Most Common Cellular Type of Rectosigmoid Malignancy and Its Percentage
The most common cellular type of rectosigmoid malignancy is adenocarcinoma, accounting for approximately 95-98% of all rectosigmoid cancers. 1
Histological Classification of Rectosigmoid Malignancies
Adenocarcinomas represent the overwhelming majority of rectosigmoid cancers, with several subtypes recognized:
Conventional adenocarcinoma (NOS - not otherwise specified) - This is the most common subtype, typically arising from adenomas 1
Specific adenocarcinoma subtypes:
- Mucinous adenocarcinoma (11.6% of cases) - Characterized by >50% of tumor comprised of extracellular mucin pools containing malignant glands or individual tumor cells 1, 2
- Signet-ring cell adenocarcinoma (4.6% of cases) - Characterized by >50% of tumor demonstrating signet-ring cell morphology with intracytoplasmic mucin displacing and indenting nuclei 1, 2
- Medullary carcinoma - Shows solid sheets of malignant cells with indistinct cell boundaries, vesicular nuclei, prominent nucleoli, and inflammatory infiltrates 1
- Serrated adenocarcinoma - Demonstrates glandular serrations, often slit-like, with low nuclear to cytoplasmic ratio 1
- Micropapillary adenocarcinoma - Shows small, rounded clusters of tumor cells within stromal spaces mimicking vascular channels 1
- Adenoma-like adenocarcinoma - At least 50% of invasive tumor has adenoma-like appearance with villous architecture 1
Molecular and Genetic Characteristics
Most rectosigmoid adenocarcinomas are characterized by:
- Chromosomal instability pathway (CIN) 1
- Microsatellite instability (MSI) is rare in rectal cancer (only a few percent) 1
- Approximately one-third are associated with aberrant DNA methylation 1
- Common genetic alterations include:
- APC gene mutations (tumor suppressor)
- Inactivation of P53 and TGFβ pathways
- Activation of oncogenes like KRAS and PI3CKA
- BRAF mutations are rare in rectal cancer 1
Clinical Implications
The histological subtype has significant implications for prognosis and treatment:
- Signet-ring cell adenocarcinoma is associated with worse stage-for-stage survival compared to conventional adenocarcinoma 1
- Medullary carcinoma is almost invariably associated with microsatellite instability and generally has a good prognosis 1
- Micropapillary adenocarcinoma is associated with adverse pathological features including extramural venous invasion and lymph node metastasis 1
- Adenoma-like adenocarcinoma is associated with a good prognosis 1
Diagnostic Considerations
When evaluating rectosigmoid malignancies, histopathological examination should include:
- Assessment of surgical margins (proximal, distal, and circumferential)
- Examination of regional lymph nodes (at least 12 nodes recommended)
- Evaluation of vascular and nerve invasion
- Histological grading based on gland formation 1
Understanding the cellular type of rectosigmoid malignancy is crucial for determining prognosis and guiding treatment decisions that will ultimately affect morbidity, mortality, and quality of life outcomes.