The hypothesis underlying this study is that treatment outcome in molecular radiotherapy is dependent on the absorbed doses delivered rather than on the radioactivity administered. For up to 50 patients, post treatment dosimetry will be performed to establish the range of absorbed doses and associated uncertainties delivered to thyroid remnants, residual disease, and healthy organs from an ablative administration of I-131 radioiodine.

Imaging systems in participating centres will be characterised (system volume sensitivity, partial-volume effect and dead-time) and set up for quantitative imaging to allow for collation of data. Patients will be assigned to one of two dosimetry gamma camera scanning schedules. For scanning schedule 1 (Standard of Care) a single standard-of-care scan will be acquired according to local protocol with regards to time post-radioiodine administration. A minimum of 20 patients will be assigned to scanning schedule 2 (Standard of Care + Additional Imaging), for which a standard-of-care scan and three additional SPECT-CT scans will be acquired at 244 hours, 4812 hours, 7212 hours and 12048 hours post-radioiodine administration.

SPECT imaging datasets will be reconstructed with attenuation and scatter corrections to allow quantitative dosimetry calculations. Time integrated activity will be derived from analysis of the quantitative SPECT data and combined with relevant dose factors to obtain absorbed dose estimates for each target tissue. The subset of patients with additional imaging will be used to establish the pharmacokinetic properties of radioiodine in each tissue type (biological retention half-life) to enable determination of time integrated activity in patients with a single standard-of-care scan.

A minimum of 3 whole-body retention measurements will be performed per day during the patient's stay in hospital, approximately every 2-6 hours, according to local standard of care procedures. At each external measurement time point the quantified level of radioactivity in the whole body will be recorded.

Patients will be followed-up at their standard-of-care clinic visits to assess the success of treatment as well as for short to mid-term toxicity. Results from standard of care biochemical, radiological and haematological assessments (TSH, thyroglobulin, anti-thyroglobulin antibodies, ultrasound, serum creatinine, haemoglobin, white cell count, neutrophil count and platelet count) will be recorded to assess response to treatment. Statistical analysis of the relationship between the absorbed doses and outcome data will be performed.