A Retrospective Observational Study Comparing Two Different Treatment Options in Thyroid Cancer Patients With T4 Tumours

Inclusion Criteria: Historical records from patients who meet all of the following criteria will be eligible for inclusion in this retrospective evaluation: 1. Male or female patients living or deceased, aged 18 years or older at the time of first ablation for thyroid cancer. 2. Diagnosed with a well-differentiated T4 tumour (defined as a primary tumour of any size that extends beyond the thyroid capsule \[this may be referred to as TNM classification T4, N0-1, M0-1\]). This excludes unusual histological types such as oncocytic (Hurthle cell carcinoma), tall cell, sclerosing or cribriform thyroid cancers. 3. Undergone a near-total or total thyroidectomy on or after 01 January 2000 4. Undergone first ablation of thyroid remnants with high activity 131I (≥28 mCi or ≥ 1.036 GBq). 5. Undergone first 131I ablation for the T4 tumour using either Thyrogen (0.9 mg IM on 2 consecutive days) or THW stimulation. This excludes use of non-standard Thyrogen regimens (i.e., had a regimen of Thyrogen other than the standard 0.9 mg IM on 2 consecutive days or received Thyrogen in combination with THW). 6. Historical records are available confirming ablation results by: * DxWBS using small activity (≥2 mCi or 74 MBq) of iodine 131I (or 123I) performed at least 6 months after administration of the first ablation activity of 131I and/or * Stimulated Tg measured at least 6 months after administration of the first ablation activity of 131I. Exclusion Criteria: Historical records from patients who meet any of the following criteria will be excluded from this retrospective evaluation: 1. Received propylthiouracil, methimazole, vitamins or supplements containing kelp or iodine (taking a multivitamin that does not contain iodine or kelp is acceptable), medications that significantly affect iodine handling such as high dose corticosteroids, high dose diuretics, or lithium in the 45 days before administration of first ablative activity of 131I. 2. Received any iodine-containing contrast agents within 3 months prior to first ablative activity of 131I administered. 3. Used amiodarone within the 2 years prior to first ablative activity of 131I administered.