Colorectal adenocarcinoma, primarily affecting older adults, is the most common colorectal malignancy characterised by a polyp-to-cancer progression sequence, with histopathological features of glandular tissue invasion and potential for local and distant metastasis, commonly to the liver
Description
Colorectal adenocarcinoma is a malignant neoplasm originating from the glandular epithelium of the colon or rectum. It is the most common type of colorectal cancer, and the third most common cancer worldwide.
Pathogenesis
The pathogenesis of colorectal adenocarcinoma is typically through the adenoma-carcinoma sequence, where normal colonic mucosa undergoes dysplasia forming an adenoma, which then progresses to carcinoma. Key molecular changes include mutations in the APC gene, KRAS, and TP53.
Subtypes
- Tubular adenocarcinoma: The most common subtype, accounting for 80-86% of all cases. It is characterised by well-differentiated glands that resemble normal colonic architecture.
- Mucinous adenocarcinoma: Represents 10-15% of all cases. Defined by the presence of extracellular mucin composing at least 50% of the tumour. Generally carries a worse prognosis due to a higher likelihood of advanced stage at diagnosis and resistance to adjuvant chemotherapy.
Epidemiology, Risk Factors & Associations
- Colorectal adenocarcinoma is more common in older adults (over 50 years) and slightly more common in males.
- Risk factors include diet high in red or processed meat, smoking, alcohol, obesity, and lack of physical activity.
- Associations include familial adenomatous polyposis (FAP) and hereditary non-polyposis colorectal cancer (HNPCC or Lynch syndrome).
Clinical Features
- Early disease may be asymptomatic.
- Symptoms can include changes in bowel habits, rectal bleeding or blood in the stool, abdominal discomfort, and unexplained weight loss.
Complications
- Local spread can lead to bowel obstruction and perforation.
- Distant metastases, most commonly to the liver and lungs.
Pathological Features
Histopathology
- Macroscopic: Adenocarcinomas often appear as a circumferential ‘napkin-ring’ or polypoid mass.
- Microscopic: Characterised by glandular structures with varying degrees of dysplasia.
Serology
- Elevated CEA
Biochemistry
- FOBT: Detection of trace blood in stool
- The Australian National Bowel Cancer Screening Program invites 50 – 74 years old a home test kit every 2 years.
- A positive result requires further diagnostic testing such as colonoscopy
- A negative result means the participant returns to regular screening every 2 years
Radiological Features
General Features
- Typically demonstrates a circumferential or polypoid mass causing luminal narrowing.
- Pericolonic fat stranding may be present in advanced cases.
CT
- Non-contrast: Mass or thickening of the bowel wall.
- Contrast-enhanced: Enhancing mass or wall thickening. The “apple-core” sign can be seen with circumferential tumours.
MRI
- T1: Hypointense mass or wall thickening.
- T2: Hyperintense mass or wall thickening.
- T1 C+: Enhancement post-contrast.
- DWI/ADC: Restricted diffusion.
PET FDG
- Colorectal adenocarcinomas are typically FDG-avid.
Grading and Staging
Grading is based on the degree of glandular differentiation. Staging is done according to the TNM system.
Diagnosis
Diagnosis is confirmed by colonoscopy with biopsy and histological examination. Carcinoembryonic antigen (CEA) levels can be used for monitoring.
Differential Diagnosis
- Colonic adenomas: These are the benign precursors to adenocarcinomas and may appear as a bulky lobulate mass that usually does not cause luminal narrowing or obstruction (unlikely colorectal carcinoma). Malignant potential of adenomas however increase with size (> 2cm approx. 53% association with malignancy)
- Other colonic malignancies such as lymphoma or carcinoid: These may present with similar symptoms and signs, but usually have different imaging features.
Management
- Management usually involves a multidisciplinary approach with surgery, chemotherapy, and radiotherapy.