Gross Morphology
Colorectal cancers, while all originating from either adenomatous polyps or flat dysplastic lesions, demonstrate distinct gross morphologies depending on their anatomical location. In the right (proximal) colon, tumors commonly present as polypoid or fungating exophytic masses. These growths may not cause overt symptoms initially, but chronic occult bleeding from these lesions often leads to iron deficiency anemia as an early clinical manifestation.
On the other hand, left-sided (distal) colon cancers more frequently exhibit an annular or constrictive morphology, giving rise to the characteristic “apple-core” or “napkin-ring” appearance on imaging. This configuration narrows the bowel lumen and often causes bowel dysfunction, including constipation, diarrhea, or intestinal obstruction. In severe cases, patients may experience bowel perforation, a complication associated with a worsened prognosis.
Despite these location-dependent differences in gross appearance, both right- and left- sided tumors are generally similar under microscopic examination and have comparable prognoses in locoregional disease. However, evidence suggests that in metastatic disease, patients with right-sided primary tumors may fare worse.
Synchronous Tumors
Approximately 3 to 5 percent of CRC patients develop synchronous tumors, which are defined as two or more separate primary cancers that are not extensions or metastases of each other and are separated by normal colon tissue. When excluding individuals with Lynch syndrome (hereditary nonpolyposis colorectal cancer, HNPCC), this rate drops to about 2.5 percent. Prognostically, synchronous tumors are on par with solitary cancers when the highest disease stage at diagnosis is considered.
Histopathological Characteristics
Carcinoma Types and WHO Classification
The overwhelming majority of malignant colorectal tumors are carcinomas, predominantly adenocarcinomas (over 90%). Uncommon histological variants include neuroendocrine tumors, lymphomas, mesenchymal tumors, and hamartomas. The fifth edition of the WHO classification outlines the major histological subtypes of colorectal cancer.
Certain histological subtypes carry important prognostic implications:
- Signet ring cell carcinoma is a rare, aggressive variant marked by intracellular mucin that displaces nuclei, giving the cell a “signet ring” These tumors typically present at advanced stages (III or IV), frequently involving peritoneal carcinomatosis, and are often linked to microsatellite instability (MSI) and Lynch syndrome.
- Medullary carcinoma, a non-gland-forming variant composed of large eosinophilic polygonal cells with dense lymphocytic infiltration, is often seen in tumors with deficient mismatch repair (MMR) and is associated with a favorable prognosis.
- Mucinous adenocarcinoma, characterized by extracellular mucin constituting ≥50% of the tumor, accounts for 11–17% of CRCs, with a predilection for the right colon. These tumors often exhibit reduced responsiveness to chemotherapy and chemoradiation, although findings are not unanimous, especially in cases involving appendiceal neoplasms.
- Adenosquamous carcinoma, which shows squamous differentiation, is exceedingly rare (0.05–0.2%) and is associated with higher mortality compared to typical adenocarcinomas.
- Neuroendocrine differentiation may be found in about 10% of colorectal cancers, particularly in poorly differentiated These can range from well- differentiated carcinoid tumors, which have a better prognosis, to poorly differentiated neuroendocrine carcinomas, which carry a poor outlook. The significance of focal neuroendocrine features within adenocarcinomas remains uncertain.
Tumor Grading
The degree of differentiation, or how closely tumor cells resemble normal glandular structures, influences prognosis. Well- and moderately differentiated tumors show organized gland formation, indicating low-grade malignancy, while poorly differentiated or undifferentiated tumors display solid growth patterns, high mitotic rates, and pleomorphism, denoting high-grade malignancy.
Two main grading systems are employed:
- The College of American Pathologists (CAP) and AJCC/UICC use a four-tiered system based on gland formation:
- Grade 1: >95% gland formation (well-differentiated)
- Grade 2: 50–95% (moderately differentiated)
- Grade 3: <50% (poorly differentiated)
- Grade 4: No gland or mucin formation (undifferentiated)
- The WHO’s fifth edition advocates a simplified two-tiered system:
- Low grade: Well and moderately differentiated
- High grade: Poorly differentiated and undifferentiated
Most prognostic studies favor the two-tiered approach due to its statistical robustness.
Immunohistochemistry (IHC) and Molecular Markers
Diagnostic Markers
Immunohistochemical staining is pivotal in diagnosing CRC and distinguishing it from other malignancies:
- CK20 and CDX2 are highly sensitive and specific for colorectal adenocarcinomas.
- CK7/CK20 profiles are useful in differentiating between appendiceal tumors and mucinous ovarian cancers.
- Notably, medullary carcinomas with MSI may lack CK20 and CDX2 expression and instead express markers such as calretinin, CK7, SATB2, and CDH17.
Mismatch Repair Deficiency (dMMR)
IHC also allows detection of MMR protein deficiencies, which can have significant clinical implications:
- Sporadic MMR deficiencies result in loss of staining in tumor cells while normal mucosa retains expression, acting as an internal Such tumors are often associated with a better prognosis, influencing treatment decisions in stage II colon cancer.
- Inherited MMR defects, as in Lynch syndrome, can also be identified through IHC by the absence of specific proteins (MLH1, PMS2, MSH2, MSH6), guiding further germline testing.
- Tumors with dMMR or MSI-high status often respond favorably to immunotherapy targeting PD-1/PD-L1 pathways due to their elevated neoantigen load and immune cell infiltration.
- Furthermore, dMMR status may mitigate the negative prognostic effects associated with KRAS, NRAS, or BRAF
