Proposed Peritoneal Mesothelioma Staging Could Fill Critical Gap
Research & Clinical Trials
Written by Sean Marchese | Edited by Amy Edel
A research team has proposed a new way to stage malignant peritoneal mesothelioma, a rare cancer that spreads across the lining of the abdomen. Cancer staging is how doctors describe how advanced a disease is. Staging essentially answers: “How big have the tumors grown? And how far have they spread?”
But for peritoneal patients, the traditional mesothelioma staging systems don’t work well because this cancer behaves differently, as peritoneal surgeon Dr. Mohamed Abdelgadir Adam at the University of California San Francisco explained in a conversation with us. It spreads throughout the abdominal lining rather than forming distinct tumors that grow and spread to lymph nodes like most other cancers do.
The new approach uses a data-driven “survival tree” model that analyzes which combinations of tumor characteristics most strongly predict how long patients survive. That means patients within each stage have more similar outcomes, making it easier for doctors to explain what to expect and plan appropriate treatment.
The work was shared in the 2026 edition of the Society of Surgical Oncology and Advanced Cancer Therapy journal. The results are based on more than 20 years of data from 172 patients with peritoneal mesothelioma who underwent cytoreductive surgery with heated chemotherapy, known as HIPEC.
Why Staging In Peritoneal Mesothelioma Has Been So Hard
Cancer staging is supposed to act like a shared language. It helps doctors estimate how aggressive a cancer may be, compare outcomes across hospitals and decide which treatments or clinical trials make the most sense.
But peritoneal mesothelioma doesn’t behave like many other cancers that fit neatly into a TNM framework, which tracks tumor size, lymph node spread and metastasis to other organs. “Peritoneal behaves in a diffuse way,” Dr. Adam explained in his conversation with us.
“You don’t find one source. It’s all over,” he said. He added that traditional TNM categories often don’t translate well to disease largely confined to the abdominal cavity.
That mismatch is widely recognized in the medical literature. This asbestos-related cancer rarely spreads to lymph nodes or distant parts of the body like many other cancers do.
The PCI Helped, But It Still Left Gaps
In real-world care, many specialists already rely on the Peritoneal Cancer Index, a scoring system that estimates how much tumor is present throughout the abdomen. PCI is typically calculated across 13 regions and ranges from 0 to 39, with higher scores reflecting more extensive disease.
A 2011 staging system tried to adapt the TNM framework, using PCI to represent the “T” or tumor spread. But PCI still only measures how much tumor is present, not how fast it’s growing or how likely it is to respond to mesothelioma treatment.
Dr. Adam told us his team kept seeing cases where “what we currently go by” didn’t fully explain why some patients did unexpectedly poorly after aggressive surgery. And others with seemingly advanced disease might still deserve a closer, more data-driven look.
New Model Considers How Tumors Behave, Not Just How Much Is There
In the published work, researchers describe using a computer program to build the “survival tree.” The program identified factors that best predicted survival (in what’s called “recursive feature selection”) and then grouped patients based on those factors.
Patients in the same stage now had similar outcomes, while patients in different stages had clearly different outcomes. The model found several factors that reduced mesothelioma survival: how quickly tumor cells divide (mitotic rate), certain aggressive cell types (biphasic or sarcomatoid), more widespread cancer (higher PCI) and worsening symptoms.
In speaking with us, Dr. Adam emphasized one finding in particular: “Mitotic rate is a very important predictor.” He described the speed of tumor spread as even more informative than mesothelioma cell types in their modeling. He explained the goal isn’t to replace a doctor’s judgment but to better understand how different tumor features combine to affect survival.
Four Stages With Sharper Survival Separation
Using the survival tree approach, the researchers identified 4 distinct prognostic stages based on combinations of tumor characteristics. Each stage showed clear differences in how long patients survived after treatment.
- Stage 1: Low mitotic rate (≤3)
- Stage 2: Low mitotic rate and PCI >10
- Stage 3: High mitotic rate (>3) and epithelioid subtype
- Stage 4: High mitotic rate and biphasic or sarcomatoid subtype
Median overall survival for Stage 1 hasn’t been reached yet, reflecting continued survival among these patients. Median overall survival was 42 months for Stage 2, Stage 3 was 22 months and Stage 4 was 5 months.
The team compared those results to a more traditional system. In that older framework, median survival didn’t separate cleanly by stage. For example, Stage 3 showed a higher median survival than Stage 2 in this dataset, which is one reason the researchers called for better ways to identify high-risk patients..
What This Could Change For Patients Considering Surgery With HIPEC
Dr. Adam was clear that the staging system “wouldn’t impact” how cytoreductive surgery plus heated chemotherapy is performed. He explained the impact would “only [be] on patient selection,” especially when deciding who is most likely to benefit from tumor-removing surgery with HIPEC versus other approaches.
He added that, in their model, patients in Stages 1 and 2 and potentially Stage 3 appeared most likely to benefit from CRS with HIPEC. He also cautioned that Stage 4 patients weren’t candidates for HIPEC, because the tumors may be too aggressive for the risks of major surgery to make sense.
At the same time, he emphasized urgency. Peritoneal mesothelioma “is very aggressive,” he said, and delays can be costly for people who might do well with prompt evaluation from a mesothelioma specialist. “It would be a shame to delay someone who would do well,” he noted, because that can “minimize their survival potential.”
What Patients Should Know About The Limits Right Now
Even strong early results come with caveats. The study cohort included patients treated surgically at a single center, which means the model reflects outcomes among people who were already selected for CRS with HIPEC.
Dr. Adam also pointed to the practical challenge of building predictive models for rare cancers. “It’s an orphan cancer,” he said, and even large referral programs may only see relatively few cases in a short time window, which can affect model accuracy and generalizability.
Because of that, he urged caution about using the model in routine practice before broader validation. He described the work as a step toward more objective, consistent decision-making rather than a final answer in treatment planning.
What Happens Next
Dr. Adam said the next phase is external validation and expansion to a multi-institutional dataset. He described collaborations across a U.S. HIPEC network and said the team hopes to begin multi-center work once validation efforts are completed, with a goal of scaling the project into something big enough to support more consistent staging across hospitals.
If the model holds up across centers, he believes it could give patients clearer expectations and help them advocate for the right next step at the right time. “If validated, we can put you in boxes that are more accurate about the severity of the disease,” he said, so people can make decisions that fit their situation and access appropriate care without unnecessary delay.
