Sirolimus in Combination With Metronomic Chemotherapy in Children With Recurrent and/or Refractory Solid and CNS Tumors

Inclusion Criteria: * Participants with any of the following tumors who have experienced relapse following front-line therapy, or who are refractory to front-line therapy, and participants with tumors that carry a poor prognosis and have no known standard curative therapy * Brain tumors of all World Health Organization (WHO) grades, except diffuse intrinsic pontine glioma (DIPG) - enrollment in the brain tumor stratum is closed * Extracranial solid tumors including histiocytoses * Participants must have had a histologic verification of malignancy at original diagnosis or relapse, except in participants with optic pathway gliomas, or participants with pineal tumors and elevations of serum or cerebrospinal fluid (CSF) alpha-fetoprotein (AFP) or beta-human chorionic gonadotropin (beta-HCG) * Tissue blocks or slides must be sent * Participants must have radiographically measurable disease at the time of study enrollment to be eligible. Patients with neuroblastoma who do not have measurable disease but have metaiodobenzylguanidine (MIBG+) evaluable disease are eligible. Measurable disease in patients with CNS involvement is defined as tumor that is measurable (≥ 10 mm) in two perpendicular diameters on MRI and visible on more than one slice. For all patients, tumors that are located in a previously irradiated area may be considered measurable if the lesion has shown tumor growth after radiation or has been biopsied and proven to have active disease. * Participant's current d…