery, is an advanced non-invasive imaging technique relying on diffusion tensor imaging (DTI) and algorithms to reconstruct three-dimensional nerve fiber pathways by analyzing directional movement of water molecules in tissues The accuracy of DTI Tractography heavily relies on sophisticated protocols and highly developed technical parameters, which also includes the post-processing aspects. For instance, the number of gradient diffusion directions, b-values affecting the signal-to-noise ratio (SNR) and tract fiber length play a key role, as studied in Nordbrøden's thesis focusing on providing an optimized protocol for periprostatic fiber nerve tracking. Due to inherent complexity, despite being a promising tool, application of tractography for NVB mapping preoperatively has not been well studied yet. To improve functional outcomes, surgeons should be able to rely on a patient-tailored anatomical map to provide personalized nerve-sparing surgery.

Moreover, three-dimensional virtual model (3DVM), reconstructed from classical preoperative MRI done during the diagnostic pathway, are gaining popularity in RARP. Various formats are described; for instance, Virtual Reality (VR) uses a fullly digital immersive environment, whereas Augmented Reality (AR) overlays the 3D model onto the intraoperative view. However, integration of such a tool into clinical practice faces many challenges at each step of the procedure. First, when generating the model, suboptimal image quality and the difficult segmentation task needed for surface rendering (SR) could generate anatomical inaccuracy. Secondly, in AR, the 3DVM needs to be registered to the patient's anatomy (landmarks) using manual or semi-automatic techniques, with fully-automatic registration being the subject of ongoing research . Nonetheless, Schiavina et al (2021) using a manual registration accurately managed a real-time identification of the index lesion with the downside of having a dedicated 3DVM manipulator changing the NS plan in 38.5%.

In this context, given the absence of histopathological validation possible for Tractography, the rationale of this pilot study is to leverage the benefits of DTI imaging for nerve identification and to translate them directly during the surgery.

The trial relies on two distinct parts.

• First, pre- and postoperative imaging will allow assessment of the neurovascular structures before and after the procedure, to evaluate the volumetry and integrity of the neurovascular bundles and how surgery impacted this. These results will be correlated with patients' functional outcome.
• Second, the images acquired with the preoperative imaging will be reconstructed in a 3D Tractography Virtual Model (3DTVM) that will be used for intraoperative AR fusion. In this context, the surgeon could have a visual guide on the imaging data directly in the operative room (OR). No change in the surgical technique will occur by the use of this technique within this pilot study.

As this pilot study relies mostly on DTI MRI, a non-invasive modality that does not use a contrast agent, and excludes fluorescent dye administration or electrical stimulation, participant burden is limited to the need for this acquisition. As to the first part, according to the Ordinance on Human Research with the Exception of Clinical Trials dated 20 September 2013 and entered into force on 1st January 2014, and particularly Article 7, MRI scans without a contrast medium is associated with minimal risks and burdens and hence falls into the scope of Category A. As to the second part, the AR guidance is purely a non-invasive software-based overlay of MRI data onto the surgeon's view where no additional biological material collection is needed. There will be no new incisions or instruments beyond standard procedure (same surgical steps, trocar placement, dissection planes, etc.). Therefore, this second part is also associated to minimal risks and burdens and falls as well into the scope of Category A.

To address all these challenges, our institution is particularly adequate for many reasons. First, RARP is a procedure performed at an expert-level according to the European Cancer Center Certification Program. Second, prior to each surgery, the case is discussed in a multi-disciplinary setting with proofreading of the MRI by an expert uro-radiologist, Dr. Thomas De Perrot (according to European Society of Urogenital Radiology (ESUR) and EAU Section of Urologic Imaging (ESUI); more than 1000 interpreted MRI). The NS approach can therefore be safely chosen prior to surgery based on radiologic-clinical parameters. Pr. Felix Kurz is a renowned specialist in peripheral nerve imaging while Pr. Michele Diana is a pioneer in AR surgery and multi-level imaging surgical guidance . In addition, the Diagnostic Department, together with the CIBM Center for Biomedical Imaging, received the state-of-the-art MAGNETOM Cima.X (Siemens Healthineers AG, Erlangen, Germany) in June 2025. This advanced 3.0-T MRI system offers the highest magnetic gradients available in routine clinical imaging-reaching up to 200 mT/m. These exceptional gradients are specifically engineered for high-performance diffusion MRI measurements, reducing acquisition times and improving image quality through higher signal-to-noise ratio. In addition to all of the above, the HUG surgical department has performed Robotic AR surgery since 2011.

Regarding the "sex and gender" dimensions, the study concerns patients with PCa.