Abstract
Ganglioneuroblastic transformation in olfactory neuroblastoma (ONB) is an exceptionally rare phenomenon. We document the case of a patient with a poorly differentiated sinonasal malignancy that recurred following treatment with chemoradiotherapy and showed ganglioneuroblastic transformation. Although the index tumour showed neuroendocrine differentiation, it did not have the typical clinico-pathological features associated with ONB. We highlight the diagnostic difficulties in establishing an accurate diagnosis for undifferentiated sinonasal tumours and present evidence that the index tumour was an ONB. The current report is only the third case of ONB showing complete ganglioneuroblastic transformation.