不规则颅后窝肿块即髓母细胞瘤，在小脑中线附近并且延伸至脑干之上的第四脑室。它是“小圆蓝细胞”肿瘤，最长发生于儿童。
The irregular posterior fossa mass that is seen here near the midline of the cerebellum and extending into the fourth ventricle above the brainstem is a medulloblastoma. This is one of the "small round blue cell" tumors and it most often occurs in children.