History: A 5-year-old girl presented with headache and progressive gait abnormality for a month duration.Images
Findings CT: There is a large heterogeneous mass within the posterior fossa. There is suggestion of an intratumoral solid component as well as a surrounding cystic component. There is extensive edema in the posterior fossa. There is crowding of the posterior fossa with almost complete effacement of the fourth ventricle and with supratentorial ventriculomegaly. There is white-matter hypoattenuation adjacent to the frontal and occipital horns of the lateral ventricles suggesting transependymal cerebrospinal fluid (CSF) flow. MRI: There is a mixed solid and cystic mass centered in the midline cerebellar vermis. There are intratumoral foci of susceptibilities, likely microhemorrhages and/or microcalcifications. The cystic component demonstrates thin peripheral enhancement and contains small layering hematocrit level. Local mass effects include effacement of the fourth ventricle, supratentorial hydrocephalus with minimal transependymal CSF flow, decreased pontine-mamillary distance, effacement of the prepontine cisterns, and downward herniation of the cerebellar tonsils. There is indentation of dorsal brainstem without frank invasion.
Differential diagnosis Diagnosis: Juvenile pilocytic astrocytoma Pathophysiology At histologic examination, pilocytic astrocytoma classically manifests in a noteworthy biphasic pattern composed of a combination of loose glial tissue punctuated by numerous vacuoles, microcysts, occasional macrocysts, and compacted piloid tissue. The piloid tissue component is composed of dense sheets of elongated bipolar cells that demonstrate fine fibrillary (hair-like) processes, a highly distinctive feature, and typically an abundance of Rosenthal fibers. Slow growth is the rule for most pilocytic astrocytomas. However, some tumors, particularly those of the optic nerve and chiasm, may show a propensity for periods of accelerated growth. Epidemiology Pilocytic astrocytoma is the most common pediatric cerebellar neoplasm and the most common pediatric glioma, constituting 85% of all cerebellar astrocytomas and 10% of all cerebral astrocytomas in this age group. Overall, it accounts for 0.6% to 5.1% of all intracranial neoplasms and 1.7% to 7% of all glial tumors. The optic pathway tumors have a strong association with neurofibromatosis. Clinical presentation Patients usually present with possible symptoms of headache, vomiting, gait disturbance, blurred vision, diplopia, and neck pain. Imaging features CT: Most cerebellar and cerebral pilocytic astrocytomas have a well-demarcated appearance with a round or oval shape smaller than 4 cm in size, cyst-like features, smooth margins, and occasional calcifications. Most tumors are located near the ventricular system, and almost all enhance, typically intensely, on postcontrast images obtained after intravenous administration of contrast material. MR: Pilocytic astrocytoma is typically isointense to hypointense relative to normal brain with T1-weighted pulse sequences and hyperintense compared with normal brain with T2-weighted pulse sequences. Four predominant imaging patterns of pilocytic astrocytoma have been described: mass with a nonenhancing cyst and an intensely enhancing mural nodule (21% of cases), mass with an enhancing cyst wall and an intensely enhancing mural nodule (46%), necrotic mass with a central nonenhancing zone (16%), and predominantly solid mass with minimal to no cyst-like component (17%).
附神经医学影像联盟幕下肿瘤5+2原则： 
儿童小脑肿瘤要先想到毛细胞型星形细胞瘤(大多数表现为囊实性肿块，T1WI呈低信号，T2WI呈高信号，实性部分呈明显强化，囊壁可有或无强化)和髓母细胞瘤(呈稍长T1,等或稍长T2信号，DWI高信号，可囊变，钙化少见，常呈中等不均匀强化)。 成人小脑肿瘤要想到血管母细胞瘤(绝大多数呈大囊小结节，结节明显强化，瘤周或瘤内可见血管流空影)，如儿童发现血管母细胞瘤要考虑到VHL综合征可能。老年人小脑肿瘤要想到转移瘤(出血坏死多见，增强多样化)和淋巴瘤(呈等/稍长T1，等/稍长T2信号，DWI高信号，增强扫描均匀强化；免疫力低下年龄可相对年轻，病灶内出血坏死常见)。然后还有相对少见的节细胞胶质瘤(常见于幕上，偶尔发生于幕下，多呈边界清楚的囊状肿块，伴有部分钙化的壁结节，病变表浅时常可见骨质改变，幕下病变更容易造成相邻脑实质萎缩)和小脑发育不良性节细胞瘤(长T1长T2信号，无强化。可见“条纹征”特征性表现。)内容来源：AuntMinnie.com，神经医学影像联盟 |
