Epidemiology of nasopharyngeal cancer an overview
Nasopharyngeal cancer is a malignant tumor involving the nasopharyngeal epithelial lining. Study of epidemiology of nasopharyngeal carcinoma makes an interesting reading. Extensive epidemiological studies conducted during the last 40 years have thrown lot of insights in the etiopathogenesis of this disease. This is one of the malignant conditions the causation of which a virus has been implicated (Epstein Barr virus). NPC (hereafter in this article nasopharyngeal carcinoma will be addressed thus) has demonstrated unique epidemiological features which include the most obvious regional, racial and familial predispositions.
Nasopharyngeal carcinoma is unique in its virtually endemic distribution confined to certain areas of the globe. Classically it is most common in the southern parts of China. In fact the Guangdong province of southern china shows the highest incidence (up to 100 / 100000) That is the reason for its another name "Canton tumor". In the year 2002 alone China accounted for nearly 50% of newly diagnosed cases of NPC. Studies have also revealed significant differences in the incidence rates of NPC between Northern and southern parts of China.
Incidence of NPC is significantly higher in males than in females. It is about 3 times more common in males than females. Male predominance has been documented in both endemic and low incidence areas.
Age distribution varies according to the incidence rates. In low incidence areas it increases with age, where as in endemic areas the incidence increased after the age of 30 and peaked at 50 years of age. In medium incidence areas the incidence showed a minor peak at adolescent age groups.
Studies have shown that the incidence of NPC's is maximum in xanthoderms (yellow skinned individuals). Melanoderms come next (dark skinned races) while caucasians (white skinned) were found to be least prone.
Study of data pertaining to incidence of NPC in migrants proved that people from endemic areas retained their predisposition to NPC even when they migrate to low incidence areas like U.S.
Incidence of NPC shows discernible familial predisposition. It is more so in families living in high incidence areas like southern china. Studies have shown that there has been a progressive decline in the incidence of NPC even in endemic areas. Studies have revealed a specific decline in the incidence of keratinizing squamous carcinoma because of decreasing incidence of smoking in these areas. On the contrary Non keratinizing squamous cell carcinoma variety did not show any decline in incidence. Considering the high incidence of Epstein Barr virus infection, the incidence rate of NPC is not so alarming. This could be attributed to the fact that in addition to EB virus infection other factors are also necessary. NPC should hence be considered to be a multifactorial disease.
Etiological factors (possible):
1. EB virus infection
2. Salty fish and pickled food: Ingestion of salty fish and pickled food have been attributed as one of the causative features. These food stuffs contain N-nitrosamine which is a known carcinogen. It has also been shown increased ingestion of fresh vegetables and fruits can reduce the risk of NPC due to beneficial effects of antioxidant and antinitrosamine components.
3. Smoking / dringking: Have shown to be risk factors involved particularly in Non keratinizing varieties of squamous cell carcinoma.
4. Hereditary susceptibility: Studies have shown tha HLA - A2 - Bw46 and B17 have shown increased incidence of NPC by 2 - 3 fold.
5. Risks posed by traditional chinese medicine:Studies have shown that increased use of chinese traditional medicine have increased the risk of NPC by 3 fold. Some of the ingrediants used in chinese traditional medicine have been shown to activate the latent EB virus infection
6. Exposure to carcinogens like formaldehyde, wood dust, and Nickel have shown to increase the risk of NPC
7. More recently inviduals with chronic upper air way disease are more prone to risk of acquiring NPC because bacteria present in the upper airway converts nitrate to nitrite which is a carcinogen