Imetelstat for Children With Refractory or Recurrent Solid Tumors and Lymphoma

Background: \- Imetelstat is a cancer treatment drug that may slow or stop tumor growth. It may also prevent tumors from spreading to other parts of the body. Researchers want to see if it can be a safe and effective treatment for children who have solid tumors or lymphoma that have not responded to other treatments. Objectives: \- To see if imetelstat is a safe and effective treatment for children who have solid tumors or lymphoma that have not responded to other treatments. Eligibility: \- Children and adolescents between 1 and 21 years of age who have solid tumors or lymphoma that have not responded to other treatments. Design: * Participants will be screened with a physical exam, medical history, and imaging studies. Blood and urine samples will also be collected. * Participants will receive imetelstat on the first and eighth day of a 21-day cycle of treatment. * Treatment will be monitored with frequent blood tests and imaging studies. Tumor biopsies may also be performed. * Participants will keep taking the study drugs for up to a total of 18 cycles as long as the disease does not progress and there are no severe side effects....