Systemic mastocytosis: A study of trends, survival and second primary malignancies among adults in the United States.

e18597 Background: Systemic mastocytosis is a rare hematologic disorder characterized by the clonal proliferation and accumulation of aberrant mast cells in one or more organ systems, primarily driven by KIT mutations. Due to its rarity, its true epidemiological burden in the United States is a subject of ongoing research. The objective of our study was to estimate prevalence and trends in systemic mastocytosis among adults in the United States between 2000-2021, describe survival trends and the relationship between systemic mastocytosis and the development of second primary cancers in this population. Methods: This study is a retrospective cohort study utilizing data from the SEER Research Data on adult (≥20years) patients with systemic mastocytosis diagnosed between 2000-2021. Descriptive analyses were done on sociodemographic characteristics and their relationship with systemic mastocytosis. Univariate and multivariable logistic regression analyses were used to examine sociodemographic correlates and multivariate logistic and linear regression analyses were used to examine the impact of systemic mastocytosis on the development of second primary malignancy. Results: The age adjusted incidence rate of systemic mastocytosis in this time frame was 0.1 per 100,000 and was evenly distributed among males and females. Incidence was highest among the non-Hispanic white population (0.1 per 100000) when compared with the Hispanic (0.04 per 100,000) and black population (0.02 per 100,000). The incidence rate has generally increased in the last two decades. Female patients had significantly better survival than male patients, with a hazard ratio (HR) of 0.46 (95% CI: 0.35, 0.61, p < 0.01) after adjusting for confounders. Older patients also had decreased survival, with each 5-year increase in age associated with a 55% higher risk of mortality (HR: 1.55, 95% CI: 1.46, 1.64, p < 0.01). Race, origin, income, and metropolitan status did not show significant associations with survival. 14% of the cohort developed a second primary malignancy. Of these, 58 (45.7%) developed hematologic malignancies, and 43 (33.9%) had two or more new types of malignancies. The odds of developing a second primary malignancy increased by 1.23 (95% CI: 1.15–1.31, p < 0.01) for every 5-year increase in age. Female patients had lower odds of developing secondary malignancy with an adjusted odds ratio of 0.58 (95% CI: 0.39–0.87, p = 0.01). Conclusions: Our study reveals an increasing incidence of SM, now at 0.1 per 100,000, with incidence remaining disproportionately higher among non-Hispanic White individuals. Consistent trends reveal worse survival in males and older patients. The risk of a second primary malignancy is particularly elevated in these patients and represents a focus for clinical intervention and future studies.