Cancer-associated myocarditis outside of immunotherapy: National burden and inpatient outcomes in the National Inpatient Sample, 2018–2023.

e23218 Background: Immune checkpoint inhibitor myocarditis is increasingly recognized in oncology, yet myocarditis occurring outside of immunotherapy remains poorly characterized. Understanding the inpatient burden and outcomes of non-immunotherapy myocarditis may broaden recognition of high-acuity cardiac phenotypes during cancer hospitalizations. Methods: A serial cross-sectional analysis was conducted using adult hospitalizations with a principal diagnosis of malignancy in the 2018 to 2023 Healthcare Cost and Utilization Project National Inpatient Sample with discharge-level survey weighting. Myocarditis was identified using ICD-10-CM I40* in any diagnosis field. Because a specific immune-related adverse event indicator was unavailable, a conservative non-immunotherapy proxy cohort was defined by excluding hospitalizations with T45.1X5* codes for adverse effects of antineoplastic and immunosuppressive drugs. Outcomes included in-hospital mortality, shock (R57*), mechanical ventilation (ICD-10-PCS 5A1935Z, 5A1945Z, 5A1955Z), length of stay, and hospitalization cost. National estimates accounted for survey stratification and clustering. Survey-weighted multivariable logistic regression evaluated adjusted associations. Results: Among an estimated 5,788,489 adult cancer hospitalizations nationally, myocarditis prevalence was 0.00397%. In the non-immunotherapy proxy cohort, representing approximately 5,446,609 hospitalizations, myocarditis prevalence was 0.00340%. Compared with hospitalizations without myocarditis, non-immunotherapy myocarditis was associated with substantially worse unadjusted outcomes, including higher in-hospital mortality (24.32% vs 4.35%), mechanical ventilation (24.32% vs 2.58%), and shock (10.81% vs 1.46%). Resource utilization was markedly higher, with longer length of stay (24.76 vs 6.45 days) and higher hospitalization cost ($193,612.80 vs $28,706.34). In adjusted analyses within the non-immunotherapy proxy cohort, myocarditis was not independently associated with in-hospital mortality (odds ratio 1.62, 95% CI 0.21 to 12.50, P = 0.643) or mechanical ventilation (odds ratio 3.02, 95% CI 0.40 to 22.95, P = 0.286), reflecting limited precision due to rarity. Conclusions: Myocarditis occurring outside of immunotherapy is rare during cancer hospitalizations but identifies an extreme-acuity inpatient phenotype marked by high unadjusted mortality, frequent shock and mechanical ventilation, and substantial resource utilization. Although adjusted associations are limited by rarity, these findings underscore the clinical importance of early recognition and escalation for suspected myocarditis in hospitalized oncology populations.