James Johnston, MD
CancerCare Manitoba

Hairy cell leukemia is one of the rarest leukemias and in the USA the incidence is 3.2-3.3/million population/year. [1,2] The median age at diagnosis is 58 years and the disease is four-times more common in men than in women. [2] The disease is more common in Whites than in Blacks or Asians [1, 2]. The likelihood of developing the disease increases up to the age of 40 years and then begins to plateau with the median age at diagnosis being 58 years (57 years for men and 63 years for women [2]. Over the past 30 years the incidence of this disease has not changed.

Because of the rarity of this disease there have been few studies related to the aetiology of this disease. The cause of the disease is unknown but risk factors may include exposure to pesticides, herbicides and petrol or diesel fuel [3]. There is no association with cigarette smoking, alcohol or coffee consumption Employment in farming or woodworking is of borderline significance. A familial predisposition has also been suggested due to the clustering of cases within families, but whether this is related to genetic or environmental factors is unknown. [4].[5]There has been no consistent human leukocyte (HLA) linkage in these rare cases of familial HCL.

With the development of pentostatin and cladribine, most patients have a prolonged survival and there has been interest in whether these patients will have an increased risk of second malignancies. [6] This could be because HCL patients are inherently predisposed to developing another malignancy or it could be related to immunosuppression secondary to the leukemia or to the nucleoside analogs used to treat the disease. Many studies have been carried out, primarily from single institutes or based on patients entered on clinical trials, with disparate results. The largest population based study has been carried out on 3104 HCL patients identified from 1973 through 2002 through the National Cancer Institute’s Surveillance, Epidemiology and End Results (SEER) Program . The average duration of follow-up was 6.5 years (range, 2 months to 29.3 years). Patients had a 1.24 increased incidence of a second malignancy, compared to the general population. The cumulative risk of a second cancer at 10, 20 and 25 years following diagnosis was 13.2%, 24.5% and 31.9%, respectively. Patients had a significantly increased risk of developing Hodgkin’s disease, non-Hodgkin’s disease and thyroid cancer. The risk of thyroid cancer was in the first 5 years following diagnosis while the risk of lymphoma was still observed after 10 years. Interestingly the patients had a reduced risk of death from lung cancer, cardiovascular and cerebrovascular disease suggesting that these patients smoked less than the general population. However, there was an increased incidence of infections following the diagnosis of HCL, for up to 10 years with the highest risk in the first year following diagnosis.