APPENDICURE

Why moderately differentiated appendiceal cancer keeps getting lost in the high-grade bucket, and why that matters for patients

A member of the Appendicure community raised a concern recently that we want to address head-on. She had been reading study after study about appendiceal cancer and noticed something that kept happening. The literature would describe results for “high-grade” disease, and moderately differentiated tumors would be quietly folded into that category. Sometimes it was explicit. More often it was not. Her tumor was moderately differentiated, which meant that every time she tried to understand her own prognosis or treatment options, she was looking at data that had been averaged with patients whose disease behaved very differently from hers.

She is not wrong, and she is not alone. This issue shows up constantly in both the scientific literature and in how oncologists talk to patients. We think it is worth pulling apart why it happens, what the evidence actually says, and why moderately differentiated appendiceal cancer deserves to be discussed on its own terms.

Three tiers, one name problem

The grading system most U.S. pathologists use for appendiceal mucinous tumors comes from the AJCC 8th edition, and it has three tiers. Grade 1 is well-differentiated. Grade 2 is moderately differentiated. Grade 3 is poorly differentiated, often with signet ring cells. The international specialist consensus group, PSOGI, uses a parallel three-tier system for pseudomyxoma peritonei. The World Health Organization has at times used a two-tier scheme, which is part of where the confusion starts.

In practice, what happens is this. Researchers designing a study will group patients by grade, but to get sample sizes large enough to analyze, they often collapse grades 2 and 3 together under the label “high-grade.” Clinicians then read those studies and start using “high-grade” in conversation with patients, without specifying whether they mean moderately or poorly differentiated. A moderately differentiated tumor and a poorly differentiated tumor with signet ring cells get discussed as if they were the same disease, even though pathologists trained their whole careers to distinguish between them.

The reason the three-tier distinction exists is that the three grades behave differently. Treating grade 2 like grade 3 means looking at a bleaker prognosis than the data warrants. Treating grade 2 like grade 1 means underestimating the risk. Neither is fair to the patient trying to plan.

The Taggart study, in plain terms A 2017 study from specialized surgeons looked at 265 patients with mucinous appendiceal adenocarcinoma who underwent CRS/HIPEC. When they broke the results down by the three grade tiers, the differences were striking. Five-year overall survival:   •  Well-differentiated (grade 1): 94 percent   •  Moderately differentiated (grade 2): 71 percent   •  Poorly differentiated (grade 3): 30 percent Five-year disease-free survival showed a similar pattern: 66 percent, 21 percent, and 0 percent. The authors concluded that moderately differentiated MAA has a clinical course distinct from both well- and poorly-differentiated disease, and that grouping it with either one loses important prognostic information.

Why the middle tier gets erased

There are a few reasons moderately differentiated disease keeps getting collapsed into the high-grade bucket, and none of them are sinister. They are mostly practical.

Statistical power is one. Appendiceal cancer is rare, and within the appendiceal cancer population, any individual grade is rarer still. When a study starts with a few hundred patients and needs to analyze survival curves, splitting the group into three can leave too few patients in each arm to detect meaningful differences. Combining grades 2 and 3 produces a larger group and a cleaner statistical signal, even though it obscures what the signal actually means.

Historical grading systems are another. For a long time, the appendix was graded using a two-tier system borrowed from colorectal cancer, low-grade and high-grade. Older studies used that framework, and the literature that followed them often kept it for continuity. The three-tier AJCC system is the current standard, but the language of the older system lingers in clinic notes, in discharge summaries, and in how doctors explain things to patients at the bedside.

Terminology overlap makes it worse. When a pathologist writes “high-grade cytologic features” in a report, that phrase can describe a moderately differentiated tumor. When a clinician says “high-grade appendiceal cancer” in a consultation, that phrase can mean either grade 2 or grade 3. The same two words can refer to different diseases depending on who is using them.

What the middle tier actually looks like

Moderately differentiated mucinous appendiceal adenocarcinoma has distinct features. Under the microscope, it shows infiltrative invasion, meaning the tumor cells push into the wall of the appendix and beyond in a way that low-grade mucinous neoplasms do not. The cells have high-grade cytologic features, meaning they look more abnormal than low-grade cells, with enlarged nuclei and increased mitotic activity. But they do not have the signet ring cell component that defines grade 3 disease, or they have it only in small amounts below the threshold that would reclassify the tumor.

Clinically, moderately differentiated disease tends to carry a higher risk of lymph node involvement than low-grade mucinous neoplasms. It responds differently to systemic chemotherapy than low-grade disease, which is often chemotherapy-insensitive. Candidacy for cytoreductive surgery with HIPEC depends on factors that include grade, and the decision-making for a grade 2 patient can look quite different from the decision-making for a grade 3 patient even when the peritoneal burden is similar.

