Hairy cell leukaemia: a heterogeneous disease?

The US National Cancer Institute’s Surveillance, Epidemiology and End Results program was used to develop aetiological clues for hairy cell leukaemia (HCL). Descriptive techniques (age‐adjusted incidence trends, age‐specific incidence rates (IR), and age distributions‐at‐diagnosis) were supplemented with mathematical models (two‐component mixture, generalized linear regression, and age‐period‐cohort). There were 2856 cases of HCL diagnosed during 1978–2004 (IR 0·32/100 000 person‐years). IRs were nearly 4‐fold greater among men than women and more than 3‐fold higher for Whites than Blacks. Temporal trends were stable over time. Age‐specific IRs increased rapidly until approximately 40 years then rose at a slower pace. The age‐specific IR curves reflected bimodal early‐ and late‐onset age distributions‐at‐diagnosis (or density plots), with some variation by gender. Among both men and women, a two‐component mixture model fitted the data better than a single density or cancer population. Age‐period‐cohort models confirmed statistically significant age‐related effects after full adjustment for temporal trends (calendar‐period and birth‐cohort effects). In summary, age incidence patterns (rates and bimodal densities) suggested that HCL is a heterogeneous disease, consisting of at least two underlying subgroups and/or cancer populations by age‐at‐onset. Distinct early‐ and late‐onset HCL populations may reflect different age‐related causal pathways, risk factor profiles, and/or stem cells of origin.