124 I PET/CT in Patients with Differentiated Thyroid Cancer: Clinical and Quantitative Image Analysis

Abstract

Background:

Although radioactive iodine (RAI) imaging/therapy is one of the earliest applications of theranostics, there remain a number of unresolved clinical questions as to the optimization of diagnostic techniques/protocols and improvements in patient-specific treatment planning strategies. The objectives of this study were to determine the imaging characteristics and clinical feasibility of ¹²⁴I positron emission tomography/computed tomography (PET/CT) for the determination of extent of disease and evaluation of RAI kinetics in its physiologic and neoplastic distribution in patients with differentiated thyroid cancer (DTC).

Methods:

The study was designed as a prospective phase II diagnostic trial of patients with confirmed DTC. Following adequate preparation, patients received 2 mCi ¹²⁴I in liquid form and sequential whole-body PET/CT imaging was performed at five time points (2–4 h, 24 ± 6 h, 48 ± 6 h, 72 ± 6 h, and 96 ± 6 h post-administration). All patients who had ¹²⁴I imaging subsequently underwent RAI treatment with ¹³¹I, with administered activities ranging from 100 to 300 mCi. Post-treatment scans were obtained 5–7 days after RAI treatment. A by-patient and by-lesion analysis of the ¹²⁴I images was performed and compared with the post-treatment ¹³¹I scans as well as F-18 FDG PET/CT images. Quantitative image analysis was also performed to determine the total functional volume (mL), activity per functional volume (μCi/mL), and cumulated activity (μCi/h) for remnants, salivary glands, and nodal metastases.

Results:

Fifteen patients (6 women; M _age = 57 years; range 29–91 years) were enrolled into the study. Forty-six distinct lesions were identified in these 15 patients on ¹²⁴I PET/CT images, with a sensitivity of 92.5%. In addition, ¹²⁴I identified 22.5% more foci of RAI-avid lesions compared with the planar ¹³¹I post-treatment scans. This study demonstrates different kinetic profiles for normal thyroid remnants (peaked at 24 h with mono-exponential clearance), salivary glands (peaked at 4 h with bi-exponential clearance), and metastatic lesions (protracted retention), as well as individual variations in functional volumes and thus cumulated activities.

Conclusions:

¹²⁴I PET/CT is a valuable clinical imaging tool/agent, both in determining the extent of disease in the setting of metastatic DTC and in the functional volumetric and kinetic evaluation of target lesions.