BMC Cancer 2005, 5:33 http://www.biomedcentral.com/1471-2407/5/33 referred for treatment. For this reason and because treat- ment is completely free of charge [16], no case of pre- sumed childhood leukemia should be missing from the records. Children from other countries (Nicaragua, Hon- duras, etc) were excluded from the numerator. The hospitals included in the Mexico City study were the Pediatric Hospital of the Centro Médico Nacional "Siglo XXI" (HP) and the General Hospital of the Centro Médico Nacional "La Raza" (HG). Children who were attended in the pediatric hematology service and who resided in Mex- ico City were included in both hospitals. There are accu- rate records on the population that has a right to this service because the population attended by this institu- tion in Mexico City is formed by workers whose personal data are registered. Only those children who were Mexi- can nationals and whose parents were residents of Mexico City were included in the numerator. Diagnosis Once diagnosed with presumed leukemia, a child was referred to one of the hospitals where trained staff did a comprehensive follow-up of the case to either discard or confirm the diagnosis of leukemia. In the BB, infirmary staff trained for that activity carried out this work. In all cases, the diagnosis was confirmed with bone marrow smear, and histochemical tests (myeloperoxidase, sudan black B reaction, esterases, periodic acid Schiff (PAS) reac- tion, and acid phosphatase) were performed to differenti- ate the types of leukemia. In the cases in El Salvador, the staff of St. Jude Hospital confirmed some diagnostic tests [16]. The types of leukemia were divided according to the five- group morphological classification of the international classification of childhood cancer. Only four of the types were found in this study: a) Acute lymphoid leukemia (ALL) (9820–9827, 9850); b) acute myeloid leukemia (AML), the preferred terminology instead of "acute nonl- ymphocytic leukemia" [18] (9840, 9841, 9861, 9864, 9866, 9867, 9891, 9894, 9910); c) chronic myeloid leuke- mia (CML) (9863, 9868); and e) unspecified leukemias (UL) (9800–9804) (International Classification of Dis- ease for Oncology, ICD-O2; 1990) [19]. The codes were reviewed with the Child-Check program [19] to corrobo- rate that there were no duplicate records or inconsistent data. A database was generated to record age, sex, resi- dency, year of diagnosis, and clinical manifestation of the patients. Populations Because, in El Salvador, only children under the age of 12 are attended at the BB, our analysis was limited to that cohort, instead of that conventionally used in a study of this type, i.e., children younger than 15 years [2,10]. The denominators to calculate the rates were obtained by every year from the General Direction of Statistics and Census (Dirección General de Estadística y Censos), which is the only government source that calculates the estimates of population in El Salvador. The population younger than 12 was 1,756,513; 1,764,422; 1,787,064; 1,808,946, and 1,829,146 for the years 1996 to 2000, respectively. The last available census in El Salvador dates from 2000. In Mexico City, the denominators were obtained from the Coordination of Planning and Medical Information of the IMSS, which is the only government source that calculates all the populations with access to medical attention pro- vided by the IMSS [20]. This information is updated every year by the IMSS. The recorded population younger than 15 years in Mexico City was 786,754; 832,199; 765,268; 806,043; and 881,887 for the years 1996 to 2000, respec- tively. The recorded population of children in Mexico City appeared to fluctuate from year to year because only that population of workers who are under contract with a company are entitled to the care provided by the IMSS. Thus, the denominator reflects the change in the number of jobs created or lost each year. Analysis Annual incidence rates (AIR) were calculated by age group, sex, and each type of leukemia. Also, the average annual incidence rate (AAIR) was calculated by sex and total for the whole study period. These rates were stand- ardized by age through the direct method with the world standard population [21], with the AIR and AAIR reported per million of children under 15 years old. The AAIRs were calculated by using the total of cases found in the study period as numerator and the sum of the populations found in each year of the study as denominator. Cases were stratified by age and assigned to one of four age strata: 1) <1 year; 2) 1–4 years; 3) 5–9 years, or 4) 10– 14 or 10–11 years, for Mexico City and El Salvador, respectively. Therefore, for El Salvador, it was not possible to calculate the AAIR accurately for the 0–14 year-old age group. In the study, an average rate over the whole period and for the 0– 11 year-old age group was calculated. These rates were standardized by age by direct method and also with the world standard population. Results During the study period, there were 375 children diag- nosed with leukemia in El Salvador and 238 in Mexico City. Of total leukemias in El Salvador and in Mexico City, ALL represented 80.5% and 76.9%; AML, 16.8% and Page 3 of 9 (page number not for citation purposes)