Application of isotopes in medicine

Does ionizing radiation have medical applications?

Ionizing radiation has multiple applications in the field of medicine. The specialty called radiology uses X-rays from a cathode ray tube to carry out multiple types of diagnostic radiological examinations. In the specialty of nuclear medicine, different types of non-encapsulated isotopes are handled (in liquid or gaseous form) that are administered to the patient or used in the laboratory in analytical tests with eminently diagnostic purposes. In the field of therapy, ionizing radiation is used to treat malignant tumors, giving rise to the specialty called radiotherapy.

In addition to these three specialties, ionizing radiation from radioactive isotopes is widely used in the field of medical research, having carried out a large number of kinetic and metabolic studies in human and animal physiology using radiotracers.

The great development of these specialties is due, on the one hand, to a better knowledge of the physics and applications of radiation and, on the other, to continuous advances in equipment for their production, detection and use. The most sophisticated equipment is expensive and requires highly specialized multidisciplinary staff to operate, including not only physicians but also radiophysicists, radiopharmacists and chemists who work closely together. This means that these services are sometimes only available in large medical centers that serve large population centers. At present, in Spain there are, both at the public and private health level, multiple centers that have state-of-the-art equipment and well-qualified personnel.

What is radiodiagnosis?

It is the diagnostic method that consists of obtaining images of the organism by means of X-ray equipment. The elementary X-ray tube consists of an incandescent filament (cathode) that produces electrons, which are accelerated in a vacuum causing them to collide with an anticathode, originating electromagnetic radiation called X-rays. All this is contained in a glass ampoule, included in a lead-lined envelope, except for the radiation exit hole.

Since the discovery of X-rays, enormous improvements have been achieved both in the equipment used and in the means of protection, having greatly expanded the indications for this specialty. Studies of the skeleton, chest, abdomen, nervous system, digestive tract, bile ducts, urinary system, vessels, heart, etc. are currently possible, so that no organ escapes this type of examination.

The radiological image is produced when the X-ray beam passes through the area to be explored and the X-rays are absorbed differently by the tissues, obtaining an emerging beam that presents variations in intensity, which are made visible by imaging systems. on the screen, then called radioscopy exploration or impressing a film that once revealed gives rise to an X-ray.

The most basic radiology facilities perform only simple radiographic studies of the bones, chest, kidneys, or bile ducts. The most complete facilities have a tilting table to place the patient in different positions, making it possible to take X-rays at any time desired.

In large hospitals there are specialized radiology teams for examinations that allow the visualization of the vessels of the circulatory system through the injection of iodinated contrast agents, being very useful in brain, cardiac, limb and abdominal examinations. Radiology used to control biopsy taking, cyst evacuation or therapeutic maneuvers, as well as catheterization, are even more delicate explorations that require highly specialized personnel.

Computerized axial tomography or CT is a radiological means that consists of the reconstruction by means of a computer of the slices of an organ or explored area produced by a very fine X-ray beam that rotates around it.

Mammography is the radiological technique used to explore the breasts in women, allowing the diagnosis of benign or malignant breast lesions, even of very small dimensions.

Dental radiology uses special equipment and procedures such as intraoral x-ray films or tubes or x-rays
mouth panoramas.

What is nuclear medicine?

Nuclear medicine is a medical specialty with a relatively short history, some 35 years, that uses ionizing radiation from radioisotopes or radionuclides to carry out morphological and functional studies of numerous organs, as well as for radioanalytical determinations of numerous substances contained in the organism. To carry out studies on patients, it is necessary to introduce a small amount of radioactive substance called a radiopharmaceutical into the body through different routes, generally intravenous or digestive, inhalation, etc. These substances, due to their special affinity, are fixed in the organ to be studied, emitting gamma radiation that is detected by equipment called a gamma camera, whose detector is placed on the organ to be explored, receiving the photons from the radiopharmaceutical.

These signals are transformed into electrical impulses that are modulated, amplified and processed by means of a computer attached to the equipment, which allows the spatial representation of the organ, called scintigraphy, on a screen or X-ray plate, paper or the visualization of images successive steps of the same for the study of a certain function. Recently there are cameras that allow the obtaining of sections of the organ according to the three directions of space, which improves the quality of the studies and the diagnostic sensitivity.

In some centers there is equipment called PET (positron emission tomography), which uses radionuclides that emit positrons instead of photons as in classical nuclear medicine methods. The quality of the images obtained with this equipment is superior to that of conventional equipment but, at present, due to its high cost and complicated technology, since it is necessary to have a cyclotron to produce isotopes with ultrashort half-lives of the order of minutes or hours, there are only equipment marketed in countries with a high level of medical technology. Spain currently has several of these teams, with their main applications in the fields of oncology, cardiology and neurology.

