Elsevier

Clinical Positron Imaging

Volume 2, Issue 1, January–February 1999, Pages 41-46
Clinical Positron Imaging

Original article
Radiation Dose Assessment for I-131 Therapy of Thyroid Cancer Using I-124 PET Imaging

https://doi.org/10.1016/S1095-0397(99)00004-7Get rights and content

Abstract

The goal for this work was to develop a method to determine the feasibility of estimating absorbed dose distribution of I-131 thyroid therapy using I-124 PET images of residual thyroid lesions with the dose constraint of 200 cGy to blood, that is a surrogate for bone marrow toxicity. A dose response study has been carried out on 3 patients with papillary thyroid carcinoma. Those patients were given 15-37 MBq of I-124 along with 74-185 MBq of I-131. PET imaging was performed 2–4 hour and then at 24 hour and either 48 hour, or 72 hour post-infusion. Lesion masses were computed from PET images using an adaptive thresholding technique. The definition of the boundary enabled determination of the iodine activity within the lesion. Time-activity curves were fitted to estimate the cumulated activity and therefore the absorbed dose per MBq administered. Daily blood and total body counts were performed on the patients using a multichannel analyzer with windows set for both I-131 (364 keV) and I-124 (511 keV). Cross-talk corrections from one isotope into the alternate window was determined using a standard of each respective isotope. At maximum-tolerated-activity (MTA) that delivers 200 cGy radiation dose to the blood, the dose to lesions from I-131 varied from 0.04 to 2.44 cGy/MBq (1.57–90.48 rads/mCi) with effective half-lives for I-124 ranging from 0.58 to 1.86 days. The three-dimensional absorbed dose distribution in the thyroid lesions was calculated by convolving the activity values with an I-131 point-source kernel using a Fast Hartley Transform. The calculated mean absorbed dose distribution was displayed as isodose lines on PET images that can be used to refine the amount of administered activity. PET with I-124 may improve the absorbed dose estimates from radioiodine therapy with I-131 in the treatment of thyroid cancer. The capability of estimating I-131 mean absorbed dose distributions from serial I-124 PET images can lead to patient-specific treatment planning for thyroid therapy.

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.

References (16)

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