How is colon cancer diagnosed ?

EIn France, approximately 47,000 colorectal cancers are diagnosed each year, including 15,000 cases of rectal cancer. The diagnosis of colorectal cancer often occurs late, despite the organized screening program in France for the general population aged over 50 and at average risk.

Typically, diagnosis follows a clinical examination performed by a general practitioner during a routine consultation for abdominal pain or after the presence of blood in the stool is noted. In addition to the clinical examination, further tests may be prescribed.

Blood tests, such as checking for a decrease in red blood cells (anemia) due to iron deficiency (iron deficiency anemia), or evaluating liver function, will be conducted based on a blood sample. The blood test for Carcinoembryonic Antigen (CEA) will also be performed.

While the CEA level alone is not diagnostic for colorectal cancer, it is valuable for monitoring disease progression and response to treatments.

The various exams

Une coloscopie et différents examens d’imagerie médicale [scanner abdominal, radiographie des poumons, Imagerie par Résonance Magnétique (IRM), Tomographie à Émission de positons (PET-scan) …] pourront également être prescrits.

If polyps or abnormal masses are observed during colonoscopy, they will be removed for examination under a microscope by a pathologist to determine the nature of the cells (histological examination). This will identify whether the cells are normal or cancerous. If cancerous cells are present, it will also define their exact type and origin (histological type); more than 90% of tumors originate from the colon lining tissue, known as adenocarcinomas.

Histological examination will also determine the grade of the cancer cells, which refers to how abnormal they appear compared to normal cells. The grade helps to guide treatment strategies and assess the prognosis of the disease.

Expert perspective

Classified from 1 to 3 or 4, the higher grades indicate more abnormal cancer cells that differ significantly from normal cells and are more aggressive.

Grade 1 cancers (well-differentiated cancers) have cells that closely resemble normal cells and multiply slowly. Grade 3 cancers (poorly differentiated cancers) have highly abnormal cells that multiply rapidly.

Grade 2 cancers (moderately differentiated cancers) exhibit characteristics between those of grades 1 and 3.

Research may also involve testing for mutations in specific genes such as KRAS or BRAF, with the aim of potentially targeting these mutations in future treatments (see further below). Testing for deficiencies in DNA repair mechanisms (MMR-D tumors) or microsatellite instability (MSI tumors) is also conducted to explore potential treatments involving immunotherapy.

Extension assessment

If colorectal cancer is confirmed, an extension assessment will be conducted so that the doctors convening at a Multidisciplinary Team Meeting (MDT) (see further below) can address the following questions :

  • Is the cancer operable ?
  • If operable, is it justified to proceed with treatment before surgery (neoadjuvant treatment) ?
  • Has the disease spread to distant sites ?
  • What is the most suitable treatment for the disease ?
  • Is the patient fit to receive the planned treatments ?

To answer these questions, the most common sites where colorectal cancer metastasizes will be explored :

  • The liver through ultrasound or CT scan
  • The lungs through chest X-ray or thoracic CT scan
  • A PET scan may also be performed. This involves injecting a radioactive substance intravenously that binds to cancerous tissues for visualization.

These various examinations—clinical, histological, radiological, biological, and molecular—will first clarify which segment of the colon or rectum is affected by the disease :

  • In about 50% of cases, it affects the rectum and sigmoid colon
  • In 15% of cases, it affects the cecum
  • In another 15% of cases, it involves the ascending colon and right colic flexure
  • In 13% of cases, it affects the descending colon and left colic flexure
  • In 8% of cases, it involves the transverse colon (Source: arcagy.com) (Source arcagy.com)

The extension assessment will also determine the stage of the disease.

Expert perspective

Determining the stage of cancer is primarily of therapeutic interest, as it guides the selection of the most appropriate treatment for the patient.

It also helps predict the likely course of the disease. The stage of cancer is determined based on three criteria. The first criterion depends on the characteristics of the tumor (T). In the case of colorectal cancer, an important characteristic is the depth of the tumor through the wall of the colon or rectum. The second criterion depends on the number of affected lymph nodes (N for Node). The third criterion depends on the presence of metastases and the number of organs affected by them (M). These three criteria define the stage of cancer and classify cancers according to an internationally recognized TNM classification .

Colorectal cancers at stage 1 are localized tumors only within the inner layer of the colon or rectum. Stage 2 cancers involve different layers of the colon or rectum. Stage 3 tumors have cancer cells in lymph nodes near the colon or rectum. Stage 4 tumors have spread to distant organs from the colon or rectum, forming metastases, particularly in the peritoneum, liver, or lungs.

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Article updated on Nov 14, 2024

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