Inclusion Criteria:
* Age ≥ 36 months and \< 22 years
* Pathologically proven malignant cerebellar brain tumor (including medulloblastoma, glioblastoma multiforme, giant cell glioblastoma, anaplastic astrocytoma, primitive neuroectodermal tumor, ependymoma, atypical teratoid/rhabdoid tumor, germ cell tumor, or other high-grade malignant tumor) which is progressive or recurrent despite standard care including surgery, radiotherapy, and/or chemotherapy. A pathologically proven secondary malignant cerebellar tumor without curative treatment options is eligible.
* Lesion must be ≥ 1.0 cm ≤ 3.0 cm in diameter and surgically accessible as determined by MRI. Larger tumors may be surgically debulked and treated if ≤ 3.0 cm after debulking
* Patients must have fully recovered from acute treatment related toxicities of all prior chemotherapy, immunotherapy or radiotherapy prior to entering this study.
* Myelosuppressive chemotherapy: patients must have received their last dose at least 3 weeks prior (or at least 6 weeks if nitrosurea)
* Investigational/Biologic agents: patients must have recovered from any acute toxicities potentially related to the agent and received last dose ≥ 7 days prior to entering this study (this period must be extended beyond the time during which adverse events are known to occur for agents with known adverse events ≥ 7 days). For viral therapy, patients must have received viral therapy ≥ 3 months prior to study entry and have recovered from all acute toxicities potentially related to the agent.
* Monoclonal antibodies: The patient must have received last dose ≥ 21 days prior.
* Radiation: Patients must have received their last fraction of craniospinal radiation (\>24 Gy) or total body irradiation ≥ 3 months prior to study entry. Patients must have received focal radiation to symptomatic metastatic sites or local palliative radiation ≥ 28 days prior to study entry.
* Autologous bone marrow transplant: Patients must be ≥ 3 months since transplant prior to study entry.
* Normal hematological, renal and liver function (absolute neutrophil count \> 1000/mm3, platelets \> 100,000/mm3, prothrombin time (PT) or partial thromboplastin time (PTT) \< 1.3 x control, creatinine within normal institutional limits OR creatinine clearance \>60 mL/min/1.73 m2 for patients with creatinine levels above institutional normal, total bilirubin \< 1.5 mg/dl, transaminases \< 3 times above the upper limits of the institutional norm)
* Patients \< 16 years, Modified Lansky performance score ≥ 60; patients ≥ 16 years, Karnofsky performance score ≥ 60
* Patient life expectancy must be at least 8 weeks
* Written informed consent in accordance with institutional and FDA guidelines must be obtained from patient or legal guardian
Exclusion Criteria:
* Any treatment outside the allowable guidelines outlined in section 5.1.
* Diffuse, widespread, abnormal tumor pattern involving 3 or more lobes of the brain
* Acute infection, granulocytopenia or medical condition precluding surgery
* Pregnant or lactating females
* Diagnosis of encephalitis or CNS infection \< 3 months prior, or receiving ongoing treatment for encephalitis, CNS infection or multiple sclerosis
* Tumor involvement which would require ventricular or brainstem inoculation or would require access through a ventricle in order to deliver treatment
* Required steroid increase within 1 week prior to G207 inoculation or patients requiring \>2 mg of dexamethasone daily
* Known HIV seropositivity
* Concurrent therapy with any drug active against HSV (acyclovir, valacyclovir, penciclovir, famciclovir, gancyclovir, foscarnet, cidofovir) or any immunosuppressive drug therapy (except dexamethasone or prednisone).
* Other current malignancy
* Concurrent anticancer or investigational drug
