Observational Study to Evaluate Vandetanib in RET -/+ Patients With Metastatic Medullary Thyroid Cancer

Inclusion Criteria: 1\. Signed informed consent 2. Male or female aged 18 years or above 3. Histological diagnosis of MTC 4. Patients with symptomatic and aggressive sporadic MTC, who have unresectable, locally advanced/metastatic disease. (The factors considered by the investigator to determine a patient's disease to be symptomatic and aggressive will be recorded in the CRF). 5. Measurable disease: * assessment confirmed within the 12 weeks previous to start of treatment, and * defined according to RECIST 1.1: at least one lesion, not irradiated, that can be accurately measured as ≥10 mm in the longest diameter (except lymph nodes which must have short axis ≥15 mm) with CT or MRI and which is suitable for accurate repeated measurements. Measurable lesions with calcifications should not be assessed as target lesions unless no other measurable lesion is available. 6. Known definite RET mutation status (definition according to section 3.2). The status should be: * for patients prescribed with vandetanib: positive or negative * for patients not prescribed with vandetanib: negative RET mutation status must be determined from a tumour sample obtained within 18 months prior to enrollment. It is strongly recommended that a tissue sample obtained within 6 months prior to enrolment is used. 7. For patients newly prescribed vandetanib 300 mg, the prescription should be issued according to marketing authorisation and following the vandetanib Summary of Product Characteristics (SmPC) (Appendix B). The starting dose could be reduced to 200 mg in patients with moderate renal impairment * Exclusion criteria 1. Current or planned inclusion/participation in a clinical trial 2. Patients already receiving vandetanib or who have received vandetanib for their MTC before the study first visit 3. Contraindications according to the vandetanib SmPC (not applicable for patients who do not receive vandetanib): (a) Patients with a QT interval corrected for heart rate (QTc) interval over 480 msec: (i) Congenital long QT syndrome (ii) Concomitant use of vandetanib with the following medicinal products known to also prolong the QT interval and / or induce Torsades de pointes: Arsenic, cisapride, erythromycin intravenous (IV), toremifene, mizolastine, moxifloxacin, Class I A and III antiarrhythmics (b) Currently pregnant or breast feeding (c) Hypersensitivity to the active substance or to any of the excipients (d) Severe renal impairment: creatinine clearance \< 30 ml/minute calculated by Cockcroft-Gault formula. (See Appendix D). (e) Serum bilirubin greater than 1.5 x the upper limit of reference range (ULRR) (f) Potassium, magnesium or calcium outside the normal laboratory range