Diagnostic Value and Interobserver Reproducibility of Histopathological Features in Fumarate Hydratase-Deficient Uterine Leiomyomas

Background/Objectives: Fumarate hydratase-deficient leiomyomas (dFH-LMs) are a rare subtype of uterine smooth muscle tumors (USMTs) with implications for hereditary leiomatosis and renal cell carcinoma (HLRCC). Although several morphologic clues have been proposed, their diagnostic reproducibility is poorly defined. We aimed to determine the diagnostic significance of histopathologic features associated with fumarase deficiency and the reproducibility of key morphologic criteria for defining dFH-LMs. Methods: A retrospective analysis was performed on 45 USMTs that were initially classified as atypical leiomyomas (ALMs). The cohort comprised patients aged 21 to 75 years who had surgery at one tertiary medical care center from 2016 to 2025. Hematoxylin–eosin (H interobserver reproducibility was substantial (κ = 0.63). Staghorn-like vessels occurred in 73.1% of dFH-LMs vs. 26.3% of ALMs (p = 0.02) and diffuse nuclear atypia (84.6%) was also more frequent in dFH-LMs (p = 0.01). Patients with dFH-LMs were significantly younger than those with ALMs (Median, 34 vs. 41 years). Conclusions: Eosinophilic globules, staghorn-like vessels and diffuse nuclear atypia have been shown to have high diagnostic value and could be considered morphologic indicators of dFH-LMs. The substantial interobserver reproducibility of eosinophilic globules makes this feature particularly promising for routine clinical practice.