UCSF Proposes First Peritoneal Meso Staging System
Dr. Mohamed Adam's survival tree model uses mitotic rate, PCI, and histology to stage peritoneal mesothelioma and guide CRS-HIPEC decisions.
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 | Tumor biology | Treatment implication |
|---|---|---|
| 1 | Low mitotic rate (three or fewer per 10 high-power fields) | Strongest predictor of favorable outcomes; optimal candidates for CRS-HIPEC |
| 2 | Low mitotic rate combined with PCI scores above 10 | Surgery remains beneficial, though tumor burden is higher |
| 3 | High mitotic rate (more than three) with epithelioid histology | CRS-HIPEC may still offer benefit, but outcomes are less favorable |
| 4 | High mitotic rate with biphasic or sarcomatoid histology | 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.
Cytoreductive surgery removes all visible tumors from the abdominal cavity. Immediately afterward, heated chemotherapy is circulated through the abdomen to kill remaining microscopic cancer cells. This combination is the standard of care for resectable peritoneal mesothelioma and is offered at specialized centers including UCSF.
What This Changes for Patients
For people diagnosed with peritoneal mesothelioma, the new staging system offers several practical benefits. Families can understand expected outcomes based on specific tumor characteristics, not just broad categories. Surgical decisions become more targeted: Stage 1 and 2 tumors respond well to CRS-HIPEC, while Stage 4 cases may be better served by systemic therapy. The framework also gives different treatment centers a standardized language for comparing results.
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.
References
UCSF Department of Surgery. Dr. Mohamed Abdelgadir Adam - Surgical Oncology.
https://surgicaloncology.ucsf.edu/bio/mohamed-abdelgadir-adam-md
UCSF Department of Surgery. Malignant Mesothelioma.
https://surgery.ucsf.edu/condition/malignant-mesothelioma
ClinicalTrials.gov. Randomized, Double-blind Study Comparing Tremelimumab to Placebo in Subjects with Unresectable Malignant Mesothelioma (DETERMINE).
https://clinicaltrials.gov/study/NCT01843374
Reader Q&A
Frequently Asked Questions
What is peritoneal mesothelioma?
Peritoneal mesothelioma is a cancer of the abdominal lining caused by asbestos exposure. It accounts for 10 to 20% of mesothelioma cases. People with peritoneal mesothelioma generally have better outcomes than those with pleural (lung) mesothelioma, particularly when treated with CRS-HIPEC.
Why didn't peritoneal mesothelioma have a staging system before?
The disease is rare, affecting roughly 500 people per year in the US. Unlike pleural mesothelioma, which uses the TNM system, peritoneal cases spread diffusely across the abdominal cavity in patterns that don’t fit traditional staging frameworks. Clinicians relied on the PCI score, which measures tumor volume but not biology.
What is mitotic rate and why does it matter?
Mitotic rate measures how quickly cancer cells are dividing. A low mitotic rate (three or fewer per 10 high-power fields) indicates slower-growing tumors that respond better to surgical treatment. In the UCSF model, mitotic rate was the single strongest predictor of survival.
Where can patients get CRS-HIPEC?
CRS-HIPEC is performed at specialized cancer centers with high-volume surgical programs. UCSF Helen Diller Comprehensive Cancer Center is one of the leading institutions for peritoneal surface malignancies.
What is the life expectancy after cytoreductive surgery?
Cytoreductive surgery with HIPEC for peritoneal mesothelioma shows median overall survival of 53 months in people with the disease. Studies report 5-year overall survival rates exceeding 69% for peritoneal mesothelioma and up to 80% with added chemotherapy. For peritoneal metastases from other cancers like colorectal, median survival ranges from 22 to 52 months, with 5-year rates of 27-60% depending on tumor burden and resection completeness. Outcomes vary by cancer type, disease extent, and treatment details.
What are the three types of mesothelioma?
The three main histological cell types of mesothelioma are epithelioid, sarcomatoid, and biphasic. Epithelioid mesothelioma is the most common, occurring in 50-70% of people with the disease, and shows better response to treatment. Sarcomatoid accounts for 10-20% of cases and has the poorest prognosis, while biphasic (20-30% of cases) contains a mix of both cell types and prognosis depends on the dominant subtype. Rare subtypes include desmoplastic, deciduoid, and lymphohistiocytoid. These classifications guide treatment planning based on cell behavior under microscopy.
Is cytoreductive therapy chemotherapy?
Cytoreductive therapy refers to different treatments depending on context. In blood cancers like myeloproliferative neoplasms, cytoreductive therapy uses medications to reduce abnormal blood cell levels. In abdominal cancers, “cytoreductive surgery” (CRS) is a surgical procedure that removes visible tumors, often combined with hyperthermic intraperitoneal chemotherapy (HIPEC), where heated chemotherapy is delivered directly into the abdominal cavity. The chemotherapy component is separate from the surgery itself. So cytoreductive therapy is not chemotherapy alone, but rather a multimodal approach that may include surgery, chemotherapy, or both depending on the cancer type and treatment protocol.