The CMPDs are primarily seen in the adult population, with a peak in the fifth to seventh decades of life. The combined annual incidence of the CMPDs is approxi-

Table 45.2 Operational classification of chronic myeloid disorders

Chronic myeloid leukemia Myelodysplastic syndrome IPSS—low risk IPSS—intermediate risk 1 IPSS—intermediate risk 2 IPSS—high risk Chronic myeloproliferative disease Essential thrombocythemia Polycythemia vera

Myelofibrosis with myeloid metaplasia Atypical chronic myeloid disorder Chronic neutrophilic leukemia Chronic eosinophilic leukemia Hypereosinophilic syndrome Chronic basophilic leukemia Juvenile myelomonocytic leukemia Chronic myelomonocytic leukemia Transient myeloproliferative/leukemia syndrome of Down syndrome

Systemic mast cell disease

Otherwise undefined myeloproliferative disorder

IPSS, International Prognostic Scoring System.

mately 6-9/100,000 people.6 The first epidemiologic data were based on a large population study published by Prochazka and Markowe.10 The incidence of PV in Europe and North America is similar, with eight to ten cases per million individuals per year.11 PV varies from 2 cases per million individuals per year in Japan to 13 per million per year in Australia.6

In a population-based study in Olmsted County, Minnesota, from 1935 to 1989, the age- and sex-adjusted incidence rate of PV was 1.9/100,000 person-years.12 In the same county, in another population-based study from 1976 to 1995, the authors reported an approximate annual incidence of 2.5, 2.3, and 1.3 per 100,000 for ET, PV, and de novo agnogenic myeloid metaplasia (AMM), respectively. In a report from the United Kingdom, from 1984 to 1993, 2376 cases of CMPDs, including ET, PV, and AMM, were reported.13 The standardized incidence rate was 2.27/100,000 person-years for all three conditions. The incidence rate per 100,000 for PV, ET, and AMM was 0.92, 0.79, and 0.57, respectively. Although in adults the true incidence of chronic idiopathic fibrosis is not known, it is estimated to be between 0.5 and 1.5 per 100,000 individuals per year.914 The true incidence for chronic neutrophilic leukemia (CNL) is unknown, as fewer than 100 cases have been reported.6 In a study of 660 cases of chronic leukemias of myeloid origin, not one single case of CNL was observed.15 Chronic eosinophilic leukemia (CEL) and hypereosinophilic syndrome (HES) are rare diseases. As it is often difficult to distinguish between the two, the true incidence of these conditions is unknown. It appears that approximately 10-20% of all cases of CPMDs may be unclassifiable.6

PV appears to be more common in men than in women, with reported male-to-female ratios ranging from 1.2 to 2.2 in various studies.11-1316 In the 1000 cases of ET in a study by McNally et al., there was no difference between the sexes. An age-specific (30-50 years) female preponderance in ET was seen, however. More males were affected by AMM. Males are more commonly affected by CEL and HES than females, with a male-to-female ratio of around 9:1 in HES.6

CMPDs are diseases of older individuals, with low rates until age 50, and a peak in incidence from 60 to 80 years of age.111316 The maximum incidence of PV exceeded 20/100,000 person-years in the Olmsted County data. The median age of presentation is similar in ET, PV, and AMM, being 55-65 years. The mean age at diagnosis of PV has been increasing steadily since the 1920s.131617 A few cases of PV diagnosed under the age of 40 have been reported.18 The age-specific mortality calculated by Prochazka and Markowe10 showed a sharp increase beginning in the early 40s, with the age-specific mortality reaching a maximum in the 75-and 84-year age group. CNL has been reported in both adolescents and older adults.19 20 HES may present at any age, though its peak incidence is in the fourth decade of life.21

Racial and ethnic factors influence the incidence of PV. PV is significantly less common in blacks than in whites. It is also more common in individuals of Jewish origin (Ashkenazi) than of non-Jewish origin.112223

Familial occurrence of PV has been reported, with a 6% incidence of patients enrolled in the protocols of the Polycythemia Vera Study Group, and in some sporadic cases.24'25

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