Original articleRadiation Dose Assessment for I-131 Therapy of Thyroid Cancer Using I-124 PET Imaging
Introduction
I-131 radioiodine therapy has been used for the treatment of differentiated cancer of thyroid for several decades.1, 2 Unlike external beam therapy planning, the estimation of the radiation dose actually delivered to the lesion by radioiodine, is usually ignored,3 probably because the task involves daily measurement of neck uptake either by a collimated thyroid probe or a gamma camera. To estimate the therapy dose to thyroid lesions, two general approaches have been used2: (1) The maximum “safe” dose method was developed at Memorial Sloan Kettering Cancer Institute. It is based upon administering an activity of I-131 that will deliver no greater than 200 cGy to blood that, through clinical experience, was found to avoid the onset of bone marrow depression.1 (2) Standard “fixed” activity in that, a standard activity, between 3.7 and 7.4 GBq, is given to each patient dependent upon the practice of the institute.
We subscribe to the approach (1), that first requires the oral administration of a tracer activity (15 to 195 MBq) of I-131 to determine the patient specific uptake and clearance kinetics. Our methodology involves determining the maximum tolerated dose (MTD) or more correctly the activity that will deposit 200 cGy to blood. This is given by the sum of the ray dose from activity in the blood and ray dose resulting from the whole body activity that enables the clinician to estimate of the radiation dose to each thyroid lesion per unit activity administered. The physician may then prescribe an activity corresponding either to a required dose (e.g., 30,000 cGy) to the lesion, or should this not be achievable without exceeding a dose to blood of 200 cGy, the maximum tolerated activity (MTA). For our thyroid cancer patients, the administered activity of I-131 is tailored to the uptake within the functioning mass of thyroid lesion, consistent with dose constraints to bone marrow. The greatest uncertainty in the estimation of absorbed dose to thyroid lesions, results from our ignorance of the lesion mass. Traditional planar views provide 2-D projection images of the functioning thyroid lesions, that are not suitable for lesion mass estimation. Although calculation of lesion mass is possible with single-photon emission computed tomography (SPECT), the spatial resolution is typically 1 cm vs. 4 mm with positron emission tomography (PET). The partial volume effects associate with SPECT render mass estimates less than 20 g inaccurate.4
A study has been performed by Pentlow et al.5 to investigate the feasibility of I-124 imaging with PET. This group showed that I-124 can be used for quantitative measurements of activity of lesions and normal organs even though I-124 has gamma rays in cascade with positron emission. Other studies have demonstrated that I-124 can be used as a tracer for I-131 in antibody therapy6 and thyroid cancer.7 The use of serial I-124 PET images has advantages over conventional dosimetric analysis with I-131, because the higher spatial resolution PET images enable a more accurate measurement of thyroid lesion volume.
In this study, we investigated the potential of PET to determine the absorbed dose for thyroid lesions under the constraint of 200 cGy to blood. The combination of quantitative pharmacokinetic analysis of clearance and PET imaging can play an important role in improving the dosimetry of radionuclide therapy.
Section snippets
Lesion Volume Calculation
Although, the full-width half-maximum (FWHM) is slightly greater for I-124 PET scans than F-18, PET images of thyroid cancer using I-124 are generally of much higher contrast than conventional PET scans with fluorodeoxyglucose (FDG), due to the specific radioiodine uptake characteristics of the thyroid lesions. Nevertheless the area of each region of I-124 uptake on each slice, is highly dependent upon the threshold and window setting under which the images are viewed on the screen. To select a
Results
Three patients underwent dosimetry studies with I-124 and I-131. Clearance data for these patients is given in Figure 2. Cumulated activity is then derived as the area under the curve. The results of calculation of therapy doses are summarized in Table 1. The estimate of lesion mass using the adaptive thresholding method applied to the PET images are given in column 4. The effective half-lives expressed as a single- or double-exponential fit to the lesion ROI data from serial PET scans. The
Discussion
We have developed a thyroid lesion dosimetry method using an adaptive thresholding technique8 to determine the boundaries of lesions so as to accurately and reproducibly estimate their volume. The cumulated lesion activity was obtained from serial I-124 PET images and the Fast Hartley Transform used to generate 3D dose distributions. The method has been implemented on 3 patients on a proof-of-principal basis. It is now possible to generate 3D dose distributions of thyroid lesions and to
Acknowledgements
This work was partially supported by DOE# 95ER62039 and the Gerschel Foundation.
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