Introduction
Gastric carcinoma is one of the most common causes of death worldwide. The only treatment available for treating this disease is surgical resection, which is dependent on the stage of cancer. Assessment of gastric cancer with imaging is essential for pre-operative, post-operative, and chemotherapy response assessment. Magnetic resonance imaging is useful for tumor staging, evaluation of treatment response, analyzing histological features, obtaining treatment guidance, and determining prognosis. The prognosis for patients with gastric cancer has improved in recent years due to advancements in imaging and treatment techniques.
What Is the Epidemiology of Gastric Cancer?
The rate of gastric cancer development increases with age. Patients are often diagnosed at the age of 50 to 70 years. Gastric cancer occurrence is high in South American and Asian countries like Korea, China, and Japan. However, Asian countries have been showing a declining trend in gastric cancer incidence in recent years. The factors responsible for the reduction are increased screening and reduction in Helicobacter Pylori (bacteria) infection.
What Are the Risk Factors for Developing Gastric Cancer?
- Diet factors.
- Helicobacter Pylori gastritis (redness or swelling in gastric lining caused by bacterial infection).
- Atrophic gastritis (long-standing redness and swelling along with thinning of gastric lining).
- Pernicious anemia (reduction in red blood cells due to defective absorption of vitamin B12).
- Adenomatous gastric polyps (abnormal growth of stomach epithelium that can turn cancerous)
- Hereditary factors.
What Are the Features of Gastric Cancer?
- Gastric cancer is predominantly adenocarcinoma (cancer occurring in the digestive glands of the stomach) type.
- The tumors appear ulcerated (death and cast off of surface cells) and polypoid (resembling polyp).
- Depending on the depth of invasion, gastric cancer is classified as early or advanced.
- Early gastric cancer is the superficial type that involves the mucosa (innermost layer of gastric mucosa) and submucosa (second layer). The tumor lesions are small and enclosed with ulcerated thickening of the gastric wall.
- Advanced gastric cancer involves deeper layers like muscularis propria (third layer) and deeper layers. The tumor appears ulcerating, polypoid, diffusely, or ulcerative infiltrating.
What Are the Symptoms Of Gastric Cancer?
Symptoms for early-stage gastric cancer are:
- Indigestion with stomach discomfort.
- A feeling of bloating (gas build up in stomach) following eating.
- Loss of appetite.
- Heartburn (discomfort in the chest that worsens after eating).
- Mild nausea (urge to vomit).
Symptoms of advanced-stage gastric cancer are:
- Blood in the stool (feces).
- Unexplained weight loss.
- Vomiting.
- Stomach pain.
- Ascites (excess accumulation of abdominal fluid).
- Jaundice (yellowing of the skin).
- The trouble with swallowing.
How Is Gastric Cancer Diagnosed?
- Endoscopic Ultrasonography: Endoscopy and high-frequency ultrasound are used to evaluate the gastric tract and surrounding structure. However, the imaging technique exhibits limitations in the depth of penetration.
- Multidetector Computed Tomography: It is considered the best technique for staging stomach cancer. The technique is useful in non-invasive diagnosis of tumor extent, location, and metastasis (spread of cancer cells to other organs).
- Virtual Gastroscopy: The imaging procedure generates a three-dimensional view of the endoluminal (hollow region within the stomach) area through endoscopy. The images are obtained when air is utilized as a distending agent in CT imaging, and virtual gastroscopy images are reconstructed using commercially available tools. This imaging is useful for the characterization and diagnosis of early-stage gastric tumors. The longer image processing time and the requirement for experienced imaging personnel are some of the limitations.
- Magnetic Resonance Imaging (MRI): This imaging tool is used to obtain soft tissue images and stage gastric cancer. The images can be obtained in multiple angles, directions, and parameters. Recent advancements in MRI have helped with fast and high-resolution imaging. The limitation of motion artifacts (defects) caused by gastric peristalsis (movement of gastric muscles) or patient breathing can be overcome by filling the stomach cavity with water, using oral contrast media, using scopolamine, or using breath-holding.
