Background: * Medullary thyroid carcinoma (MTC) is common in people with a genetic disorder called multiple endocrine neoplasia (MEN). * Vandetanib is an experimental drug that blocks a defective protein receptor (rearranged during transfection (RET) receptor) found on the surface of cancer cells in people with MEN. It is thought that this protein is a primary cause of MTC in people with MEN. Objectives: * To study the activity of Vandetanib in children and adolescents with MEN-related MTC by measuring the change in tumor size, in blood levels of proteins produced the tumor (calcitonin and carcinoembryonic antigen (CEA) and in tumor-related diarrhea. * To determine the safety and tolerability of Vandetanib in children and adolescents. * To study how the body handles Vandetanib in children and adolescents. * To determine the effect of Vandetanib on the survival of children and adolescents with MTC. Eligibility: -Children and adolescents 5 to 18 years of age with MTC whose tumor cannot be surgically removed or has grown back after treatment or has metastasized (spread beyond the thyroid gland). Design: * Patients take Vandetanib once a day in 28-day cycles. The first patients enrolled in the study are started on a low dose of Vandetanib to determine tolerability. * Patients have periodic blood tests, electrocardiograms, and blood pressure measurements to look for side effects of Vandetanib. * Blood tests and imaging scans (magnetic resonance imaging (MRI), computed tomography (CT), bone and octreoscan) are done every 8 weeks for the first 32 weeks of treatment and then every 16 weeks for the duration of the treatment period. * Patients who have tumor-related diarrhea keep a daily record of the number and consistency of bowel movements.
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Maximum Tolerated Dose (MTD)
Timeframe: During Cycle 1 and Cycle 2, approximately 56 days
Overall Percentage of Participants With an Objective Response Defined as a Complete Response (CR) or Partial Response (PR)
Timeframe: Patients were evaluated for response after every 2 cycles x 4 (prior to cycles 1, 3, 5, 7, and 9) and then after every 4 cycles X 1 (prior to cycle 13) and then every 6 cycles (prior to cycle 19, 25, 31, etc). Response was followed for an median of 59 mon