Background: Primary myelofibrosis (PMF) carries a poorer prognosis compared to other BCR-ABL-negative myeloproliferative diseases (MPD), and there is increased risk of early mortality due to blast transformation, thrombosis, bleeding complications, and progression of disease. Prior studies report greater incidence of second primary malignancy (SPM) among MPD patients, although the true risk in PMF has not been elucidated. We performed a large database study to evaluate the risk of SPM in PMF patients and analyzed the effects of sociodemographic factors on the risk of SPM. Methods: We used the Surveillance, Epidemiology, and End Results (SEER) database to identify all patients with a histologic diagnosis of PMF from 2009 to 2018. SPM was defined as any subsequent malignancy that developed at least 1 year after the diagnosis of PMF. Using multiple primary standardized incidence ratio (SIR) session of the SEER*Stat software (version 8.3.9), we calculated SIR and absolute excess risk (AER) of SPM for the entire cohort of PMF and also stratified based on age, sex, race, marital status, receipt of chemotherapy, follow-up duration, and year of diagnosis. We generated the 95% confidence intervals (CI) and p-values assuming Poisson distribution of the observed incidences of SPM. Results: A total of 5,273 patients with PMF were included in the analysis, of which 342 patients (6.4%) developed SPM. SPM occurred at SIR of 1.97 (95% CI 1.77-2.18, p<0.05) and AER of 151.87 per 10,000 population. A significantly higher incidence of melanoma (SIR 1.96, 95% CI 1.14-3.14, p<0.05), lymphoma (SIR 3.45, 95% CI 2.31-4.96, p<0.05), and leukemia (SIR 26.87, 95% CI 22.69-31.59, p<0.05) was observed. There was no statistically significant difference in SIR based on sex, race, marital status, follow-up duration, and receipt of chemotherapy. The risk was significantly higher in patients ≤60 years vs patients >60 years (SIR 2.34, 95% CI 1.89-2.86 vs SIR 1.86, 95% CI 1.65-2.10, p 0.01) and for PMF diagnosed after 2009 vs ≤2009 (SIR 2.64, 95% CI 2.26-3.07 vs SIR 1.61, 95% CI 1.40-1.85, p<0.001). Conclusions: Patients with PMF are at a high risk of developing SPM, especially leukemia and lymphoma. Data suggests higher incidence of SPM in patients aged ≤60 years and in the decade after 2009. The impact of ruxolitinib, which was approved in 2011, on the incidence of SPM deserves further study.

# represents p<0.05.