- Diffusion-Weighted MRI: It measures the mobility of water molecules within tissues. Tumors, being highly cellular, show restriction in water diffusion. The tumors present high signal intensity in diffusion-weighted images.
- PET (Positron Emission Tomography)/CT: It is used to detect gastric cancer with nodal and distant metastasis. However, the technique is not useful for imaging in early-stage gastric cancer or assessment of primary tumors.
What Is the Role of MRI in the Assessment of Gastric Cancer?
MRI provides T1 and T2 weighted images for gastric cancer assessment.
- Diagnosis: MRI provides better soft tissue contrast compared to other imaging techniques. The imaging technique can accurately stage tumors. The limitations of MRI in gastrointestinal imaging due to longer acquisition time and increased risk for motion artifacts have resulted in its lower utilization. The use of intravenous contrast and breath-holding techniques has enabled us to overcome limitations and obtain better imaging. MRIs are used in patients who are contraindicated for CT use due to contrast allergy or concern for radiation exposure. To diagnose gastric cancer, an MRI is performed when stomach distention is observed due to the intake of water or effervescent granules (solid agents containing medicines).
- TNM (Tumor, Nodes, and Metastasis) Staging: The MRI imaging exhibits high sensitivity and specificity in tumor diagnosis and extent of nodal involvement. Diffusion-weighted imaging with MRI is a useful tool for TNM imaging if the use of CT is not possible. A contrast agent (ferumoxtran-10) used with MRI is useful in identifying metastatic lymph nodes. It can also be useful in differentiating between benign and metastatic tumors. However, the diagnosis of nodal metastasis with MRI remains controversial as the shape and size of lymph nodes are used as diagnostic criteria that may not be accurate.
- Chemotherapy Treatment Response: Imaging is useful to evaluate treatment response. The tumor size reduction is difficult to diagnose with conventional imaging in the early stage of treatment. However, diffusion-weighted imaging combined with MRI can detect slight changes in tumors after chemotherapy initiation. A five-point pathological grade system called tumor regression grade (amount of residual tumor after therapy) is used as an indicator to analyze response to treatment. Apparent diffusion coefficients (diffusion rate of water molecules within tissues) were increased after therapy and are useful indicators for therapy effectiveness. The diffusion coefficients are inversely proportional to tumor regression grade. MRI, along with diffusion-weighted imaging, is used as a sensitive tool to analyze metastatic lymph node response to neoadjuvant chemotherapy. The apparent diffusion coefficient value after chemotherapy was elevated in patients with metastatic lymph nodes.
- Radiotherapy: The treatment with radiotherapy requires visualization of the tumor and surrounding structures at risk, which is obtained with MR imaging. Guided radiotherapy with the help of MRI provides real-time imaging, enhanced soft tissue contrast, and aids radiotherapy planning.
How Is Gastric Cancer Treated?
The treatment for gastric cancer requires a multidisciplinary approach.
- All stages of gastric cancer are treated with surgery. Depending on the extent of involvement, subtotal or total gastrectomy (partial or complete removal of the stomach surgically) may be required. In patients where resection of the tumor is not possible, endoluminal stent placement (tube placement to overcome the block in the gastric tract) or laser therapy (use of a narrow beam of light to cut and destroy tissue) may be needed.
- Early-stage cancers are removed with endoscopic mucosal resection (a minimally invasive procedure to remove tumor lesions).
- Chemotherapy uses drugs to stop the growth of tumor cells.
- Radiotherapy uses high-energy X-rays to kill tumor cells.
Conclusion
MR imaging for patients with gastric cancer is helpful for staging and evaluation of chemotherapy response. However, there is limited clinical application due to a lack of criteria for assessing nodal and metastatic involvement. Studies are ongoing to improve imaging efficacy and accuracy in diagnosis.
