Propuesta de variables e indicadores de calidad para realizar la evaluación interna de programas de estudios de educación virtual

The term nuclear medicine refers to the introduction of radioisotopes in liquid (or occasionally gaseous) form into the body for either diagnostic or therapeutic purposes, or for the study of disease. The unsealed radioactive material is administered orally, intravenously or by inhalation of gases. The scale of application of these techniques is much less than for external radiation beam procedures; nevertheless, they are still commonly used practices in health care. For example, in the UK in the year 2004, some 700 000 nuclear medicine procedures were undertaken, of which about 98 per cent were for diagnostic and 2 per cent for therapeutic purposes.

In nuclear medicine, special attention should be given to women who are breast-feeding. Depending on the procedure involved, it may be necessary to advise the patient to cease breast-feeding until it is established that the risk to the child is sufficiently low.

Precautions might also need to be taken to protect relatives, friends and others who come into contact with patients, particularly when they are discharged from hospital while still retaining radioactive material.

14.5.1 Diagnostic radioisotope tests and nuclear medicine imaging

The purpose of radioisotope diagnostic tests is the investigation of body function. By introducing radioactive tracers in a suitable chemical form into the body and observing their behaviour using external detectors, or by monitoring excretion, important information on the functioning of body organs may be obtained. The pattern of distribution of the radioactive tracer can be constructed into an image by a gamma camera, which consists of a collimated scintillation detector coupled to an array of photomultiplier tubes (see Fig. 14.6).

This basic scintigraphy technology has now been developed to include single photon emission computed tomography (SPECT). As the name implies, this is very similar to transmission CT except that the system detects g-ray photons emitted by the radioactive tracers in the body and constructs an image of a section through an organ or the whole body using one, two or even three gamma camera detector arrays or ‘heads’. The organs that can be imaged by this technique include the lungs, brain, liver, spleen, kidneys, thyroid, bone and blood. Most of these tests use suitable pharmaceuticals labelled with a radionuclide (called radiopharmaceuticals), commonly technetium-99m (Tc-99m).

The great advantage of this is that it can be obtained from a radionuclide generator. The

Nuclear medicine 179

generator typically contains 0.04 TBq of molybdenum-99 (Mo-99), which has a half-life of 66 h and decays to the pure g-emitter Tc-99m, which has a half-life of 6 h. The Mo-99 is absorbed onto tin dioxide and, as the Tc-99m daughter is produced, it is released into saline solution in the generator. The saline solution containing the Tc-99m is eluted into phials and, if necessary, combined with pharmaceuticals in preparation for administration.

Another technique used is positron emission tomography (PET). In this case, the radionuclide tracer is a positron emitter, usually fluorine-18, and the detection system detects the 0.51 MeV annihilation g-rays. These require greater shielding than the softer g radiation from some other radionuclides. In addition, owing to the very short half-lives of the radioisotopes involved, such as 110 minutes for fluorine-18, facilities that offer PET imaging generally require an on-site accelerator called a ‘cyclotron’ to produce the required radiopharmaceuticals. The installation and operation of accelerators such as cyclotrons represent additional radiation protection challenges.

Less sophisticated, non-imaging, techniques are also used. For example, a single scintillation detector placed close to the thyroid can be used to study the functioning of this organ (see Fig. 14.7).

The quantities of radionuclides involved in these tests range from tens to hundreds of MBq and the dose to the patient is generally a few mSv. With increasing use of PET scanning, particularly involving the use of F-18, the dose received by staff involved in nuclear medicine procedures requires careful monitoring and control. With appropriate

Lead collimator

Scintillation detector Sodium iodide crystal

Computer

Array of photomultiplier

tubes

Image display Patient containing

radioactive material

Figure 14.6 Schematic illustration of gamma camera used in nuclear medicine imaging.

Scaler Collimated detector

Thyroid

Figure 14.7 Thyroid radioiodine uptake test.

methods of working, fingertip and eye dose can usually be controlled and it is often the whole body dose that is limiting.

Under many circumstances the patient can be discharged immediately after the examination has been completed because the low activities and short half-lives of the radioisotopes involved do not leave a residual activity that would represent a significant hazard to other people. The radioactivity is normally reduced to a very low level within a few days by radioactive decay and excretion.

14.5.2 Radioisotope therapy

In some circumstances radiation therapy is best performed by the ingestion or injection of radionuclide solutions into the body. Specific nuclides or radiolabelled pharmaceuticals are chosen, which concentrate in the organs requiring treatment, thus minimizing the dose to the rest of the body. The majority of therapeutic procedures involve the administration of nuclides of fairly short half-life (8 days or less) and the quantity is selected so that the required dose is delivered from the time of administration until the nuclide decays or is excreted. The main applications for radioisotope therapy are the treatment of thyroid cancer and thyrotoxicosis, using iodine-131. Typically, quantities of up to about 5000 MBq are administered for the treatment of thyroid cancer, giving a thyroid absorbed organ dose of up to 100 Gy, and a dose to the whole body (effective dose) of up to 1 Sv. Treatment for thyrotoxicosis, although more common than for thyroid cancer, involves only about one-tenth of the quantity of iodine-131 and therefore one-one-tenth of the dose.

Patients containing therapeutic quantities of radioactivity should be nursed under conditions that permit easy containment of radioactivity in case of contamination. Ideally, special rooms should be provided with en-suite facilities and all surfaces designed to permit easy cleaning. Where g-emitters (such as iodine-131) are involved, the room may need to be shielded to ensure that the dose rate in adjacent areas is not significantly increased.

The ventilation system should provide an adequate rate of air change, typically 5–10 air changes per hour, and should be designed to ensure that there is no possibility of the air being recycled to other areas. Protective gowns and gloves should be worn when handling the patient, contaminated linen or excreta, and a special storage area should be provided for contaminated linen waste and samples of excreta. The Radiation Protection Supervisor (RPS) should specify any limitations on the time allowable for nursing procedures or visiting periods. Washing and monitoring facilities should be provided for use when leaving the area and regular radiation and contamination surveys should be made of the ward. Patients must only use a so-called ‘hot’ toilet that is designated for aqueous radioactive waste disposal.

Within the hospital, liquid wastes and excreta are normally routed via a dedicated drainage system, but ultimately the discharge is to the normal public sewage system where they are diluted by the much greater quantities of uncontaminated liquid from other areas.

In some cases, such as treatment for thyrotoxicosis, patients undergoing therapeutic nuclear medicine procedures may be treated as outpatients and discharged on the day of treatment. Where the administered quantity of radionuclide is higher, such as for the treatment of thyroid cancer, the patient would normally remain in hospital for a few days during which the level of retained radioactivity would rapidly reduce.

Within a hospital, the source preparation is undertaken in a radiochemical laboratory called the radiopharmacy and protection is achieved by the methods described in Chapters 8 and 9, that is by minimizing the quantities of radioactive material handled, by containing it whenever possible and by the use of good procedures and facilities. The grade of the

Summary of key points 181

laboratory should be appropriate to the radiotoxicity and the quantity of the nuclides in use. It will be recalled that in such laboratories special attention is paid to surface finishes and to ventilation. Fumehoods are an essential feature even if quite low levels are being used. It is very important to have separate facilities for diagnostic and therapeutic work as low-level diagnostic tests can be ruined by cross-contamination from highly active equipment used in therapeutic work.