After rectal cancer is diagnosed, tests are needed to find out if the cancer has spread in the rectum or has spread to other parts of the body.
The process of checking to see if the cancer has spread in the rectum or has spread to other parts of the body is called staging. The information gathered from the staging process can determine the stage of the disease. In order to develop a treatment plan, it is important to know the stage of the cancer.
The following tests may be used in the staging process:
X-ray chest x-ray: An x-ray scan of the organs and bones in the chest. x-ray is a beam of energy that passes through the body and is imaged on film, presenting images of various parts of the body.
Colonoscopy: A procedure to view polyps (small raised pieces of tissue), abnormal areas, or tumors inside the rectum and colon through a colonoscope. A colonoscope is a thin, tube-shaped instrument with a light and a lens for viewing. A colonoscope may also come with an instrument to remove polyps or to take tissue samples that are placed under a microscope to check for signs of cancer. 
Colonoscopy. A thin, glowing tube is passed through the anus and rectum and into the colon, looking for abnormal areas.
CT scan (CAT scan): A procedure that takes a set of detailed images of various parts of the body, such as the abdomen, pelvis, and chest, from different angles. These images are generated by a computer that is connected to an X-ray machine. A dye is injected into a vein or swallowed so that organs or tissues can be presented more clearly. This process is also known as computed tomography, computed tomography, or computerized radiographs.
MRI (magnetic resonance imaging): A test that uses magnets, radio waves, and a computer to generate a series of detailed images of areas inside the body. This method is also called nuclear magnetic resonance imaging (NMRI).
PET scan (positron emission tomography): A method to find malignant cells in the body. A small amount of radioactive glucose (sugar) is injected into a vein. the PET scanner rotates around the body and takes pictures of the areas of the body where the glucose is used. Malignant tumor cells appear brighter in the pictures because they are more active and take up more glucose than normal cells.
Endoluminal ultrasound of the rectum: A method used to examine the rectum and surrounding tissues. An ultrasound transducer (probe) is inserted into the rectum, causing high-energy sound waves (ultrasound) to bounce off the tissues or organs in the body and create an echo. This echo creates an image of the body’s tissues called an echogram. Doctors can use the sonogram to identify tumors. This process is also called transrectal ultrasound.
There are three ways that cancer cells can spread through the body.
Cancer can spread through the tissues, the lymphatic system, and the blood.
Tissue. Cancer spreads from the primary site to nearby areas.
Lymphatic system. Cancer moves from the primary site into the lymphatic system. It metastasizes through the lymphatic vessels to other parts of the body.
Blood. Cancer enters the bloodstream from the primary site. It metastasizes through blood vessels to other parts of the body.
Cancer cells can spread from the primary site to other parts of the body.
When cancer cells spread to another part of the body, it is called metastasis. Cancer cells break away from where they started (the primary tumor) and move through the lymphatic system or bloodstream.
Lymphatic system. Cancer cells enter the lymphatic system, cross the lymphatic vessels, and form a tumor in another part of the body (metastatic tumor).
Blood. Cancer cells enter the bloodstream, cross blood vessels, and form a tumor in another part of the body (metastatic tumor).
Metastatic tumors are the same type of cancer as the primary tumor. For example, if rectal cancer cells spread to the lungs, then the cancer cells in the lungs are actually rectal cancer cells. This disease is called metastatic rectal cancer, not lung cancer.
Many cancer deaths are due to the spread of cancer from the primary tumor to other tissues and organs. This is called metastatic cancer. This animation shows how cancer cells move from where they first formed in the body to other parts of the body.
Stage of rectal cancer:
Stage 0 (carcinoma in situ)
Stage 0 (rectal cancer in situ). Abnormal cells are seen in the mucosa of the rectal wall.
In stage 0 rectal cancer, abnormal cells are found in the mucosal layer of the rectal wall (innermost layer). These abnormal cells may form cancer cells and spread to nearby normal tissue. stage 0 is also called carcinoma in situ.
Stage I
Stage I rectal cancer. Cancer cells have spread from the mucosal layer of the rectal wall to the submucosal or muscular layer.
In stage I rectal cancer, cancer cells have formed in the mucosal layer (the innermost layer) of the rectal wall and have spread to the submucosal layer (the layer of tissue immediately adjacent to the mucosal layer) or to the muscular layer of the rectal wall.
Stage II
Stage II rectal cancer. In stage IIA, the cancer cells have spread through the muscular layer of the rectal wall to the plasma layer. In stage IIB, the cancer cells have spread through the plasma membrane layer but have not spread to the adjacent organs. In stage IIC, cancer cells have spread through the plasma membrane layer to nearby organs.
