Systemic Mastocytosis in United States: A Population Based Study
Authors: Bista Amir , Uprety Dipesh , Vallatharasu Yazhini , Arjyal Lubina , Ghimire Subash , Giri Megha , Rosenstein Lori 
Published: October 2019

Abstract
Introduction:
Systemic mastocytosis (SM) is a rare hematological disorder characterized by clonal proliferation and activation of abnormal mast cells. It can vary from an indolent form to an aggressive form including progression to leukemia. There is limited data on epidemiology, clinical characteristics and outcome of this disease in population based setting. Till date, a retrospective study by Lim, Ken-Hong, et.al which included 342 patients diagnosed with systemic mastocytosis in Mayo clinic, is the largest series of patient published so far. This study reports the clinical presentation and outcome of patients with SM but there are no population based study in United States so far. We therefore conducted this population based study to determine epidemiology, survival pattern and incidence of second primary malignancy among patients with SM.
Methods:
We used SEER 18 database (2000-2014) to select all adult patients with age 20 or above with SM. Patient population was divided into various cohorts based on age (20-59, 60-79 and 80+ years), sex, race (Caucasians, African American and Others), area of residence (rural, urban and metropolitan) and annual household income (<$25000, $25000-<$50000 and ≥$50000). Age adjusted incidence rate was calculated using 2000 US standard population using SEER stat rate session. 5-year relative survival (RS) rate was calculated using SEER stat and compared using Z test. Cox proportional hazard model was used for multivariate analysis of factors associated with relative survival using Cansurv software. MP-SIR session in SEER stat was used to calculate the risk of second primary malignancy.
Result:
The incidence was found to be 0.046 per 10000 among general population. Incidence was found to be higher among Caucasians compared to African American (0.056 vs. 0.018 per 100000). Median age at diagnosis was 55 years. Of the total 425 patients, majority were Caucasians (92.5%), age <60 years (59.3%) and from metropolitan area (84.7%) but there was equal distribution among male and females.
10 year overall survival was found to be 61.5±3.1% by KM curve. 5-year RS was found to be 74.0±2.7% for the whole population. Females had significantly better survival compared to males, 5-year RS of 84.7±3.1% vs. 62.3±4.3%, P <0.0001. Survival trended to be better for Caucasians compared to African American but didn't reach clinical significance, 74.6±2.8 vs. 50±14.8, p 0.08. Patients <60 years had significant better survival compared to 60 to 79 and 80+ years ( 5-year RS of 88.6±2.4 vs. 58.4±5.2 and 16.0±11.2 respectively with P <0.0001 for both comparison). In multivariate analysis, younger age group, female sex and Caucasian race were found to be independent predictor of better 5 year relative survival with P<0.0001 compared to their counterparts.
Patients with systemic mastocytosis were found to have higher risk of developing both solid organ as well as hematological malignancy within 5 years of diagnosis (as shown in table 1). After that the risk decreased and was comparable to general population.
Conclusion:
Our study shows that systemic mastocytosis is rare disease in general population and survival is better among specific subgroup of patients including females, younger patient and Caucasians. As the majority deaths occurred in first 5 years after diagnosis and as incidence of second primary malignancy is higher in the first 5 years, we recommend close follow up for first five years after diagnosis.

Disclosures
No relevant conflicts of interest to declare.