The fundamental advantages of nuclear medicine exploratory methods are that they are not dangerous or annoying for the patient and that they have minimal side effects, since the radiation received is equal to or less than that of routine radiological studies.

Analytical techniques called radioimmunoassays allow the detection and quantification of numerous substances that are present in very small amounts in blood or urine, and that are very difficult to detect by conventional analytical means. They are carried out thanks to an ingenious system that combines an antigen-antibody binding reaction with the labeling of one of these two components with an isotope, generally iodine-125.

Although nuclear medicine is essentially a diagnostic specialty, non-encapsulated radioisotopes can be used as a means of treatment in specific applications, thus speaking of metabolic radiotherapy. This consists of administering a relatively large dose of radioactive substance in liquid form by injection or ingestion so that it accumulates in the treated organ or site, where it acts through the radiation emitted on the tissues in close contact with it. The most frequent application is the treatment of patients with thyroid cancer or hyperthyroidism and, to carry it out, patients are generally admitted to special hospitalization units that have rooms with means of radioprotection and are attended by specialized personnel.

What are the main diagnostic applications of isotopes?

Virtually all medical specialties can benefit from the morphological, functional and analytical studies of nuclear medicine. Morphological studies can be completed with exploratory radiological and ultrasound imaging techniques or other recently acquired techniques, such as computerized axial tomography or nuclear magnetic resonance.

• Within the endocrinology specialty, thyroid or adrenal scintigraphic studies are of great interest, along with useful hormonal determinations for the study of these same organs, as well as the pituitary gland, growth problems, sexual development, fertility, diabetes, etc.
• In the specialty of cardiology, the applications focus above all on the diagnosis of alterations in the cardiac circulation that produce conditions such as angina or myocardial infarction, as well as on the diagnosis of congenital heart disease.
• Pulmonary studies allow the study of pulmonary vascularization and ventilation, which are affected in numerous diseases of the respiratory system.
• The examinations of the digestive system are very varied, including studies of the function of the esophagus and stomach, liver studies for the diagnosis of cirrhosis, cysts or tumors or studies of the bile ducts used in the presence of gallbladder infections or gallstones. Meals containing small amounts of radioactive substances may also be given to study disorders of intestinal digestion or absorption.
• The function and morphology of the kidney and urinary tract can be assessed using isotopic techniques that reveal renal processes, urinary tract obstruction, viability of kidney transplants, etc.
• In patients with trauma, infections or bone tumors, bone scintigraphy shows an exaggerated and anomalous accumulation of the injected radiopharmaceutical in the altered areas, which allows the diagnosis of these processes.
• In cancer patients, conventional nuclear medicine studies, and especially PET studies, allow staging of the tumor, which is essential for making therapeutic decisions.
• Functional studies of the central nervous system are very useful in the assessment of patients with various types of dementia, epilepsy, vascular or tumor diseases, in which studies with different isotopes allow visualization of functionally affected areas that cannot be diagnosed by other techniques. purely imaging study such as computed tomography or magnetic resonance imaging.
• Within the field of laboratory analytical applications, endocrinological hormonal studies are of great interest, as well as the determination of so-called tumor markers, which are substances produced specifically by tumors and whose presence in blood allows their diagnosis and monitoring. They are also applied in the study of patients with allergic diseases, hepatitis, anti-doping control and different hematological studies.

What are the most used isotopes in nuclear medicine?

Nuclear medicine uses different types of isotopes for its diagnostic and therapeutic applications. Their choice is conditioned by the need for them to be non-toxic, have a suitable type of radioactive emission, low energy and a short half-life, so that the absorbed dose is small. Its elimination must be rapid so that the time it remains in the body is not prolonged.

To carry out studies on patients, a pure radionuclide can be used that is fixed in the organ to be explored, as in the case of radioiodine that is taken up by the thyroid gland, or different molecules can be labeled that have a great tropism for the organ that you want to study, such as labeled colloids for liver scintigraphic studies or labeled phosphates for bone studies, in which case we speak of radiopharmaceuticals.

The isotope currently most widely used in nuclear medicine services is technetium-99, which emits gamma radiation and has a half-life of six hours, making it necessary to have generators, which are shielded containers that are usually received weekly in nuclear medicine services and that contain a parent isotope (molybdenum-99), with a longer half-life, from which the daughter isotope (technetium-99) is obtained, which is used daily for explorations .

