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&E) slides were independently examined by three pathologists for features associated with FH deficiency, including eosinophilic globules, staghorn-like vessels, diffuse nuclear atypia, "bizarre" nuclei, and prominent nucleoli with halos. Molecular status was determined by immunohistochemistry (IHC) for fumarate hydratase (FH) and S-(2-succino)-cysteine (2SC). Interobserver agreement was quantified using Fleiss's κ and Cohen's κ. Results: Loss of FH expression was detected in 26/45 tumors (57.7%). Eosinophilic globules occurred in 88.5% of dFH-LMs, but only in 15.8% of ALMs (p < 0.001). By majority consensus (≥2 of 3 reviewers), the eosinophilic globules predicted FH deficiency with a sensitivity of 88.0% and a specificity of 85.0%; 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.