Silent Damage, Delayed Symptoms: A Case of Breast Cancer Radiation–Induced Lumbosacral Plexopathy

<b>Background and Clinical Significance</b>: Radiation-induced lumbosacral plexopathy (RILP) is a rare but potentially debilitating complication of radiotherapy, typically affecting patients treated for pelvic malignancies. We report the first documented case of asymmetric RILP following radiotherapy for breast cancer. <b>Case Presentation</b>: A 64-year-old woman developed progressive left lower limb weakness, foot drop, and sensory disturbances four years after receiving locoregional radiotherapy extending to the left thoracoabdominal and lumbar areas. Electrophysiological studies revealed an asymmetric sensorimotor axonal neuropathy predominantly involving the left lower limb, without conduction block and sparing the upper limbs, whereas needle electromyography of the lower limbs showed fibrillation potentials, positive sharp waves, and fasciculations in the vastus lateralis, tibialis anterior, and medial gastrocnemius muscles on the left. Magnetic resonance imaging demonstrated edema and contrast enhancement of bilateral L2-L4 nerve roots with paraspinal muscle atrophy. Cerebrospinal fluid analysis showed albuminocytologic dissociation and elevated neurofilament levels. After exclusion of alternative diagnoses, including amyotrophic lateral sclerosis and inflammatory neuropathies, a diagnosis of radiation-induced peripheral neuropathy and RILP was made. The patient's condition stabilized with physiotherapy and symptomatic treatment. <b>Conclusions</b>: This case highlights the need for heightened awareness of RILP as a late complication of breast cancer radiotherapy, underscoring the importance of accurate diagnosis to avoid misclassification and unnecessary treatments. Clinicians should carefully integrate all clinical elements-including a thorough remote medical history-since radiation-related neurological damage may manifest many years after the initial insult.