A Population-Based Study of Incidence of Myeloproliferative Neoplasms in Sweden Between 2000 and 2012

Background

The Myeloproliferative neoplasms (MPNs) consists of the subtypes polycythemia vera (PV), essential thrombocythemia (ET), primary myelofibrosis (PMF), and MPN unclassifiable (MPN-U). The incidence rates of these diseases vary substantially between different reports, ranging from 1.15 to 4.99/100,000 person-years. However, in a recent metaanalysis, there was no significant difference in MPN incidence between Europe and North America and the variations in incidence may therefore reflect the quality of the cancer registers and reporting of MPNs. In addition, there is a limited number of reports on MPN incidence during more recent years. Therefore, we assessed the incidence of MPN based on the Swedish Cancer Register, a high-quality population-based cancer register between 2000 and 2012.

Patients and Methods

The Swedish Cancer Register was used to identify all patients diagnosed with an MPN between January 1^st 2000 and December 31^st 2012. These Swedish Cancer Registers have very high levels of quality and completeness. Between 2008 and 2012, the reporting of newly diagnosed MPN to the cancer register was >92%. Information on the Swedish population was obtained from the Human Mortality Database (www.mortality.org). Based on information from these registers, incidence rates of MPNs with 95% confidence intervals (CIs) were calculated. Confidence intervals were estimated on the log scale. In addition, the incidence rate in relation to MPN subtype, age group (18-39, 40-49, 50-59, 60-69, 70-79, and ³80 years), as well as calendar year of diagnosis was assessed.

Results

A total of 5,442 MPN patients were reported to the cancer register between 2000 and 2012. During these years, there were 1,810 incident cases of PV, 1,862 of ET, 636 of PMF, and 1,134 with MPN-U. Between January 1^st 2000 and December 31^st 2012, the population in Sweden increased from 8,861,426 to 9,555,893 inhabitants.

The overall annual incidence rate of MPN was 5.83 (95% CI 5.68-5.99)/100,000 persons. The incidence rate of PV was 1.94 (1.85-2.03), ET 2.00 (1.91-2.09), PMF 0.68 (0.63-0.74), and MPN-U 1.22 (1.15-1.29) per 100,000 person-years. In addition, there was a strong correlation between age and incidence of MPN with incidence rates being substantially higher among the older age groups (Table).

The overall incidence rate of MPNs increased during the study period, from 5.06 (4.55-5.62)/100,000 person-years in the year 2000 to 5.98 (5.45-6.55)/100,000 person-years in 2012. The incidence rate of PV was similar throughout the study period, the incidence was 2.05 (1.74-2.42)/100,000 person-years in 2000 and 2.12 (1.81-2.47)/100,000 person-years in 2012. The annual incidence rate of ET and PMF increased, from 1.62 (1.34-1.95) to 2.49 (2.15-2.87) per 100,000 persons for ET and from 0.36 (0.24-0.53) to 0.86 (0.67-1.10) per 100,000 persons for PMF between 2000 and 2012. Conversely, the incidence of MPN-U decreased, 1.03 (0.81-1.29) to 0.52 (0.38-0.71)/100,000 person-years between 2000 and 2012.

Summary and Conclusions

In this large population-based study, the incidence of MPN was higher than previously reported in both European and North American studies. As earlier lower incidence rates likely are an effect of limited coverage of cancer registers, there may be an underreporting of MPNs in many European and American countries. The increase in MPN incidence rates during the study period may reflect increasing life expectancy of the Swedish population, improved reporting to the cancer register as well as changes in the classification and diagnostic systems. Similarly, the decrease in incidence of MPN-U is also likely a result of improved diagnostics during more recent years. In conclusion, the MPN incidences rates reported here are presumably more accurate compared to earlier reports due to the high level of coverage and accuracy of the Swedish registers.