Hypertrophic olivary degeneration mimicking tumor recurrence after brainstem radiosurgery: A diagnostic pitfall in neuro-oncology.

Background: Hypertrophic olivary degeneration (HOD) is a rare trans-synaptic degeneration of the inferior olivary nucleus resulting from disruption of the dentato-rubro-olivary pathway, also known as the Guillain- Mollaret triangle. Unlike most degenerative processes, it produces olivary hypertrophy rather than atrophy. In oncology patients with prior posterior fossa or brainstem involvement, this condition can simulate tumor recurrence or radiation necrosis, leading to unnecessary interventions if unrecognized. Case Description: A 38-year-old woman with a history of human epidermal growth factor receptor 2-positive invasive ductal carcinoma of the breast and previously treated brainstem metastasis presented with progressive right facial paresthesias and gait instability. Neurological examination revealed dysmetria, impaired rapid alternating movements, and gait ataxia without motor or sensory deficits. Magnetic resonance imaging (MRI) demonstrated a nodular enhancing pontine lesion due to the treated metastatic lesion and a new enlargement involving the rostral aspect of the left central tegmental tract with associated hypertrophic change of the inferior olivary nucleus, consistent with unilateral HOD. Magnetic resonance spectroscopy and perfusion imaging excluded neoplastic recurrence. A multidisciplinary neuro-oncology board concluded that the findings were secondary to radiation-induced tract injury. The patient continued maintenance chemotherapy and received symptomatic management and rehabilitative therapy, with serial MRI showing stability. Conclusion: HOD should be considered in patients with prior brainstem surgery or radiotherapy who develop delayed cerebellar signs and characteristic MRI findings. Recognizing its benign and self-limited nature is crucial to prevent misdiagnosis as tumor recurrence and avoid unnecessary oncologic or surgical interventions. Integration of advanced MRI techniques and multidisciplinary evaluation allows precise diagnosis and tailored management, ensuring appropriate treatment and long-term follow-up.