Colorectal cancer is a disease that begins in the colon or the rectum, which are parts of the large intestine. It is the third most common cancer diagnosed globally, representing about 10% of all cancer cases. The disease typically starts from small growths called polyps on the inner lining of the colon or rectum. While many polyps remain harmless, certain types can slowly develop into cancer over several years.
The risk of developing colorectal cancer increases with age, with most cases occurring in individuals over 50. Lifestyle factors, such as diet, physical activity levels, and smoking, can influence risk. In its early stages, colorectal cancer often produces no symptoms, which underscores the importance of screening. As the disease progresses, it can grow into the deeper layers of the intestinal wall and potentially spread to other parts of the body.
Visualizing Polyps and Tumors During a Colonoscopy
The primary method for directly viewing the inside of the colon and rectum is a colonoscopy. This procedure uses a colonoscope, a long, thin, flexible tube with a small camera and light at its tip, to transmit images to a monitor. To prepare for the procedure, the colon must be completely empty, which requires following a special diet and using strong laxatives. During the examination, the physician carefully guides the scope through the entire length of the large intestine.
A healthy colon lining appears smooth, pink, and uniform in texture, with visible blood vessels running beneath the surface. The folds of the colon are well-defined and regular, indicating the absence of inflammation, polyps, or other abnormalities. Any deviation from this baseline prompts closer inspection.
Polyps are growths protruding from the colon’s lining and are a common finding during a colonoscopy. A benign polyp can be a small, mushroom-shaped growth attached to the colon wall by a stalk, known as a pedunculated polyp. These are often easily removed during the procedure.
Other polyps have a higher likelihood of becoming cancerous. A sessile polyp is flat and grows directly on the colon wall, making it harder to remove. Another type, the villous adenoma, often has a cauliflower-like surface and is associated with a higher risk of cancer. The size of a polyp is also a factor, as larger polyps are more likely to be pre-cancerous or cancerous.
A cancerous tumor’s appearance is distinct from a benign polyp. An early-stage cancer can appear as a large, ulcerated mass with raised, irregular edges. It may be friable, meaning it bleeds easily when touched by the colonoscope. The surface can be nodular and hard, unlike the soft tissue of a normal colon lining. These features alert the physician to a high probability of cancer, prompting a tissue sample, or biopsy, for confirmation.
Staging Cancer with CT, MRI, and PET Scans
Once a diagnosis of colorectal cancer is confirmed, the next step is staging, which is the process of determining the extent of the cancer’s growth and spread. This information guides treatment decisions. Advanced imaging technologies provide detailed pictures of the inside of the body, offering a cross-sectional view of tissues and organs that a colonoscopy cannot see.
A computed tomography (CT) scan is frequently used for staging colon cancer. It combines a series of X-ray images taken from different angles to create detailed cross-sectional pictures. For staging, CT scans show the primary tumor’s size and whether it has grown through the colon wall. They also identify if the cancer has spread to nearby lymph nodes or distant organs like the liver.
Magnetic resonance imaging (MRI) is particularly useful for staging rectal cancer. An MRI uses powerful magnets and radio waves to create highly detailed images of soft tissues. For rectal cancer, an MRI of the pelvis shows how far the tumor has penetrated the rectal wall and its proximity to other structures. This detail is important for surgical planning to remove the tumor while preserving function.
A positron emission tomography (PET) scan detects cancer cells that have spread to distant parts of the body. The patient receives an injection of a radioactive sugar substance, which metabolically active cancer cells absorb at a high rate. A PET scanner detects the radiation from this substance, creating an image that highlights these “hot spots.” This can identify metastases not visible on CT or MRI scans.
The Microscopic Appearance of Cancer Cells
While imaging provides large-scale views of tumors, a definitive diagnosis is made at the cellular level through histology, the microscopic examination of tissue. A pathologist analyzes tissue from a biopsy or surgically removed tumor to make the diagnosis.
Under a microscope, normal colon tissue has an organized structure. The cells form well-defined glands arranged in straight, parallel rows. The individual cells are uniform in size and shape with small, regular nuclei, an architecture characteristic of healthy tissue.
In contrast, colorectal cancer cells lose their organized structure, forming irregular, crowded glands that invade the colon wall. The cancer cells themselves are abnormal, varying widely in size and shape. Their nuclei are often large, dark, and irregular. This disorganized and invasive growth is the hallmark of malignancy.
The pathologist also assigns a grade to the cancer based on how abnormal the cells appear. Low-grade (well-differentiated) cancer cells still resemble normal colon cells and form glandular structures. High-grade (poorly differentiated) cancer cells look very abnormal and have lost their original features. This grade indicates how aggressive the cancer is likely to be.
Imaging to Monitor Treatment and Detect Recurrence
After diagnosis and staging, imaging continues to be important for managing colorectal cancer. Scans are used to track treatment effectiveness and to watch for any signs of recurrence after treatment is complete.
During treatments like chemotherapy or radiation, doctors use imaging to monitor the tumor’s response. CT or PET scans are performed at intervals to measure changes in the tumor’s size. A shrinking tumor indicates effective treatment, while a tumor that does not change or grows may require a different approach.
After treatment, patients enter a surveillance phase with regular follow-ups and imaging to check for recurrence. CT scans are used to monitor the original cancer site and check for spread to organs like the liver and lungs. This schedule is most intensive in the first few years post-treatment, when recurrence risk is highest, allowing for early detection of new growths.