At the nasal vestibule, keratinising squamous cell carcinoma is the most common malignancy [130, 167, 245]. Due to early recognition and easy access to treatment, they usually have a more favourable prognosis than their counterpart of the sinonasal region.
Sinonasal keratinising squamous cell carcinoma represents up to 45-50% of the malignant tumours of this region in several series [93, 259]. At the Hospital Clinic of the University of Barcelona, where non-ke-ratinising squamous cell carcinomas (see Sect. 2.11.2 on cylindrical cell carcinomas) are grouped separately from keratinising squamous cell carcinomas, the latter account for only 27% of the sinonasal malignancies. They predominate in males and the great majority are seen in patients aged over 50 years. The maxillary antrum, the lateral nasal wall and the sphenoidal sinuses are the most common sites, whereas the frontal and sphenoid sinuses are rarely involved. These tumours grow by local extension, infiltrating the neighbouring structures, but lymph node metastases are rare . For neoplasms circumscribed to the nasal cavity the 5-year survival is slightly above 50% , whereas in neoplasms of the maxillary antrum the 5-year survival may be as low as 25% .
The occupational epidemiology of sinonasal squamous cell carcinoma has been strongly related to exposure to nickel [141, 243, 252, 253] and to a lesser extent to chromium, isopropyl alcohol and radium . As in other territories of the respiratory tract, a definite association between sinonasal squamous cell carcinoma and cigarette smoking has been documented [26, 146]. Chronic sinonasal inflammation is considered a predisposing factor. A case of carcinoma of the maxillary antrum after thorotrast exposure has been reported . Nitrosamines and to a lesser extent formaldehyde are strong nasal carcinogens in laboratory rodents [44, 155].
Keratinising squamous cell carcinomas originate in the respiratory sinonasal mucosa from areas of pre-existing squamous metaplasia and manifest the same range of histological appearances as those arising in other sites. They are characterised by the proliferation of malignant epithelial cells with squamous differentiation and intercellular bridges. Malignancy is graded according to the degree of differentiation, cellular pleomorphism and mitotic activity. They are divided into well-differentiated, moderately differentiated and poorly differentiated forms. Well-differentiated carcinomas are uncommon in this territory and when encountered need to be differentiated from pseudoepitheliomatous types of hyper-plasia and from verrucous carcinoma. Most conventional keratinising squamous cell carcinomas of the sinona-sal tract present as moderately or poorly differentiated tumours. Special types, such as verrucous carcinoma , spindle cell carcinoma [205, 276], basaloid-squa-mous cell carcinoma [16, 269] and adenosquamous carcinoma [10, 94] are occasionally found in the sinonasal tract. Regional lymph node involvement is seen in about 17% of sinonasal squamous cell carcinomas and distant metastases in about 1.5% .
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