Appearance
The medulloblastoma is usually a soft tumor with a blurred, soft surface and a gray-white cut, but can occasionally be sharply defined and rough. Larger tumors have central areas where actually active cells die (necroses). Microscopically, the classical medulloblastoma consists of densely packed cells with round to oval, strongly stainable (hyperchromatic) nuclei, surrounded by little cytoplasm.
Sometimes round cells with less stainable nuclei are also added. In less than one third of the cases the typical pseudorosettes, called Homer-Wright rosettes, are found. These consist of tumor cells arranged in a ring around a center of cytoplasm, in which the cell nuclei are peripheral. Many of the cells are also in the process of nuclear division (mitosis) or are dying (apoptosis).
Classification
The World Health Organization (WHO) has developed a classification that classifies brain tumors. The classification is based mainly on the tumor’s growth behavior: By definition, medulloblastoma is always classified as a grade 4 tumor because it is malignant, spreads rapidly and, if left untreated, leads to death.
- Grade 1 tumors grow slowly and are usually benign.
- Grade 2 tumors are predominantly benign, but already consist of isolated malignant cells and can also spread further. Grade 2 tumors are therefore still considered benign tumors with a tendency to degenerate.
- Grade 3 tumors are malignant brain tumors according to the tumor classification of the World Health Organization (WHO). While grade 3 tumors are already malignant, they grow somewhat more slowly than grade 4 tumors.
- Grade 4 tumors are characterized by extremely rapid growth
Causes
Medulloblastoma belongs to the group of embryonic tumors (primitive neuroectodermal tumors), i.e. it develops from embryonic, immature cells. The causes for the degeneration of the cells are still largely unexplained. In most cases, the tumor develops spontaneously.
The role of genetic factors in the development of brain tumors has become increasingly important in recent years, even though they are not relevant for the majority of brain tumors.In medulloblastomas, changes on the long arm (q-arm) of chromosome 17 are often described. This chromosome carries the p53 tumor suppressor gene, which encodes the protein p53. p53 controls the cell cycle and changes in the protein (mutations) lead to a progression of malignant tumors. But other genes also influence the complex process of tumor development. In addition, brain tumors increasingly produce growth factors and growth factor receptors, which leads to the extraordinarily rapid growth of tumors.
