[Medulloblastoma originates from the residual germ cells in the granular layer outside the posterior medullary sails, and it occurs in the cerebellar earth, accounting for 1.84%-6% of all primary intracranial tumors. The general pathology is purple-red or gray-red due to the rich vascularity of the tumor, and the tumor is relatively well-defined and brittle. Necrosis can be seen, and calcified cystic lesions are rare. [The common symptoms are headache, vomiting, gait instability and ataxia, diplopia and vision loss. Physical examination may reveal optic papillar edema, nystagmus, rotational dyskinesia, difficulty in standing with eyes closed, slanting neck and abductor nerve palsy. Increased protein is common on cerebrospinal fluid analysis. The disease progresses rapidly, usually for six months to one year. It is sensitive to radiotherapy. [1. CT: ①The scan shows a round or round-like high-density mass in the midline of the posterior cranial fossa with clear edges, surrounded by a low-density edematous band around the lesion. ②10%-15% can see speckled calcification, small cystic area and necrotic area, but large hemorrhage is rare. ③Enhanced scans show uniform and moderate to significant enhancement. ④The anterior displacement of the fourth ventricle may be curved and flattened. ⑤80%-90% of the tumors are accompanied by significant enlargement of the supratentorial ventricles. 2. MR performance: ①The cerebellar earthworm is more prevalent. T1-weighted low signal, T2-weighted equal signal or slightly high signal, with clear margins. ③The calcification, hemorrhage and necrosis are rare, and there may be a small part of cystic changes. ④Most of them are obviously and uniformly enhanced, and a few of them show moderate enhancement. ⑤Often compressing the fourth ventricle, secondary to supratentorial hydrocephalus. [Differential diagnosis] ①Ventricular meningioma: It is often difficult to distinguish between the two tumors, but ventricular meningioma is located in the fourth ventricle, and the tumor is surrounded by a circular linear cerebrospinal fluid density shadow on CT. (2) Astrocytoma: The most common brain tumor in children is astrocytoma, and it tends to occur in the cerebellum, but is mostly cystic in nature. When it presents as a solid mass, it is similar to medulloblastoma. The main differences are: medulloblastoma is denser than astrocytoma on both plain and enhanced scans; secondly, the location is different, with more than 80% of medulloblastoma in the midline, while astrocytoma may be located in the midline or in the cerebellar hemisphere, but secondary supratentorial ventricular enlargement is relatively rare. CT scan shows a round high and low mixed density lesion in the cerebellar earthworm with a clear border and mild edema around it. The fourth ventricle is occluded. Enhancement showed that the parenchymal part was obviously enhanced and the low-density area was enhanced. MR shows a round lesion in the cerebellar earthworm with slightly low signal in T1-weighted cross section and slightly high signal in T2-weighted cross section, with irregular-shaped cystic lesions in the middle and mild surrounding edema. After enhancement, the lesion was significantly strengthened with clear margins. The fourth ventricle was obviously compressed and deformed, and the supratentorial obstructive hydrocephalus was present.