**2.1 Descriptive epidemiology**

Based on World Health Organization (WHO) classification,36 subtype of NHL (21 of B-cell and 15 of T-cell type) are recognized (Ekström-Smedby, 2006). NHL is the most common in the developed world, with the highest incidence in USA, Australia and New Zealand, and Europe, and the lowest in eastern and South central Asia (Ekström-Smedby, 2006). The age standardized incidence of NHL, around the year 2000, was estimated at approximately 10- 14 per 100000 person-year in western countries, and 3 per 100000 in South central Asia (Parkin, Bray, Ferlay, & Pisani, 2005).

In recent decades, there has been a dramatic increase in NHL incidence worldwide, of about 2-4% annually (Baris & Zahm, 2000). This increase has been occurred in both males and females in all age groups except the very young and in black and whites (Weisenburger, 1994). Racial differences have not been observed in age-specific incidence curves until the age of 45 for males and 35 for females, however over these ages, NHL develops more frequently in whites than blacks (Müller, Ihorst, Mertelsmann, & Engelhardt, 2005). The highest increase was observed in western countries, but this increase is no limited to these countries, and it has been observed in eastern countries such as India, Japan, Singapore (Devesa & Fears, 1992). Several reasons including: recategorisation of borderline type of lymphoma; less histopathological misdiagnosis of NHL as Hodgkin's disease; greater use of immunohistological techniques to examine cancer of uncertain cell type and coding effects, may account for part of the increase (A. J. Swerdlow, 2003). The general trends in western countries has been a dramatic increase in incidence in young men in areas where AIDS has become common (Morton, et al., 2006).

The median age of NHL in Asian countries is significantly lower, compared to the population-based registration in western countries. The Hematological Malignancy Research Network reported that the median age of their patients was 68 years old (Smith, et al., 2010). However the median age in Asian countries is about 54 years old, in Iranian patients was 55 years old (Mozaheb, Aledavood, & Farzad, 2011), in the Korean patients 52 years (Y.-H. Ko, et al., 1998), in Taiwan 54 years (Lee, Tan, Feng, & Liu, 2005), and in a previous study in Japan 54.5 years (Aozasa, et al., 1985), but in a recent study in Japan it was 66 years (Aoki, et al., 2008). It is notable that the median age of Asian patients at the time of presentation was younger than in the western countries and it might be attributable to the

attempt to gain more insights into the differences between the Oriental and Western countries. In addition, because most different are related to etiologic factors, we also

Non-Hodgkin lymphoma is a heterogeneous group of B-cell and T-cell neoplasm that arise primarily in the lymph nodes with varied clinical and biologic feature. Current classification system include the Revised European-American Lymphoma (REAL) classification and the World Health Organization (WHO) classification of hematopoietic and lymphoid neoplasms (Alexander, et al., 2007). The distribution of NHL types varies internationally (Anderson, Armitage, & Weisenburger, 1998). Epidemiological investigation of the NHL and its etiology

Based on World Health Organization (WHO) classification,36 subtype of NHL (21 of B-cell and 15 of T-cell type) are recognized (Ekström-Smedby, 2006). NHL is the most common in the developed world, with the highest incidence in USA, Australia and New Zealand, and Europe, and the lowest in eastern and South central Asia (Ekström-Smedby, 2006). The age standardized incidence of NHL, around the year 2000, was estimated at approximately 10- 14 per 100000 person-year in western countries, and 3 per 100000 in South central Asia

In recent decades, there has been a dramatic increase in NHL incidence worldwide, of about 2-4% annually (Baris & Zahm, 2000). This increase has been occurred in both males and females in all age groups except the very young and in black and whites (Weisenburger, 1994). Racial differences have not been observed in age-specific incidence curves until the age of 45 for males and 35 for females, however over these ages, NHL develops more frequently in whites than blacks (Müller, Ihorst, Mertelsmann, & Engelhardt, 2005). The highest increase was observed in western countries, but this increase is no limited to these countries, and it has been observed in eastern countries such as India, Japan, Singapore (Devesa & Fears, 1992). Several reasons including: recategorisation of borderline type of lymphoma; less histopathological misdiagnosis of NHL as Hodgkin's disease; greater use of immunohistological techniques to examine cancer of uncertain cell type and coding effects, may account for part of the increase (A. J. Swerdlow, 2003). The general trends in western countries has been a dramatic increase in incidence in young men in areas where AIDS has

The median age of NHL in Asian countries is significantly lower, compared to the population-based registration in western countries. The Hematological Malignancy Research Network reported that the median age of their patients was 68 years old (Smith, et al., 2010). However the median age in Asian countries is about 54 years old, in Iranian patients was 55 years old (Mozaheb, Aledavood, & Farzad, 2011), in the Korean patients 52 years (Y.-H. Ko, et al., 1998), in Taiwan 54 years (Lee, Tan, Feng, & Liu, 2005), and in a previous study in Japan 54.5 years (Aozasa, et al., 1985), but in a recent study in Japan it was 66 years (Aoki, et al., 2008). It is notable that the median age of Asian patients at the time of presentation was younger than in the western countries and it might be attributable to the

describe some of them**.** 

**2. Non-Hodgkin Lymphoma** 

**2.1 Descriptive epidemiology** 

(Parkin, Bray, Ferlay, & Pisani, 2005).

become common (Morton, et al., 2006).

may result in a better understanding and hence prevention.

lower frequency of lymph node type lymphoma, and higher frequency of T-cell lymphoma, which comes as follows.

Geographically related variation in the incidence of histopathologic distribution and clinical feature of NHL are well recognized (Shih & Liang, 1991). T-cell leukemia lymphoma occurs more frequently in southwest Japan, and the Caribbean basin (Takatsuki, 1990), northeast of Iran (Mashhad) (Abbaszadegan, et al., 2003); follicular lymphoma (FL) occurs less frequently in eastern countries (Intragumtornchai, et al., 1996; Mozaheb, et al., 2011; Ohshima, Suzumiya, & Kikuchi, 2002), and Immunoproliferative Small Intestinal Disease (IPSID) is the most prevalent in the Middle east and Africa (Khojasteh & Haghighi, 1990).
