UCSF Proposes First Staging System for Peritoneal Mesothelioma

Researchers at UCSF have proposed the first formal staging system for peritoneal mesothelioma, filling a gap that has hampered treatment decisions for decades. Unlike its pleural counterpart, peritoneal mesothelioma has never had its own TNM-based classification, leaving clinicians to rely on indirect measures.

The new system, developed by Dr. Mohamed Abdelgadir Adam and published in 2026 through the Society of Surgical Oncology, uses a data-driven survival tree model built from more than 20 years of surgical data at UCSF.

How the Staging System Works

The model draws on 172 people who underwent cytoreductive surgery (CRS) combined with hyperthermic intraperitoneal chemotherapy (HIPEC) at UCSF. Rather than relying on the Peritoneal Cancer Index (PCI) alone, the new system integrates three key variables that together predict survival more accurately.

The four proposed stages:

• Stage 1: Low mitotic rate (three or fewer per 10 high-power fields), the strongest predictor of favorable outcomes. These individuals are optimal candidates for CRS-HIPEC.
• Stage 2: Low mitotic rate combined with PCI scores above 10. Surgery remains beneficial, though tumor burden is higher.
• Stage 3: High mitotic rate (more than three) with epithelioid histology. CRS-HIPEC may still offer benefit, but outcomes are less favorable.
• Stage 4: High mitotic rate with biphasic or sarcomatoid histology. In these cases, the risks of surgery generally outweigh the potential gains.

Why Staging Matters

Peritoneal mesothelioma accounts for roughly 10 to 20% of all mesothelioma cases. It develops in the lining of the abdomen rather than the lungs and carries a significantly better prognosis than pleural disease, with median survival around 53 months and a 47% five-year survival rate for people who undergo CRS-HIPEC.

Despite these better outcomes, the absence of a formal staging framework has created challenges. Without standardized stages, it has been difficult to compare outcomes across centers, counsel families on prognosis, or consistently identify who will benefit most from surgery.

The PCI scoring system, which divides the abdomen into 13 regions and grades tumor burden from 0 to 39, has served as a practical tool. But PCI captures tumor volume only. It says nothing about how fast the cancer is growing or which cell type is present, both of which strongly influence survival.

What This Changes for Patients

For people diagnosed with peritoneal mesothelioma, the new staging system offers several practical benefits:

• Clearer prognosis: Families can understand expected outcomes based on specific tumor characteristics, not just broad categories
• Better surgical decisions: Stage 1 and 2 tumors respond well to CRS-HIPEC, while Stage 4 cases may be better served by systemic therapy
• Standardized language: Allows different treatment centers to compare results using the same framework

The system also helps identify candidates for clinical trials. UCSF is currently enrolling people with unresectable peritoneal and pleural mesothelioma in a Phase 2b trial of tremelimumab, with approximately 564 patients participating globally.

Limitations and Next Steps

The model is based on data from a single high-volume center where patients are carefully selected for surgery. Dr. Adam has acknowledged that external validation is the critical next step. Multi-institutional testing through the US HIPEC Collaborative network is underway.

If validated, this staging system could become the national standard, replacing ad hoc classification approaches and allowing more precise, personalized treatment planning for peritoneal mesothelioma.

People diagnosed with this disease should ask their surgical team about PCI scoring, mitotic rate, and histological subtype before making treatment decisions. Evaluation at a HIPEC-specialized center can help determine which stage applies and whether surgery is the right option.