Rectal cancer is diagnosed after a physical examination and colonoscopy by a colorectal surgeon. Once diagnosed, determining the stage of the disease is of vital importance. Rectal cancer staging is a critical step to understand the extent of the disease's spread and to determine the most appropriate treatment strategy.

The main tests used in rectal cancer staging are:

* Digital Rectal Examination (DRE): This involves the specialist physician examining the inner surface of the rectum with a gloved, lubricated finger. This examination can help assess the tumor's location, size, and its distance from the anus. However, tumors located further from the anus may not be detected by this method. If an abnormality is found, endoscopy is recommended for further evaluation.
* Flexible Sigmoidoscopy: This involves inserting a flexible tube with a camera through the anus and rectum to examine the lower part of the large intestine. This test can support the diagnosis and provide initial information about the tumor's location and spread.
* Rigid Sigmoidoscopy: This examination is performed by inserting a rigid, optical scope through the anus into the rectum. It is usually performed by gastroenterologists or general surgeons. Rigid sigmoidoscopy offers the advantage of more precisely measuring the tumor's distance from the anus.
* Complete Blood Count (CBC): This test evaluates the general condition of the blood. Low red blood cell levels (anemia) may indicate blood loss due to the tumor, while high white blood cell levels may be a sign of an infection or inflammation associated with the tumor.
* Tumor Marker Tests: These measure the levels of substances (tumor markers) that cancer cells can secrete into the blood. Markers such as Carcinoembryonic Antigen (CEA) and Carbohydrate Antigen (CA) can be used in the follow-up of rectal cancer. High levels can indicate cancer, but can also rise in benign conditions, so they are not diagnostic on their own and are generally used to monitor response to treatment.
* Computed Tomography (CT) Scan: Provides detailed imaging of the chest, abdomen, and pelvis. This allows for assessment of metastasis to lymph nodes or distant organs such as the lungs and liver. It can also indirectly help determine the tumor's distance from the anus.
* Magnetic Resonance (MR) Imaging: Provides highly detailed images of the tumor's relationship with surrounding muscles, organs, and other tissues in the rectum, the layers of the rectal wall, and the status of regional lymph nodes. This is critical for local staging and surgical planning.
* Chest X-ray: A simple imaging method used to check if rectal cancer has spread to the lungs (metastasis).
* Endoscopic Ultrasonography (EUS): Combining endoscopy and ultrasound, EUS provides very high-resolution images of the layers of the rectal wall and adjacent lymph nodes. This is invaluable for assessing how deeply the tumor has invaded the rectal wall and regional lymph node involvement.
* Positron Emission Tomography (PET) Scan: Aims to detect metabolically active cancer cells in the body. It is based on the principle that radioactive glucose, administered intravenously, is taken up more by cancerous cells. It is particularly useful for investigating distant metastases or recurrences.
* Molecular and Genetic Tests: These tests examine the genetic characteristics of cancer in tissue samples taken from the tumor. Tests such as KRAS and MSI (Microsatellite Instability) are performed to evaluate the tumor type, its potential response to targeted therapies, and inherited risks such as Lynch syndrome.