Procedures for use in dosimetry of patients undergoing I-131 therapy of benign thyroidal disease. Report of the SEFM. Working group on radiation dose estimations after administration of radiopharmaceuticals

Abstract

This document presents a report of the patient dosimetry written by the Nuclear Medicine Task Group of the Spanish Society of Medical Physics. It is dedicated to hyperthyroidism treatments with ¹³¹I. The document describes how to apply the international procedures to determine the absorbed dose estimation on an individual patient basis and using the typical equipment available in a Nuclear Medicine department, i.e. planar gamma camera, thyroid-uptake system and external-exposure survey-instrument.

Basically this report focuses on how to obtain in each patient the three parameters needed to estimate the absorbed dose in the patient thyroidal uptake: the uptake mass, the uptake of activity at each time, and the effective half-life.

To estimate the uptake mass a procedure with the planar gamma camera is proposed.

To estimate the uptake of activity at any time after the ¹³¹I administration four methods are included: Three of them use the image obtained with a planar gamma camera and the main difference is the calibration procedure. One uses a calibration factor obtained from a phantom following the recommendation of the European Association of Nuclear Medicine (EANM). The second uses a calibration factor obtained in air following the Medical Internal Radiation Dosimetry committee recommendation (MIRD Pamphlet 16). The third follows a procedure based in the classic calibration of INa (Tl) detectors, with an intrinsic efficiency determined with a calibration measurement and with an individual geometric factor calculated for each patient. The fourth procedure is based on the direct measurement of the activity with an external counting thyroid uptake device. Two methods are included for each instrument (gamma camera or thyroid uptake system): single image or measurement and dual conjugate views or measurements.

The time-activity curve is obtained fitting the experimental measurements performed with the gamma camera or with the uptake counter. If very few values of these experimental data are available, an external exposure survey can be used to estimate the temporal evolution of the uptake of activity within the patient.

The document also includes estimations of absorbed doses in other organs of the patient and tables with values of fetus/embryo absorbed doses in pregnant women undergoing ¹³¹I hyperthyroidism treatment.