KEY POINTS
- The report describes two patients with suspected radiation-induced lumbosacral plexopathy (RILSP) after contemporary prostate radiotherapy, combining neurologic assessment, imaging and retrospective plexus dosimetry. RILSP remains a diagnosis of exclusion, and prostate-specific evidence is largely limited to case reports.
- Case 1 received salvage VMAT with 45 Gy/25 fractions to pelvic nodes, 66.6 Gy to the prostate fossa and 70.2 Gy to a 2-cm gross recurrence. Four weeks later, unilateral pain and weakness progressed to bilateral neurologic decline and eventual wheelchair dependence despite decompression, vitamin B12 replacement and empiric IVIg.
- The lumbosacral plexus had not been prospectively contoured. Retrospective analysis showed Dmax 72.3 Gy, V60 12.9%, V65 6.4% and V70 1.8%, with the maximum dose localized to the inferior left sacral plexus near S4. Re-optimization reduced Dmax to 69.8 Gy and V60 to 8.4%, but target geometry prevented complete hotspot removal.
- Case 2 received pelvic and abdominal nodal RT to 46 Gy/23 fractions, followed by sequential boosts to 60 Gy and 78 Gy to the prostate/seminal vesicles while receiving leuprolide and enzalutamide. One month after RT, he developed bilateral neuropathic pain, proximal weakness and recurrent falls.
- Plexus exposure in Case 2 was substantially lower: Dmax 58.6 Gy, V50 18.2%, V55 9.7% and V60 2.3%, with no measurable exposure above 65 Gy and no focal boost convergence. Following rehabilitation and discontinuation of enzalutamide, neurologic function improved substantially and near-independent ambulation returned.
- Across published symptomatic pelvic RT cohorts summarized by the authors, reported lumbosacral plexus Dmax ranged from 59–73 Gy. Case 1 therefore sat near the upper end of previously reported symptomatic exposures, whereas Case 2 was near the lower end; these comparisons are descriptive and do not represent validated dose constraints.
- The authors have changed their own practice to prospectively contour the lumbosacral plexus and aim for Dmax 60–66 Gy when feasible, particularly where pelvic nodal targets or focal boosts converge near the plexus. They explicitly note that this is an institutional response to these cases rather than a validated tolerance threshold.
CLINICAL TAKEAWAY
Lumbosacral plexus dose may deserve more attention in prostate plans involving pelvic nodes, salvage boosts or PSMA-guided target expansion, particularly when high-dose regions converge near the plexus. The contrast between these two cases makes focal Dmax biologically plausible as a risk factor, but two cases cannot establish a tolerance dose or prove radiation as the sole cause of neurologic injury.