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Introduction
Therapy with 131I, based on the concentration of radioactive iodine in thyroid tissue, seeks to completely eliminate all thyroid remnant tissue remaining after surgery as well as any functioning thyroid metastases in the body. The most common method used to ablate thyroid remnants is the use of fixed levels of activity, which may result in the administration of insufficient levels of activity to cause complete ablation or overdosage of iodine to the subject. With large administrations of radioactive iodine, the possibility that subjects require unnecessary hospital stays, with their attendant costs, as the optimum amount of activity was not given, based on individual patient anatomical and biokinetic data.
An alternative to the use of fixed activities is the administration of tracer levels of 131I to determine the characteristics of each patient, including the thyroid uptake and elimination half-times for radioactive iodine in the remaining thyroid tissue 1. Such data may be used to establish more exactly the activity needed for ablation. To individualize the dosage in this manner, one needs to know with reasonable accuracy the mass of thyroid tissue to be treated, which is best evaluated with three dimensional imaging techniques. Knowledge of the distribution and retention of iodine in other tissues of the body is also desirable. The best image analysis techniques employ Single Photon Emission Computed Tomography (SPECT) or Positron Emission Tomography (PET), with anatomic images supplied by Computed Tomography (CT) or Magnetic Resonance Imaging (MRI) 2, 3 devices.
In this work, we define a protocol for dose planning for thyroid ablation therapy in patients with differentiated thyroid cancer, using quantification of SPECT images. As such techniques may be difficult to implement in routine clinical use, we also show how a simple computer program can facilitate the application of such calculations with minimal data input by the user.
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