Technetium is easily combined with carrier molecules that allow the study of a wide variety of organs such as the skeleton, heart, liver and spleen, bile ducts, digestive tract and brain. In addition to technetium, other gamma emitters with a short half-life are used, such as thallium-201 for cardiac studies, gallium-67 for tumor detection, Indium-111 for inflammatory processes, iodine-131 and 123 for thyroid and kidney studies, and xenon-133 for pulmonary studies.

For PET studies, the most used radiopharmaceutical is fluorine deoxyglucose labeled with fluorine-18.

In analytical studies called radioimmunoassays (RIA), iodine-125 is mainly used and sometimes tritium.

In therapeutic applications known as metabolic therapy, iodine-131 is fundamentally used in liquid form for the treatment of patients with thyroid cancer or hyperthyroidism, in which case the doses administered are much higher than in the case of diagnostic applications, for so the patient is usually admitted to the hospital for a few days. The use of pure beta emitters in applications such as treatment of arthritis or bone metastases does not require hospitalization, since beta emission, due to its low penetration capacity, does not cause radioprotection problems for the patient or their relatives.

What is radiotherapy?

Radiotherapy is the medical specialty that uses the administration of ionizing radiation for curative purposes for the destruction of malignant tissues or tumors. Over eighty years ago, two French physicians, Bergonie and Tribondeau, demonstrated that the radiosensitivity of cells is directly related to their differentiation and ability to reproduce, with less differentiated cells and those with a faster growth rate being more sensitive. Given that the cells that make up malignant tumor tissues usually meet these conditions, these tumors can be subjected to the action of radiation that will cause the death of the tumor tissues, surviving the surrounding healthy tissues that are more radioresistant because they are composed of more resistant cells. differentiated and slower growing.

In the treatment of malignant tumors, radiotherapy can be used alone or associated with other therapeutic means such as surgery or chemotherapy. The decision on the type of treatment is made based on a series of factors such as radiosensitivity of the tumor, location and tumor volume, degree of evolution of the disease, general condition of the patient, opportunity for irradiation and the technical modality used.

The study of the characteristics of the tumor cells, location and tumor extension allows, once this form of treatment has been decided, to plan the type of irradiation, calculation of the total dose, form of administration and possible fractionation with rest intervals that may facilitate treatment. Progressive reduction of the tumor favoring the elimination of dead cells and allowing better repair of the surrounding tissues.

In addition to curative purposes, radiotherapy can be used as palliative therapy in cases of incurable patients, in which the tumor mass produces obstructions or compressions of other organs that worsen the patient's quality of life. In these cases, the administration of radiation produces a decrease in tumor volume, relieving the patient's symptoms and improving their quality of life, which makes this type of treatment a first-order indication in these patients.

The radiation therapy modalities used are given different names in relation to the characteristics of the radiation and the equipment that generates it.

What is teletherapy?

Teletherapy (tele: far) is the form of radiotherapy that uses radiation from a generator located at a certain distance from the area to be irradiated. This irradiation modality comprises a wide range of equipment. Conventional or orthovoltage radiotherapy, rarely used, is performed using low or medium energy X-ray equipment. The most widely used high-energy or megavoltage equipment today includes the cobalt pump and linear accelerators.

Low energy X-ray equipment is used more for skin treatments, so that the maximum doses are achieved on the surface with little irradiation of the deeper tissues. With medium energies greater depths are reached, expanding the possible indications. Different types of filters are used to reduce the softer radiation that would affect the skin uselessly.

Of the supervoltage equipment, the most used is the so-called cobalt bomb, which contains a cobalt-60 source of one to two centimeters in diameter that is located in an armored casing that prevents the exit of radiation, except for a small hole. diaphragmed to provide targeted radiation. Cobalt-60 has an approximate half-life of five years and produces high-energy radiation (1.2 MeV) capable of irradiating large, deep-seated tumors. The head of the equipment can be directed in any direction in line with the patient's treatment table, according to pre-planning.

Linear accelerators are high-energy teletherapy equipment (greater than 3 MeV) that usually work with electrons, which are accelerated and released by making them travel through an accelerator tube where a very high-frequency electromagnetic field pulls them forward at all times. the points of its trajectory. These equipments allow choosing the appropriate energy according to the type of tumor or depth. Exposure times are short, with the advantage that they only emit radiation at the time of use, and through various filters the dose in the tumor volume is optimized. They have a high initial and maintenance cost.

There is some very sophisticated equipment to apply special radiotherapy techniques in places where surgery is difficult to access. The techniques are called radiosurgery and are applied with special accelerators or with radiation-emitting equipment with multiple cobalt-60 pellets ("gamma-knife").

