Second malignancies among older patients with classical myeloproliferative neoplasms treated with ruxolitinib.

Polycythemia vera (PV), essential thrombocythemia (ET), and myelofibrosis (MF) are classical Philadelphia chromosome-negative myeloproliferative neoplasms (MPNs) associated with an increased risk of development of second primary malignancies (SMs). The reasons for solid and lymphoid SMs are unclear, but a therapy-related effect has been invoked. Although an increased risk of nonmelanoma skin cancers is associated with the use of ruxolitinib (RUX), its influence on the development of other forms of SMs remains controversial. We conducted a retrospective cohort analysis to assess associations between RUX and SMs among older patients diagnosed with MPN from 2012 to 2019 in the Surveillance, Epidemiology, and End Results-Medicare-linked database. We identified 6043 patients (2396 with PV, 2944 with ET, 703 with MF) with a median age of 76 years (interquartile range [IQR], 71-82). After a median follow-up of 3.66 (IQR, 2.25-5.17) and 3.02 years (IQR, 1.84-4.75) for 513 RUX users and 5530 nonusers, respectively, 469 patients developed an SM: 383 solid and 86 lymphoid. In the multivariable proportional subdistribution hazard regression model with death as a competing risk, the risk of developing any SM did not differ by RUX use status (hazard ratio [HR], 0.96; 95% confidence interval [CI], 0.65-1.42;