Mitotic Figure is the Most Important Prognostic Factor for Gastric Leiomyoma
The mitotic figure count is the most significant prognostic factor for gastric leiomyoma, with high mitotic rates strongly correlating with aggressive behavior and poor outcomes regardless of other factors.
Understanding Gastric Leiomyoma
Gastric leiomyomas are benign smooth muscle tumors that arise from either the muscularis mucosae or muscularis propria within the wall of the stomach. They are relatively rare in the stomach compared to gastrointestinal stromal tumors (GISTs), which were often misclassified as leiomyomas in older literature 1.
True gastric leiomyomas are characterized by:
- Well-differentiated smooth muscle cells
- Positive staining for smooth muscle actin and desmin
- Negative staining for CD117, CD34, and S100 protein
- Hypoechoic, well-circumscribed masses on EUS examination
Key Prognostic Factors in Order of Importance
1. Mitotic Figure Count (Most Important)
- High mitotic rates (>5 per 50 HPF) dramatically increase risk of malignant behavior 2
- For gastric tumors of the same size (5-10 cm), the difference in risk between low mitotic rate (≤5 per 50 HPF) and high mitotic rate (>5 per 50 HPF) represents a 15-fold increase in risk of metastasis 2
- Even for smaller tumors (2-5 cm), the risk of metastasis varies significantly with mitotic rate, from 1.9% with low mitotic rates to 16% with high mitotic rates 2
2. Tumor Size
- While important, size has less pronounced impact on prognosis than mitotic rate 2
- A small tumor (<2 cm) with high mitotic rate has worse prognosis than a larger tumor (2-5 cm) with low mitotic rate 2, 3
- For uterine leiomyosarcomas, tumor size >5 cm is associated with poorer survival 4, but mitotic count remains more significant for gastric leiomyomas
3. Vascular Invasion
- Significant but not as critical as mitotic count 5
- Associated with metastasis-free interval in early-stage disease 5
4. Nodal Invasion
- Least important of the options
- Lymph node metastasis is uncommon in smooth muscle tumors of the GI tract 1
- Nodal dissection is not routinely recommended during surgery 1
Risk Assessment for the Case Presented
For the patient with a 9×7 cm anterior gastric wall mass diagnosed as leiomyoma:
- The large size (9×7 cm) places the tumor in a higher risk category
- However, the mitotic count would be the most critical factor to assess
- If the mitotic count is low (≤5 per 50 HPF), the prognosis is significantly better despite the large size
- If the mitotic count is high (>5 per 50 HPF), the risk of aggressive behavior increases dramatically
Management Implications
- Complete surgical excision with negative margins (R0) is the standard treatment 1
- For tumors with high mitotic rates, closer follow-up is warranted
- The pathology report should include:
- Three-dimensional tumor measurement
- Mitotic count per 5 mm² (equivalent to 50 HPF)
- Presence/absence of necrosis
- Distance between lesion and margin 1
Conclusion
When evaluating the prognosis of a gastric leiomyoma, the mitotic figure count should be considered the most important factor, followed by tumor size, vascular invasion, and lastly nodal invasion. This is supported by multiple guidelines and studies showing that mitotic activity has the most dramatic impact on clinical outcomes, regardless of other factors.