Stage II rectal cancer is divided into stage IIA, stage IIB, and stage IIC.
Stage IIA: Cancer cells have spread through the muscular layer of the rectal wall to the plasma membrane of the rectal wall (the outermost layer).
Stage IIb:Cancer cells have spread through the plasma membrane layer of the rectal wall (outermost layer) to the tissues surrounding the abdominal organs (visceral peritoneum).
Stage IIC: Cancer cells have spread through the plasma membrane layer (outermost layer) of the rectal wall to the surrounding organs.
Stage III
Stage III rectal cancer is classified as stage IIIA, stage IIIB, and stage IIIC.
Stage IIIA rectal cancer. The cancer has spread through the mucosal layer of the rectal wall to the submucosal layer and may have spread to the muscular layer and has spread to one to three adjacent lymph nodes or to the tissues of the adjacent lymph nodes. Alternatively, cancer cells have spread through the mucosal layer to the submucosa and to four to six nearby lymph nodes.
In stage IIIA, the cancer has spread:
through the mucosal layer of the rectal wall (the innermost layer) to the submucosa (the layer of tissue immediately adjacent to the mucosa) or to the muscular layer of the rectal wall. Cancer cells have spread to one to three nearby lymph nodes, or cancer cells have formed in the tissues near the lymph nodes; or
Reaching the submucosa (the layer of tissue immediately adjacent to the mucosa) through the mucosal layer (the innermost layer) of the rectal wall. The cancer has spread to four to six nearby lymph nodes. 
Stage IIIB rectal cancer. Cancer cells have spread through the muscular layer of the rectal wall to the plasma membrane layer, or have spread through the plasma membrane layer but not to adjacent organs; cancer cells have spread to one to three nearby lymph nodes or tissues near the lymph nodes. Alternatively, the cancer cells have spread to the muscularis or plasma membrane layer and to 4 to 6 nearby lymph nodes. Alternatively, cancer cells have spread through the mucosal layer to the submucosal layer and may have spread to the muscularis; cancer cells have spread to seven or more nearby lymph nodes.
In stage IIIB, the cancer has spread to:
the plasma membrane of the rectal wall (outermost layer) through the muscular layer of the rectal wall or to the tissues of the periphery of the abdominal organs (visceral peritoneum) through the plasma membrane. Cancer cells have spread to one to three nearby lymph nodes or have formed in the tissues near the lymph nodes; or
to the muscularis or plasma membrane (outermost layer) of the rectal wall. The cancer has spread to four to six nearby lymph nodes; or
through the mucosal layer (innermost layer) of the rectal wall to the submucosa (layer of tissue immediately adjacent to the mucosa) or to the muscular layer of the rectal wall. The cancer has spread to seven or more nearby lymph nodes. 
Stage IIIC rectal cancer. The cancer has spread through the plasma membrane of the rectal wall but has not spread to nearby organs; the cancer has spread to four to six nearby lymph nodes. Or, the cancer has spread through the muscular layer to the plasma membrane, or has spread through the plasma membrane but not to nearby organs; the cancer has spread to seven or more nearby lymph nodes. Or, the cancer has spread through the plasma membrane layer to nearby organs and one or more nearby lymph nodes or tissues near the lymph nodes.
In stage IIIC, the cancer has spread:
through the plasma layer of the rectal wall (outermost layer) to the peripheral tissues of the abdominal organs (visceral peritoneum). Cancer cells have spread to 4 to 6 nearby lymph nodes; or
Reaching the plasma layer of the rectal wall (outermost layer) through the muscular layer of the rectal wall or reaching the peripheral tissues of the abdominal organs (visceral peritoneum) through the plasma layer. The cancer has spread to seven or more nearby lymph nodes; or
Reaching nearby organs through the plasma layer (outermost layer) of the rectal wall. The cancer has spread to one or more nearby lymph nodes, or the cancer has formed in the tissue near the lymph nodes.
Stage IV:
Stage IV rectal cancer is divided into stages IVA, IVB, and IVC.
Stage IV rectal cancer. The cancer has spread through the blood and lymph nodes to other parts of the body, such as the lungs, liver, abdominal wall, or ovaries.
Stage IVA:The cancer has spread to an area or organ farther away from the rectum, such as the liver, lungs, ovaries, or distal lymph nodes.
Stage IVB:The cancer has spread to multiple areas or organs distant from the rectum, such as the liver, lungs, ovaries, or distal lymph nodes.
Stage IVC:The cancer has spread to the tissues surrounding the abdominal wall and may have spread to other areas or organs.