Sinonasal undifferentiated carcinoma (SNUC) is defined as a high-grade malignant epithelial neoplasm of the nasal cavity and paranasal sinuses, composed of small to medium-sized cells, lacking evidence of squamous or glandular differentiation and of rosette formation [48, 86, 114]. Cigarette smoking  and nickel exposure  have been associated with SNUC. Epstein-Barr virus (EBV) and the deletion of the retinoblastoma gene have been ruled out as factors involved in the development of this tumour . Ionising radiation is another aetiologic factor, as radiotherapy either for retinoblas-toma or for nasopharyngeal carcinoma has been associated with SNUC .
Sinonasal undifferentiated carcinoma occurs in both sexes over a wide age range, with a median in the 6th decade of life. It commonly originates from the ethmoidal region as a large fungating mass. Grossly, the tumours are quite frequently extensive le sions, obstructing the nasal cavity either unilaterally or bilaterally and invading the adjacent sinonasal structures (Fig. 2.10a), as well as the orbit, skull base and the brain. Microscopically, SNUC is composed of small to medium-sized, undifferentiated cells, which arise via dysplastic changes from the basal cells of the surface epithelium. The cells are polygonal with distinct borders, showing round to oval, hy-perchromatic or vesicular nuclei, with either inconspicuous or slightly prominent nucleoli, surrounded by a moderate amount of either amphophilic or eo-sinophilic cytoplasm (Fig. 2.10b). Mitotic figures are common. The tumour forms nests, cords and sheets of cells that show frequent areas of central necrosis and tendency to vascular (Fig. 2.10c) and perineural invasion. Ultrastructural studies demonstrate poorly formed desmosomes in quite a number of cells, while the presence of tiny bundles of tonofilaments is very rare. Neurosecretory granules are very rarely found. SNUC are immunoreactive with epithelial markers, such as CAM 5.2 and epithelial membrane antigen (EMA). Variable reactivity can be seen with neuron-specific enolase (NSE), whereas there is negative reactivity for synaptophysin, chromogranin and other neuroendocrine markers. SNUC are also negative for EBV [48, 127].
The two main differential diagnoses of SNUC are small cell (neuroendocrine) carcinoma (SCC) and high-grade olfactory neuroblastoma (ONB). All three entities may share some clinical and light microscopic features. However, SNUC and SCC show a marked immunoreactivity for cytokeratins that is not seen in ONB, and SNUC lacks the neuroendocrine immuno-reactivity seen in SCC and ONB. Most lesions categorised in the past as grade IV ONB are now considered to be either SNUC or SCC. This is important because SNUC and SCC have a worse prognosis than ONB. In addition, SNUC needs to be distinguished from other primary sinonasal tumours, such as solid adenoid cystic carcinoma, microcytic malignant melanoma, cylindrical cell carcinoma, primary sinonasal nasopharyn-geal-type undifferentiated carcinoma, lymphoma and others (Table 2.2).
Sinonasal undifferentiated carcinomas are very aggressive tumours. In most instances, the tumour is so large and the infiltration is so extensive that complete surgical resection cannot be achieved. Radiotherapy and chemotherapy are additional options, either single or combined. High-dose chemotherapy and autol-ogous bone marrow transplantation have been considered as a form of treatment . Prognosis of SNUC is dismal, with a median survival of 4 months to 1 year [86, 114]. In our experience survival after 2 years is less than 40%.
Fig. 2.10. Sinonasal undifferentiated carcinoma (SNUC) a Extensive neoplastic growth invading ethmoid, orbit and sphenoid at the left and also the right ethmoid. Courtesy of Prof. J. Traserra, Barcelona, Spain. b Nests of small to intermediate epithelial cells showing markedly atypical nuclei and areas of necrosis. c Frequent foci of intra-vascular invasion
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