Evaluation of dosimetry of radioiodine therapy in benign and malignant thyroid disorders by means of iodine-124 and PET

Abstract

The aim of this study was to evaluate the use of 124I positron emission tomography (PET) to determine the dosimetry of radioiodine therapy in hyperthyroidism and thyroid cancer. Phantom studies to assess the accuracy of PET were performed using an EEC phantom with spheres of different diameters filled with 3-30 MBq of 124I. Patient dosimetry was derived from PET data obtained 1-13 days after simultaneous oral administration of a therapeutic dose of 131I and a diagnostic dose of 124I. The obtained data were compared with findings from intratherapeutic probe measurements and clinical outcome. The phantom studies confirmed that 124I can be quantitated by PET (imprecision < or =10%), and volumetry is feasible for nodules <13 mm (imprecision < or =20%). Any influence of contamination with 123I or the simultaneous administration of 131I on the accuracy of the PET quantification and the probe measurements was ruled out by phantom measurements with solutions of 131I, 124I and 123I in various ratios. In autonomous nodular goitres, radioiodine uptake measured by PET varied from 25.4% to 64.3% and was not significantly different from that obtained by a scintillation probe (24.1%-73.1%, correlation coefficient r=0.91). Comparison of uptake and effective half-life in normal tissue versus autonomous nodules revealed significant differences in uptake but not in effective half-life [uptake 2.0-8.3 kBq/(ml x MBq) in normal tissue vs 12.6-29.3 kBq/(ml x MBq) in nodules; half-life 97.8-156.7 h in normal tissue vs 73.3-192.3 h in nodules]. Calculated radiation doses ranged between 177 and 633 Gy for autonomous nodules and between 47 and 126 Gy for normal tissue. In thyroid cancer patients, doses between 350 and 1,420 Gy were achieved in thyroid remnants and between 70 and 170 Gy in tumour metastases. It is concluded that 124I and PET are suitable for evaluation of the dosimetry of radioiodine therapy in benign and malignant thyroid diseases. The applied technique might be particularly useful for quantitative dose-response studies in radioiodine treatment and further investigations of stunning phenomena.

Optimized 124I PET Dosimetry Protocol for Radioiodine Therapy of Differentiated Thyroid Cancer

“…Using the reference approach (5 PET measurement points from 4 to 96 h), the present study found broadly similar radioiodine kinetics to those observed by other groups investigating 124 I PET dosimetry, albeit we noted wider ranges and higher maximum values than did those groups (3)(4)(5) The clinical course of DTC is often favorable, and standardized radioiodine therapy is well established and frequently successful. In selected patients, an individual therapy needs to be performed at the limits, and prospective dosimetry is required.…”

“…Specifically, the adapted approach used data points from only 2 (24-and 96-h) PET scans and an assumed fixed interception point of 8 h (the median for Table 2 lists the relevant statistical variables, and Figure 3C shows Lin's concordance correlation plots for this adapted approach. Lin's coefficients were identical with those of C 2 (4,24,96). Figure 4C depicts the individual absolute percentage deviation of each lesion with the adapted C 2 (24,96) versus the reference approach.…”

“…Erdi et al (3) used a 3-point model with the first scan at ;4 h and the last scan at ;24 or ;96 h after radioisotope intake. Eschmann et al (4) relied on 4 daily scans, starting at 24 h after 124 I administration. Sgouros et al (5) generally obtained 3 PET images, at ;4, ;20, and ;44 h after 124 I administration; sometimes a fourth scan was acquired at 96-144 h. However, to our knowledge, no group has yet published the results of any systematic investigation to determine the number and timing of PET scans needed for an optimized protocol that provides sufficiently reliable data on lesional absorbed dose while minimizing the quantity and time range of scans.…”

Internal Radiation Dosimetry: Models and Applications

“…In such cases, it is necessary to acquire some whole scans starting from 6-7 hours after radio-iodine therapy. (Eschmann et al, 2002) (Sgouros et al, 2004).…”

A new PET probe, 18F-tetrafluoroborate, for the sodium/iodide symporter: possible impacts on nuclear medicine

“…PET dosimetry could provide useful routine procedures for radioiodine therapy in advanced differentiated thyroid cancer and might allow safer or more effective radioiodine dose and earlier multimodal interventions than standard empirical protocols. In addition, PET can be used for dosimetric calculation in patients with Graves' disease and autonomously functioning nodule (11).…”