Ectopic olfactory neuroblastoma ‑ complete response to chemotherapy following a late recurrence: A case report

Olfactory neuroblastoma, also known as esthesioneuroblastoma, is a rare type of cancer that originates in the nerve tissue of the olfactory epithelium, located in the upper part of the nasal cavity. The present study reports the case of a female patient who was 35 years old at the time of diagnosis in 2013, with an ectopic olfactory neuroblastoma (Hyams grade III, Kadish stage B) in the right sphenoid sinus, associated with the paraneoplastic syndrome of inappropriate antidiuretic hormone secretion (SIADH). Following transnasal endoscopic surgical resection and adjuvant intensity‑modulated radiotherapy (60 Gy) in 2014, the SIADH resolved and the patient achieved complete remission. After a 10‑year latency period, in April 2024 the patient developed a late, destructive local recurrence (5.3x4.7 cm) with extension to the maxillary sinus and right orbit. Given the technical limitations for salvage surgery and the refusal of re‑irradiation, a multidisciplinary committee approved the administration of 6 cycles of systemic chemotherapy with cisplatin (75 mg/m²) and etoposide (100 mg/m²) every 21 days. Treatment began in June 2024 and concluded in September 2024. Treatment was well tolerated, with only grade 1 nausea and transient grade 2 myelosuppression. Close follow‑up with contrast‑enhanced head and neck CT in December 2025 confirmed the complete clinical and radiological response, demonstrating the total absence of the tumor lesion. As of February 2026, the patient remains asymptomatic and disease‑free. The present case highlights the rarity of ectopic sphenoid location, the usefulness of SIADH as an indirect biomarker and the high chemosensitivity of high‑grade variants in late recurrences, justifying prolonged and indefinite follow‑up.