ABSTRACT
Recently, 225Ac has received considerable attention for its use in targeted alpha therapy because it has a relatively long half-life and yields four more alpha-particles from the daughter nuclides. The performance evaluation should separately assess the distribution of 225Ac and 213Bi because daughter nuclides, including 213Bi, can cause renal toxicity, which may hinder the widespread use of 225Ac for targeted alpha therapy. In this study, we describe and validate a spectrum decomposition method for dual-isotope imaging of 225Ac and 213Bi using an alpha imaging detector. We implemented an experiment to demonstrate the feasibility of using the alpha imaging detector to obtain distribution images using therapeutic amounts of 225Ac. In addition, we designed and conducted a Monte Carlo simulation under realistic conditions based on the experimental results to evaluate the spectrum decomposition method for dual-isotope imaging. The alpha imaging detector exhibited a detection efficiency of 18.5% and an energy resolution of 13.4% at 5.5 MeV. In the simulation, the distributions of 225Ac and 213Bi were obtained in each region with a relative error of 5%. The results of this study confirmed the feasibility of the dual-isotope imaging method for discriminating alpha-emitters using small amounts of 225Ac.
ABSTRACT
PURPOSE: Serum Tg and I-131 WBS have been used to detect recurrent and metastatic thyroid cancers postoperatively. Tg is known to be more sensitive than I-131 WBS, and therefore, false-negative WBS cases with elevated Tg levels are frequently found. However, the clinical characteristics of false-negative Tg cases with positive WBS have not been clarified. MATERIALS AND METHODS: The authors evaluated 824 postoperative patients with differentiated thyroid carcinoma who underwent post-ablation/therapy I-131 WBS. Tg negativity was defined as a Tg level of < or = 2 ng/mL without TgAb under thyroid-stimulating hormone stimulation. Remission, recurrence, and metastasis were confirmed using pathologic or clinically findings. RESULTS: Fifty-two patients (6.3%) with functioning metastasis and negativity for TgAb were Tg-negative and posttherapy I-131 WBS-positive (TgN group), and 128 patients with functioning metastases were Tg positive and WBS positive (TgP group). The TgN group consisted of 45 cases of cervical/mediastinal lymph node metastases (86.5%) and seven cases of distant metastasis to lung or bone by follow-up WBS. The TgN group demonstrated significantly higher profiles of regional involvement than the TgP group (P < 0.029). In 47 patients in the TgN group, metastatic uptake disappeared in 33, ameliorated in four, and persisted in ten during follow-up. CONCLUSIONS: A significant number of differentiated thyroid cancer patients were Tg-/TgAb-negative despite a positive WBS finding. Cervical and mediastinal lymph nodes were predominant sites of metastasis in the TgN group. WBS should be undertaken routinely as a complementary modality to detect functioning recurrence and metastasis regardless of serum Tg results.
