INTRODUCTION
Acute lymphoblastic leukemia (ALL) represents the most common childhood malignancy, accounting for approximately 28 % of cancers and 80 % of all leukemias in children 1,2. ALL presentation is bimodal, with the first peak occurring in childhood, and a second peak around age fifty 1. Its incidence is reported to be higher in Hispanic children at 43 cases per million vs. 28 in non‐Hispanics 3 and in Mexico it has been reported at 49.5 cases per million.4
The pathogenesis of ALL is complex and remains elusive, exogenous and/or endogenous exposures, genetic susceptibility, biologic heterogeneity, and chance appear to have roles 5,6. Environmental risk factors including infections are an area of particular interest in leukemia’s etiology 7,8 and epidemiological investigation into the periodicity of leukemia onset, which has a long history, and the evidence for seasonal variation in incidence may provide insight into this hypothesis9,10. Many viral infections have characteristic seasonal onsets, thus investigations had suggested that ALL is related to infection or other seasonally-varying environmental risk factors 11–13.
Seasonal variation in diagnosis of children with ALL may provide evidence of an infectious etiology for childhood ALL, as seasonal climatic changes give rise to respiratory infections or gastrointestinal infections in winter, spring, and summer. This might possibly suggest the presence of environmental factors such as pesticides, which are applied frequently in rural areas. Also, spatiotemporal clusters occur, with an excess of childhood ALL cases observed in a determined geographical area at certain points in time compared with other areas and other times.14 Importantly, most studies reporting seasonal trends do so in countries with temperate-cool climates15, with a few conducted in arid or semiarid regions, which have a rather short winter season 16.
Studies addressing the seasonality of presentation in acute leukemia report contrasting results 17–20. Time trends and seasonal variations have been assessed, while some have not identified a seasonal pattern 20,21 , others have found significant seasonal influence 17,19,22. Most studies on this subject have analyzed different reference dates, including birth date, first symptom date or diagnosis date, and used different statistical methods, leading to inconsistencies and controversy in the results. The evidence obtained may contribute to learn about potential seasonally varying environmental risk factors, such as infections or pesticide exposure, especially in Latin America where this pattern is less clear, with few reports documenting this aspect of childhood and adolescence ALL.
The present study aimed to investigate the presence of seasonal variation in month of diagnosis in a homogeneous ethnic and socioeconomic group of Hispanic children and adolescents with ALL in a low-middle income population over a 15-year period.