ANALYSIS OF CLINICOPATHOLOGICAL HISTOMORPHOLOGICAL AND MOLECULAR DIFFERENCES IN RIGHT AND LEFT SIDED COLONIC CARCINOMA

This study of Sri Lankan colorectal carcinoma patients demonstrates that right-sided tumors exhibit distinct clinicopathological features associated with microsatellite instability, and while the MsPath score shows a significant correlation with mismatch repair deficiency, immunohistochemistry using a PMS2 and MSH6 panel remains the most feasible and reliable method for screening in resource-limited settings.

The Gist

🎯 The Big Picture: Two Different Kinds of "Bad Neighbors"

Imagine the colon (large intestine) as a long, 150cm hallway in a house. This hallway has two distinct neighborhoods:

1. The Right Side: The beginning of the hallway (near the kitchen/caecum).
2. The Left Side: The end of the hallway (near the bathroom/rectum).

For a long time, doctors treated cancer in both neighborhoods as the same problem. But this study, conducted by Dr. A.V. Tharangi Damayanthi at the National Hospital in Sri Lanka, discovered that these two neighborhoods actually host two very different types of "bad neighbors" (tumors).

The goal of the study was to figure out how to tell these two types apart just by looking at them under a microscope, without needing expensive, high-tech DNA tests that Sri Lanka didn't have at the time.

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🔍 The Detective Work: The "MsPath" Score

The researchers used a special checklist called the MsPath Score. Think of this like a weather forecast for cancer.

Instead of needing a satellite (expensive DNA test) to predict a storm, the MsPath score looks at the "clouds" (visual clues under the microscope) to guess if a storm is coming.

The Clues (The "Clouds"):

• Location: Is the tumor in the Right Side? (Right side = higher chance of the "special" type).
• Age: Is the patient young (under 50)?
• Shape: Does the tumor look like a messy pile of cells (poorly differentiated) or a specific "mucous" type?
• The Army: Are there lots of immune system soldiers (lymphocytes) fighting inside the tumor?
• The Reaction: Is there a "Crohn-like" reaction (a specific type of inflammation) around the tumor?

If a tumor has many of these clues, it gets a high MsPath score. This suggests it might be MSI-High (Microsatellite Instability-High).

Why does "MSI-High" matter?
Think of MSI-High tumors as rebellious cells that have a broken "spell-checker" in their DNA. Because of this, they look messy and attract a lot of immune system soldiers.

• Good News: These tumors often respond better to immunotherapy (training your own immune system to fight the cancer) and have a different prognosis than standard tumors.
• Bad News: They are harder to treat with standard chemotherapy.

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🧪 The Experiment: The "Two-Panel" Trick

In rich countries, doctors use a 4-antibody test to confirm if a tumor is MSI-High. It's like checking four different locks on a door. But in Sri Lanka, resources were tight.

The researchers asked: "Can we just check two locks (PMS2 and MSH6) instead of four?"

The Analogy:
Imagine you want to know if a car engine is broken. Usually, you check the spark plugs, the fuel pump, the battery, and the alternator (4 checks).
The researchers hypothesized that if you check the Battery and the Alternator (2 checks), and they are broken, the engine is definitely broken. If they are working, the engine is probably fine.

The Result:
They tested 46 tumors using this "Two-Panel" method.

• Success: It worked! The two-antibody test was just as good as the four-antibody test.
• Correlation: The tumors that had a high MsPath score (lots of visual clues) were the same ones that tested positive for the broken "spell-checker" (MSI-High) in the lab.

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📊 What They Found (The "Receipts")

1. Right vs. Left: The "messy, immune-fighting" tumors (MSI-High) were mostly found on the Right Side of the colon. The "standard" tumors were mostly on the Left.
2. The Score Works: The MsPath score was a great predictor. If a tumor had a high score, it was very likely to be MSI-High.
3. Reliability: Two different doctors looked at the same slides and agreed on the score almost perfectly. This means the checklist is easy to use and not confusing.
4. Cost-Effective: Using the visual checklist + the 2-antibody test is a budget-friendly way to find these special tumors in places where expensive DNA tests aren't available.

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🚧 The Hiccups (Limitations)

Every study has some bumps in the road:

• Small Sample Size: They only tested the "Two-Panel" method on 46 tumors. It's like testing a new recipe on a small family dinner; it tasted great, but we need to cook it for a whole wedding to be sure it works for everyone.
• New to the Lab: The lab team was learning how to do these specific stains for the first time. It's like a chef trying a new, complex dish; the first few tries might have had some inconsistencies.
• One Hospital: The study was only done in one hospital in Colombo. We need to see if this works in rural villages or other countries too.

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💡 The Takeaway (The "So What?")

This study is a game-changer for resource-limited settings (like Sri Lanka and many other developing nations).

The New Strategy:

1. Look at the slide: If the tumor is on the Right Side and looks "messy" with lots of immune cells, give it a High MsPath Score.
2. Test Smart: Instead of wasting money on expensive DNA tests for every patient, only run the cheap, 2-antibody test on the ones with the High Score.
3. Treat Better: If the test confirms it's MSI-High, the doctor knows to use immunotherapy instead of standard chemo.

In short: The researchers built a low-cost radar to find dangerous but treatable tumors, ensuring that patients in Sri Lanka get the right treatment without breaking the bank. It's a brilliant example of using simple observation to solve a complex medical problem.

Technical Summary

1. Problem Statement

Colorectal carcinoma (CRC) exhibits distinct biological behaviors based on tumor location (right-sided vs. left-sided). Right-sided tumors are frequently associated with Microsatellite Instability-High (MSI-H) status, which has critical implications for prognosis, Lynch syndrome screening, and immunotherapy response.

