Abstract
1187
Background: The determination of I-131 therapeutic activity for remnant ablation, adjuvant treatment or treatment for known persistent or recurrent disease in patients with differentiated thyroid cancer (DTC) remains controversial. Therapy with I-131 is performed using either empirically derived fixed activities or dosimetrically determined activity amounts. In the first approach the I-131 therapeutic activity is either 30 mCi, 100 mCi or 150 mCi and the managing physician or team makes the determination for the specific patient case. The second approach may be more optimal and patient-specific since it allows one to calculate the highest amount of activity that will be administered to the patient to achieve maximum therapeutic outcome without harming the dose-limiting organs. However, the dosimetric approach in and of itself can be employed using different methods: simplified, full whole-body, and lesional dosimetry. A dedicated survey study to establish a database of facilities in the US and outside US that perform dosimetry identified the reasons why some facilities do not perform dosimetry. The two most frequent reported reasons for not performing radioiodine dosimetry are “The literature does not show any value from dosimetry-guided I-131 activity over empiric activities” (43%, 26/61) and “It is too much work for our staff” (38%, 23/61).
Objectives: The aim of this educational exhibit is to provide an overview of the so-called simplified methods of radioiodine dosimetry, which could be more easily implemented in clinical practice compared to the more elaborate methods such as full whole-body dosimetry. Discussion: The objectives of any radiation therapy including I-131 therapy for differentiated thyroid cancer (DTC) are: (1) to deliver a sufficient radiation absorbed dose to the target in order to achieve the desired effect, whether it is cure, stabilization or palliation, and (2) to eliminate, or at least limit the radiation absorbed dose to normal tissues so as to reduce or avoid side effects. To accomplish the above, the radiation absorbed dose to tumors and target organs should be calculated. These measurements are difficult to perform and possibly subject to large errors. Second, full dosimetry such as described by Benua and Leeper [1] can be demanding for both patient and personnel. Alternative approaches to simplify the full whole body dosimetric approach and in order to determine patient-specific prescribed activities of I-131 for treatment of DTC have been proposed. These simplified methods help calculate the maximum tolerated activity to the blood (or bone marrow). These simplified methods can be listed according to the corresponding first authors: Traino, Thomas, Sisson, Atkins, Hänscheid, and Jentzen approaches. This educational exhibit will discuss each one of these simplified dosimetric methods including the performance, benefits, limitations, and any subsequent validation of each. Summary: With a better understanding of the performance, benefits, limitations, and validations, any facility that has a nuclear medicine gamma camera can perform one or many simplified dosimetric methods to determine I-131 activity for therapy of DTC. In the fast growing interest in personalized medicine and targeted radionuclide therapies, implementation of dosimetry techniques for treatment planning is becoming increasingly important, and, utilization of simplified methods for radioiodine dosimetry is a step forward towards this implementation. [1] Benua RS. Leeper RD. A method and rationale for treating metastatic thyroid carcinoma with the largest safe dose of I-131. Frontiers in thyroidology. 1986:1317-21.