Radiation-Induced Lumbosacral Plexopathy Following Prostate Radiotherapy.

Radiation-induced lumbosacral plexopathy (RILSP) is an uncommon but potentially devastating complication of pelvic radiotherapy. Most published series involve gynecologic or colorectal malignancies, and limited data exist regarding plexus dosimetry and clinical presentation following contemporary prostate radiotherapy. We report two cases of suspected RILSP following prostate radiotherapy with detailed clinical, radiographic, and dosimetric correlation. Case 1 involved a 75-year-old man who underwent salvage pelvic radiotherapy with focal boost for recurrent prostate cancer. Four weeks after treatment completion, he developed progressive unilateral pain and weakness that evolved into bilateral neurologic decline and eventual wheelchair dependence despite lumbar decompression, vitamin B12 repletion, and empiric immunotherapy. Retrospective contouring of the lumbosacral plexus demonstrated focal high-dose exposure with a maximum dose (Dmax) of 72.3 Gy and direct overlap between the ≥70 Gy isodose line and the symptomatic plexus segment. Re-optimization reduced but did not eliminate hotspot overlap because of anatomic constraints. Case 2 involved a 65-year-old man treated with definitive pelvic radiotherapy and intensified androgen deprivation therapy for node-positive prostate cancer. He developed subacute bilateral proximal weakness and neuropathic pain one month after treatment completion. Electromyography raised concern for plexopathy, but no compressive lesion was identified. Retrospective plexus contouring demonstrated substantially lower exposure (Dmax 58.6 Gy) without measurable dose above 65 Gy or focal boost convergence. Following rehabilitation and discontinuation of enzalutamide, the patient experienced substantial neurologic recovery. These contrasting cases suggest that focal high-dose geometry to the lumbosacral plexus, particularly maximum point dose, may contribute to clinically significant neurologic toxicity beyond prescription dose alone. They also highlight the importance of trajectory-based multidisciplinary reassessment when clinical progression diverges from expected imaging findings. As prostate-specific membrane antigen positron emission tomography increasingly guides target expansion and focal dose escalation in prostate radiotherapy, routine plexus contouring and attention to maximum dose may become increasingly important to reduce the risk of severe neural morbidity.