• The Gap: In Sri Lanka, molecular testing for MSI (via PCR or Next-Generation Sequencing) is expensive and largely unavailable.
• The Challenge: There is a lack of local data on the histopathological predictors of MSI-H in the Sri Lankan population.
• The Goal: To determine if routine histological features and a calculated "MsPath score" can reliably predict MSI-H status, thereby serving as a cost-effective triage tool for selecting cases for Immunohistochemistry (IHC) or molecular testing in resource-limited settings.

2. Methodology

Study Design:
A retrospective analytical study conducted at the National Hospital of Sri Lanka (Colombo) covering cases diagnosed between January 2019 and January 2021.

Cohort:

• Total Sample: 156 resected colorectal carcinoma specimens.
• Inclusion: Invasive adenocarcinoma.
• Exclusion: Neoadjuvant therapy, intramucosal carcinoma, or poor slide quality.
• Sub-sample for IHC: 46 cases (approx. 30%) were selected for MMR protein analysis based on MsPath scores (stratified above and below a cut-off of 1).

Data Collection & Assessment:

1. Histopathological Review: All H&E slides were reviewed by the principal investigator (blinded to clinical data) to assess:
   • Tumor differentiation (Poor vs. Moderate).
   • Histological subtype (Mucinous, Signet ring, Medullary, Undifferentiated).
   • Tumor-Infiltrating Lymphocytes (TILs).
   • Crohn-like lymphocytic reaction.
2. MsPath Score Calculation: A weighted score was calculated for each case based on specific coefficients:
   • Age < 50 years (0.7)
   • Right-sided location (Caecum/Ascending/Transverse) (1.6)
   • Mucinous/Signet ring/Medullary/Undifferentiated components (1.1)
   • Poor differentiation (0.6)
   • Presence of TILs (2.1)
   • Crohn-like reaction (0.5)
   • Cut-off: A score of ≥1 was used to stratify cases for further testing.
3. Immunohistochemistry (IHC):
   • Target: Mismatch Repair (MMR) proteins.
   • Panel: A two-antibody panel (PMS2 and MSH6) was used instead of the standard four-antibody panel. This was chosen based on literature suggesting that loss of PMS2 or MSH6 implies loss of their obligatory partners (MLH1 and MSH2), making a two-panel test cost-effective and accurate.
   • Interpretation: Loss of nuclear staining in tumor cells (with retained internal control staining) indicated MMR deficiency.

Statistical Analysis:

• Chi-square tests for categorical variables.
• Logistic regression to identify independent predictors of MSI-H.
• Kappa statistics to measure inter-observer agreement.

3. Key Contributions

• First Local Study: This is the first comprehensive study in Sri Lanka comparing clinicopathological features of right vs. left-sided CRC and evaluating the utility of the MsPath score in a local context.
• Validation of Two-Antibody Panel: The study supports the use of a PMS2/MSH6 two-antibody panel as a feasible, cost-effective alternative to the four-antibody panel for screening MMR deficiency in resource-constrained environments.
• MsPath Utility: It validates the MsPath scoring system as a practical tool for prioritizing cases for molecular testing in settings where universal genetic testing is not feasible.

4. Key Results

Demographics & Distribution:

• Total Cases: 156 (Median age: 64 years; 52% Female).
• Location: 41 cases (26%) were Right-sided; 115 cases (74%) were Left-sided.
• Histology: 89% were moderately differentiated adenocarcinomas.

Histopathological Differences:

• Right-sided tumors showed a higher frequency of features associated with MSI-H:
  • Tumor-Infiltrating Lymphocytes (TILs): 29% overall (higher in right-sided).
  • Crohn-like lymphocytic reaction: 23% overall (higher in right-sided).
  • Mucinous differentiation was more common in left-sided tumors in this specific cohort (71% of mucinous cases were left-sided), though mucinous features generally correlate with MSI-H in literature.

MsPath Score & IHC Correlation:

• Score Distribution: 50% of cases had a score < 1.0.
• IHC Results (n=46):
  • MMR deficiency was observed predominantly in cases with higher MsPath scores.
  • Statistical Significance: A significant association was found between MsPath score category and MMR status ($\chi^2 = 13.76$, $p = 0.001$).
  • Predictors: High MsPath score ($\ge 4.0$), mucinous differentiation, and TILs were independent predictors of MSI-H status.

Reproducibility:

• Inter-observer Agreement: Substantial agreement (Kappa value indicated high reproducibility) between the principal investigator and an independent trainee regarding MsPath scoring, confirming the reliability of the criteria.

5. Significance and Conclusion

• Clinical Impact: The study confirms that right-sided CRCs in Sri Lanka frequently exhibit histological hallmarks of MSI-H (TILs, Crohn-like reaction, poor differentiation).
• Resource Optimization: The MsPath score, combined with routine H&E examination, can effectively triage patients. Cases with high scores should be prioritized for IHC testing, reducing the burden of unnecessary molecular testing in low-resource settings.
• Feasibility of IHC: The study demonstrates that a two-antibody IHC panel (PMS2/MSH6) is a reliable, cost-effective screening method for MMR deficiency, capable of replacing the more expensive four-antibody panel without significant loss of sensitivity.
• Limitations: The study was limited by a small IHC sub-sample (n=46), single-center design, and technical variability during the initial optimization of IHC protocols.
• Future Directions: The authors recommend expanding IHC capacity, conducting multi-center studies with larger cohorts, and integrating molecular validation (BRAF mutation/MLH1 methylation) to refine the predictive model.

Final Verdict: The research provides a validated, low-cost framework for identifying MSI-H colorectal cancers in Sri Lanka, bridging the gap between advanced molecular diagnostics and resource-limited clinical practice.