Other equipment for exclusive use in research is nuclear reactors that produce neutrons and cyclotrons that produce other subatomic particles.

Accelerators, like any other type of radiotherapy, have a large number of safety devices, both for the protection of the patient and the personnel who use them. These devices, as well as the characteristics of the radiation beam, must be measured and checked periodically by the staff of each hospital.

What is brachytherapy?

Brachytherapy (brachy: short, near) is the radiotherapy modality that uses closed or sealed sources of radioactive material that are placed in contact with the tumor or introduced into the tumor. Its greatest advantage is that of concentrating the maximum dose of radiation in the tumor tissue with little irradiation of the surrounding healthy tissue, based on the fact that the dose received in the proximity of a source decreases very quickly when moving away from it. Superficial brachytherapy is called when plates of radioactive material are placed on the tumor area; endocavitary when the radioactive material is introduced into the body cavity (vagina and cervix); interstitial when surgical placement of needles, wires or radioactive seeds is performed within the tumor itself (breast, neck, prostate), and intraluminal when radiation is applied within the lumen of any of the organic ducts (bronchus, esophagus , vascular).

Although years ago the radioactive material most used in brachytherapy was radium-226, it has now been replaced by others with more suitable characteristics and less radiological risk, such as strontium-90, caesium-137, cobalt-60 and iridium-192.

In these treatment modalities, hospitalization in special units is necessary, following radioprotection standards similar to those of patients admitted to nuclear medicine units for radiometabolic treatment. The patient is discharged once the radioactive source is removed.

As one of the problems of brachytherapy, also called curietherapy, is the possible unnecessary exposure of the patient and the health personnel who prepare, transport and handle the radioactive sources, a series of methods have been devised, such as the use of simulated non-radioactive sources to the calculation of its correct position in the patient, the use of remote control controls of the radioactive sources or the automatic withdrawal of the same to a protected place in the event of any incident.

Is radioactive waste produced in medical activities with isotopes?

As a consequence of the use and manipulation of non-encapsulated isotopes in nuclear medicine for the diagnosis and treatment of patients, a small amount of short-half-lived and low-concentration radioactive waste is produced, which, however, must be managed following all the criteria and legal regulations provided.

The residues from the doses administered and that are eliminated by hospitalized patients are liquid radioactive substances. Given their short half-life, in general, after a waiting period in protected deposits, they lose a large part of their activity, and can be discharged into the drainage network after dilution, using slow and controlled discharge systems.

Solid waste comes from used calibration sources, contaminated syringes, tubes and vials used in analytical techniques, as well as products contaminated by hospitalized patients, such as bedding, pajamas and other objects whose contamination will be previously checked. They must generally be stored until they lose their activity in containers with the appropriate shielding and only if this activity persists at measurable levels, will they be removed by the National Radioactive Waste Company (ENRESA) for definitive storage in suitable places.

Regarding the gaseous waste, vapors or radioactive particles in suspension that are generated, it must be taken into account that the workers of these radioactive facilities never exceed the permitted annual inhalation limits, using adequate ventilation systems. For the expulsion of contaminated air, the possible use of dilution media or filters should be considered, in order not to exceed the maximum permitted limits for the concentration of radioactive substances in the air.

In nuclear medicine services, considered by law as second-class radioactive facilities, radiological protection standards must be followed to avoid risks of external irradiation and contamination, both for patients and for staff working in the service. Likewise, a series of dosimetric controls of contamination of surfaces, places and people must be carried out with the appropriate periodicity and a series of actions must be planned in the event of an emergency or accident.

In radiotherapy services, solid waste is generated in the form of encapsulated sources (cobalt batteries, needles, wires or seeds of radioactive material) of very low volume but of medium activity. A record must be kept of the movements of each source, hermeticity tests and actions planned in the event of incidents or accidents. The withdrawal of the service sources will be carried out by the authorized company (ENRESA).

Are medical actions planned in the event of a nuclear catastrophe?

The Basic Nuclear Energy Plan (PLANBEN) provides, in the event of nuclear emergencies at radioactive plants or facilities, guidelines for action to prevent or reduce the effects of ionizing radiation on the population, which include the action of a health group with functions , personnel and clearly defined means, which would act in conjunction with the radiological and logistics group. Its missions would be the application of prophylactic measures, the planning, classification and treatment of casualties, both in the aspects of first aid and evacuation assistance (first level of action) and the treatment of irradiated or contaminated patients in special units (second level of action) located in previously authorized hospitals that have decontamination means, radioactivity controls, planned treatment protocols and radioprotected rooms assisted by multidisciplinary personnel who are experts in this type of care activity.