Based on what the surgeon sees at the time of the surgery, the actual tumor is placed in one of the following categories (T referring to tumor):
T1 Tumor <3>3 cm in diameter
• T3a Tumor >3 cm in diameter with spread
• T3b Tumor >3 cm in diameter with definite spread into the brain stem (part of brain that controls breathing, hearing, seeing, and other important functions)
• T4 Tumor >3 cm in diameter with extension up past the aqueduct of Sylvius and/or down past the foramen magnum
In addition to "T" staging, medulloblastoma staging has been modified by including "M" staging, where the "M" stands for metastasis. Remember, this is a word that describes how far the tumor cells have spread from the original location, if at all. The M stage is determined not only by the surgeon's observations, but also in combination with MRI scans and lumbar cytology, and consists of 5 possible groups:
• M0 No evidence of metastasis
• M1 Tumor cells found in cerebrospinal fluid (by lumbar puncture and cytology study)
• M2 Tumor beyond primary site but still in brain
• M3 Tumor deposits ("seeds") in spine area that are easily seen on MRI
• M4 Tumor spread to areas outside the CNS (outside both brain and spine)
