Cancer may be suspected in a patient because of certain symptoms, or because of a doctor’s initial examination.
There are several screening tests that can be done to determine whether there are cancerous cells present in particular parts of the body, or whether a tumour is cancerous. Once that has been done, further tests can determine whether the cancer has spread to other parts of the body. This is called "staging".
But back to the diagnosis. It is extremely important that the type of cancer be diagnosed correctly, as the different types of cancer are treated in different ways.
The following methods are used to diagnose cancer, according to the Merck Manuals:
Initial examination by a doctor of any lumps, growths or skin abnormalities. Patients often go to the doctor because something has changed in the way their body functions. The doctor then decides on the next step in the possible diagnosis.
A biopsy. A small tissue sample is removed and is analysed under a microscope to see if there are any cancer cells present. Some biopsies can be done in a doctor’s surgery using local anaesthetics. If there is fluid in the tumour, a fine needle biopsy may be done, in which fluid is extracted from the tumour for analysis.
An endoscopy. A flexible plastic tube, which has a little camera on the end, is inserted into body cavities and this can help a doctor to see inside areas such as the stomach or the lower intestine.
Diagnostic imaging. These can include X-rays, a CAT (computerised axial tomography) scan (which is more precise than an X-ray, an MRI scan (magnetic resonance imaging) which is used to create detailed images of the body’s soft tissues, organs and blood vessels, and an ultrasound, which can determine of a lump is solid or fluid. All of these tests might be able to confirm the size and location of a mass, but cannot tell definitely whether they are cancerous, or not. For that, a biopsy needs to be done. Scans are often used to determine how far a cancerous tumour has spread, if at all.
Blood tests. These can be used to confirm the presence of certain cancers, such as prostate cancer. Blood tests can also be used to see if the cancer has had any affect on the kidneys, the liver or the bones.
Once the doctor has determined that a patient indeed has cancer, staging tests will determine the extent of the cancer by looking at the size and the location of the tumour, and by whether it has spread to surrounding or distant tissue in the body.