None of this is captured when moderately differentiated disease gets reported as high-grade. Patients lose the ability to find their own prognosis in the literature. Clinicians lose the ability to tailor recommendations to what the evidence actually shows for grade 2 specifically.

What patients can do

The first thing is to ask for specificity. A pathology report should say what grade the tumor is, and the grading system being used. If it says “high-grade” without further detail, that is a reasonable thing to ask about. If it says “moderately differentiated” or “grade 2,” that is meaningful and worth knowing.

The second thing is to seek pathology review by a specialist when possible. Appendiceal tumors are uncommon enough that general pathologists may not see many of them. A second opinion from a gastrointestinal or peritoneal surface oncology pathologist can confirm the grade, or occasionally revise it, and that can change the treatment plan.

The third thing is to read research with a critical eye. When a study reports outcomes for “high-grade appendiceal cancer,” look at the methods section to see how that category was defined. If grades 2 and 3 were combined, the overall numbers will be pulled downward by the grade 3 patients and will not accurately represent what a grade 2 patient should expect. If the study broke the grades out separately, the grade 2 results are the ones to focus on.

The fourth thing, and maybe the most important, is to push back when language gets imprecise. Patients have the right to know exactly what their diagnosis is, and to have it named correctly in every conversation about their care.

Where we stand

Appendicure is committed to discussing appendiceal cancer with the specificity the science requires. That means naming the three grades when we talk about mucinous appendiceal adenocarcinoma, and not letting moderately differentiated disease disappear into a category that does not describe it accurately. It means flagging when a study’s methods collapse the grades, and interpreting the results accordingly. It means producing patient education content that treats grade 2 as its own category, with its own prognosis, its own treatment considerations, and its own place in the decision tree.

The community member who raised this concern was right to raise it. Moderately differentiated disease is its own thing. It deserves to be named, understood, and studied on its own terms, and we will continue to do our part to make that happen.

Glossary

Term	What it means
Differentiation	How closely cancer cells resemble the normal tissue they came from. Well-differentiated cells look similar to normal cells and tend to grow slowly. Poorly differentiated cells look very abnormal and tend to grow quickly.
Grade	A pathologist’s assessment of how aggressive a tumor looks under the microscope. Grade 1 is well-differentiated, grade 2 is moderately differentiated, and grade 3 is poorly differentiated.
WHO	World Health Organization. Its Classification of Tumors is the international reference standard for how cancers are named and graded.
AJCC	American Joint Committee on Cancer. Its staging manual is the standard used by most U.S. pathologists and oncologists. The 8th edition uses a three-tier grading system for appendiceal mucinous tumors.
PSOGI	Peritoneal Surface Oncology Group International. A consensus body of specialists focused on peritoneal cancers, including appendiceal disease.
MAA	Mucinous appendiceal adenocarcinoma. A type of appendiceal cancer that produces mucin and commonly spreads within the peritoneal cavity.
CRS/HIPEC	Cytoreductive surgery with heated intraperitoneal chemotherapy. A specialized operation used to treat peritoneal disease from appendiceal cancer.

Questions to Ask Your Doctor

Questions to Ask Your Doctor
What grade is my tumor, specifically? Is it grade 1, grade 2, or grade 3, and which grading system is the pathologist using?
If my pathology report says “high-grade,” does that mean moderately differentiated, poorly differentiated, or both? Can you point me to the exact wording?
Is there evidence of signet ring cells in my tumor? If so, what proportion?
Has my case been reviewed by a pathologist who specializes in appendiceal or gastrointestinal cancers?
How does my specific grade affect the treatment plan you are recommending?
Am I a candidate for cytoreductive surgery with HIPEC? How does my grade factor into that decision?
Are there clinical trials open to patients with my specific grade?
If I seek a second opinion, should I have the slides sent for review, or the pathology report alone?

Selected references

Taggart MW, et al. Stratification of outcomes for mucinous appendiceal adenocarcinoma with peritoneal metastasis by histological grade. World Journal of Gastrointestinal Oncology, 2017.

Carr NJ, Bibeau F, Bradley RF, et al. The histopathological classification, diagnosis and differential diagnosis of mucinous appendiceal neoplasms, appendiceal adenocarcinomas and pseudomyxoma peritonei. Histopathology, 2017.

Davison JM, et al. Clinicopathologic and molecular analysis of disseminated appendiceal mucinous neoplasms: proposed criteria for a three-tiered assessment of tumor grade. Modern Pathology, 2014.

AJCC Cancer Staging Manual, 8th edition. American Joint Committee on Cancer.