How “benign” is cutaneous mastocytosis? A Danish registry-based matched cohort study

Abstract Background There are limited estimates of the incidence rates (IRs) of mastocytosis, and only a few studies have addressed the long-term consequences of living with these diagnoses. Previous reports have shown that systemic mastocytosis (SM) is associated with leukemic transformations and an increased risk of death as opposed to cutaneous mastocytosis (CM) and indolent SM (ISM), benign diagnoses with life expectancy rates similar to those of the background population. Objectives To study the incidence and mortality of mastocytosis. Methods A population-based matched cohort study on mastocytosis patients identified from the Danish National Health Registries from 1 January 1977 to 31 December 2014. IRs of CM, ISM and paediatric mastocytosis were highlighted. Survival estimates were compared to those of a healthy background population, using a Cox proportional hazard model. Results 1,461 patients with mastocytosis were identified. The annual IR of overall mastocytosis was 1.1 per 100,000 person years (95% CI 1.0-1.2). Among children, the IR was 1.8 per 100,000 person years (95% CI 1.6-2.1). The prevalence of any comorbidity was twice as high among mastocytosis patients as in the non-mastocytosis population (odds ratio 2.1 95% CI 1.8-2.5). The CCI-adjusted mortality among adult mastocytosis patients was HRCM 1.2 (95% CI 0.8-1.9), HRISM 1.9 (95% CI 1.4-2.5) and HRSM 4.2 (95% CI 1.9-9.4) respectively. Conclusion Based on an entire nation, with healthcare free at the point of access, we estimated an annual IR of mastocytosis and its subgroups. We discovered that ISM patients had an increased risk of death compared to the general population. Our data supported the overall benign nature of CM diagnosed after the age of two years.