A 38-year-old nulliparous woman undergoes hysterectomy for symptomatic uterine fibroids. Histopathological examination is performed. Which is the most common histological pattern seen in uterine leiomyomas?

Explanation

## Most Common Histological Pattern in Uterine Leiomyomas **Key Point:** The **fascicular pattern** (interlacing bundles of smooth muscle fibers) is the most common histological appearance of benign uterine leiomyomas, seen in the majority of cases. ### Histological Patterns of Uterine Fibroids | Pattern | Frequency | Microscopic Features | Clinical Significance | | --- | --- | --- | --- | | **Fascicular** | Most common | Interlacing bundles of smooth muscle separated by collagen; organized whorled or fascicular arrangement | Typical benign leiomyoma; no malignant potential | | **Whorled** | Common | Concentric rings of smooth muscle, resembling onion layers | Variant of benign leiomyoma; benign | | **Myxoid** | Less common | Abundant myxoid (mucoid) stroma with sparse smooth muscle cells | Benign variant; may appear degenerate on imaging | | **Cellular** | Uncommon | Increased cellularity, minimal collagen, crowded smooth muscle nuclei | May mimic leiomyosarcoma; requires careful evaluation to exclude malignancy | **High-Yield:** The fascicular pattern is characterized by **interlacing bundles of smooth muscle fibers separated by fibrous tissue (collagen)**. This is the hallmark of a benign leiomyoma and is the most frequently encountered pattern on histology. **Clinical Pearl:** The **cellular variant** of leiomyoma can be diagnostically challenging because increased cellularity and mitotic activity may superficially resemble leiomyosarcoma. However, benign cellular leiomyomas lack the diagnostic criteria for malignancy (>10 mitoses per 10 high-power fields, significant atypia, and tumor cell necrosis). ### Distinguishing Benign from Malignant Smooth Muscle Tumors ```mermaid flowchart TD A[Smooth Muscle Tumor of Uterus]:::outcome --> B{Mitotic Count<br/>per 10 HPF}:::decision B -->|0-4 mitoses| C[Benign Leiomyoma]:::action B -->|5-9 mitoses| D{Atypia Present?}:::decision B -->|≥10 mitoses| E[Leiomyosarcoma]:::urgent D -->|No| F[Benign Leiomyoma]:::action D -->|Yes| G[Leiomyosarcoma]:::urgent C --> H[Fascicular or Whorled Pattern]:::outcome F --> I[Cellular Leiomyoma<br/>Benign Variant]:::outcome ``` **Mnemonic:** **FWMC** — **F**ascicular (most common), **W**horled (common), **M**yxoid (less common), **C**ellular (uncommon but needs care to exclude malignancy). [cite:Robbins & Kumar Pathologic Basis of Disease 10e Ch 22]