Adenocarcinoma of the prostate involving the lumbosacral plexus: MRI evidence to support direct perineural spread

Background: Prostate adenocarcinoma, which may recur despite aggressive treatment, has the potential to spread to the lumbosacral plexus. This intraneural involvement is not widely known and is thought to be from direct perineural spread. We hypothesized that high-resolution imaging could provide supportive evidence for this mechanism. Methods: The clinical data and imaging studies (magnetic resonance imaging, MRI, and positron emission tomography/computed tomography, PET/CT) of patients evaluated at our institution between 2004 and 2009 for lumbosacral plexopathy due to intraneural prostate carcinoma were retrospectively reviewed. Results: Four patients presenting with painful lumbosacral plexopathy were found to have intraneural lumbosacral prostate adenocarcinoma. Two patients had involvement of the lumbosacral plexus ipsilateral to the lobe of the prostate most involved with adenocarcinoma at prostatectomy. High-resolution MRI and PET/CT studies revealed similar findings: abnormal soft tissue signal was followed from the prostate (n=1) or prostatic bed (n=3) area along the expected course of the pelvic plexus to the level of the sciatic notch, where it involved the sacral spinal nerves and sciatic nerve. Imaging findings were consistent with neoplastic infiltration, which was confirmed at biopsy in three patients. Conclusions: The potential for prostate adenocarcinoma to spread to the lumbosacral plexus has, to our knowledge, not been readily appreciated. Because the imaging findings are often subtle, we believe that intraneural lumbosacral plexus involvement may not be uncommon. This study, with the use of high-resolution MRI and PET/CT studies, supports the direct perineural spread of prostate adenocarcinoma via the pelvic plexus to the lumbosacral plexus. This mechanism could also explain cases of leptomeningeal and/or dural-based prostate metastases.