Dose optimization in nuclear medicine therapy of benign and malignant thyroid diseases

The iodine isotope I-131 has been used in nuclear medicine for several decades to treat both benign and malignant thyroid diseases. The therapy is effective and safe and insensitive to variations in the activity dosage of I-131. Individualization of therapy in order to limit the administered activity to the lowest amount necessary to successfully treat diseased thyrocytes while minimizing exposure to healthy organs requires performing dosimetry through measurement of the individual iodine kinetics. This review discusses the concepts of dosimetry used for patients with thyroid disorders and compiles information on the present evidence for superiority of individualization of therapy by dosimetric assessments. Two main concepts of individualized treatment are used for patients with differentiated thyroid carcinoma: (a) the assessment of the specific absorbed dose to the blood as a substitute for the red marrow dose in order to target at a specific blood dose from therapy and (b) the administration of the I-131 activity determined by lesion dosimetry to be necessary to achieve a fixed absorbed dose to the tumour which is known to be effective in most of the cases. The first concept is mainly used to safely administer the highest tolerable activity, thus enhancing the absorbed dose to the tumour. Increasing evidence exists that patients with advanced disease benefit from this dose optimization. The second concept becomes increasingly feasible with the improvements in dosimetry introduced by advanced imaging techniques like I-124 PET/CT, but bears the inherent risk of under-dosing the patient. Studies reporting response in tumours are not consistent regarding the absorbed dose necessary to certainly eliminate the lesion. In the treatment of benign thyroid diseases, most studies comparing regimes with calculated and estimated activity dosage did not find improved rates of cure and side effects in patients with measured kinetics. A few studies with advanced dosimetric concepts found good dose–response relations. Individualization of radioiodine therapy in the treatment of thyroid disorders still is not used to its full potential. Recently developed imaging techniques like SPECT/CT and PET/CT, enabling 3-dimensional measurement of dose distributions, allow considerable improvements in dosimetry. Prospective randomized trials with appropriate and controlled dosimetry are necessary to provide conclusive information on the value of individualized treatment planning and to identify the major confounding variables responsible for treatment failure.