Pretherapy I-123 imaging in determination of the ablation dose of I-131 in post-surgical thyroid cancer patients

Objectives To determine if pre-therapy I-123 imaging in postoperative thyroid cancer patients aid in the determination of the I-131 radioablative/treatment dosage.

Methods Histopathology reports and pretherapy I-123 images from 49 post-surgical thyroid cancer patients were retrospectively reviewed. To determine if I-123 pretherapy imaging impacts the determination of the I-131 dosage, dosages for each patient were first determined based on age, pathology and staging information. The treatment dosage was again determined after interpretation of the I-123 scan. A "fixed" dosage approach was used (100 mCi for remnant ablation, 150 mCi if cervical nodes visualized, 200 mCi if distant disease was present).

Results The I-131 dosage was unchanged in 39 patients (80%), changed in 10 patients (20%). Among the 39 concordant patients, the disease stage was I in 23, II in 5, III in 9 and IV in 2 patients. Among the 10 discordant patients, disease stage was I in 3 patients, II in 4, and III in 3. In the discordant group, the determined dosage was higher when pre-therapy imaging was used. Patients were also evaluated in terms of whether surgical nodal sampling was performed, which was done in 30/49 patients. When pre-therapy scans were used, the derived dosage was different in only 10% (3/30) of these patients. This compares to 37% (7/19) of patients in which nodal sampling was not performed.

Conclusions Pretherapy I-123 scans do not alter the I-131 therapeutic dosage in the majority of patients after thyroidectomy. The pretherapy scans appear to influence the dosage more frequently in patients without nodal staging at the time of surgery.