Cancer staging is the process of determining the extent to which a cancer has developed by growing and spreading. Contemporary practice is to assign a number from I to IV to a cancer, with I being an isolated cancer and IV being a cancer that has spread to the limit of what the assessment measures. The stage generally takes into account the size of a tumor, whether it has invaded adjacent organs, how many regional (nearby) lymph nodes it has spread to (if any), and whether it has appeared in more distant locations (metastasized). The staging system is not applicable to astrocytoma, which is instead expressed as "grade I–IV". Grade IV astrocytoma, more commonly referred to as glioblastoma multiforme, is a universally fatal primary brain cancer most commonly seen in the seventh decade of life. TNM staging system Cancer staging can be divided into a clinical stage and a pathologic stage. In the TNM (Tumor, Node, Metastasis) system, clinical stage and pathologic stage are denoted by a small "c" or "p" before the stage (e.g., cT3N1M0 or pT2N0). This staging system is used for most forms of cancer, except brain tumors and hematological malignancies. Clinical stage is based on all of the available information obtained before a surgery to remove the tumor. This stage may include information about the tumor obtained by physical examination, blood tests, radiologic examination, biopsy, and endoscopy. Pathologic stage adds additional information gained by examination of the tumor microscopically by a pathologist after it has been surgically removed. Because they use different criteria, clinical stage and pathologic stage often differ. Pathologic staging is usually considered to be more accurate because it allows direct examination of the tumor in its entirety, contrasted with clinical staging which is limited by the fact that the information is obtained by making indirect observations of a tumor which is still in the body. However, clinical staging and pathologic staging often complement each other. Not every tumor is treated surgically, so pathologic staging is not always available. Also, sometimes surgery is preceded by other treatments such as chemotherapy and radiation therapy which shrink the tumor, so the pathologic stage may underestimate the true stage. Considerations Correct staging is critical because treatment (particularly the need for pre-operative therapy and/or for adjuvant treatment, the extent of surgery) is generally based on this parameter. Thus, incorrect staging would lead to improper treatment. For some common