Lecture
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located directly beneath the superficial layers of the skin; thereafter the dose value drops fairly quickly (1 cm of soft tissue attenuates cobalt gamma rays by 5%).
Indications for remote (teletherapy) gamma-therapy:
S For radical, palliative and symptomatic treatment of tumors of internal organs.
1000 MeV. Unlike photon ionizing radiation, with proton irradiation the maximum ionization (maximum absorbed dose) is located at the end of the particle path (Bragg peak). Proton irradiation is used for RT of small intracranial lesions, as well as for treating radioresistant tumors of small diameter. Proton beams make it possible to simultaneously irradiate strictly limited tissue volumes with doses of 100-200 Gy. Neutron irradiation.
Neutron radiation Processes of interaction between neutrons and matter are determined both by the neutron energy and by the atomic composition of the absorbing medium. The absence of an electric charge in neutrons allows them to penetrate through the electron shells of atoms and approach the nucleus freely.
Sources of neutrons:
bombardment of deuterium;
mixtures of alpha emitters with beryllium or boron:
He + Be = 13C → 12C + n.
When acting on tissues, neutrons are captured by atomic nuclei, which leads to disruption of their structure and is accompanied by the emission of alpha or beta particles and gamma quanta. In addition, nuclear transformations release recoil nuclei, which possess high energy and produce intense ionization of the medium. Their ionizing capacity is close to that of alpha particles. However, the damaging effect of neutrons is significantly greater due to their high penetrating power. When irradiated with neutrons, a cell undergoes a simultaneous rupture of its DNA, which leads to its death. Since not only tumor cells but also healthy cells die, a high percentage of radiation damage is characteristic of neutrons. Of all types of ionizing radiation, fast neutrons pose the greatest radiation hazard. Fast neutrons are better slowed down by the nuclei of light elements (water, paraffin, adipose tissue). Consequently, the absorbed dose turns out to be greater in adipose tissue, which leads to radiation damage.
High penetrating power opens up prospects for the use of neutrons in the radiation therapy of malignant neoplasms.
When addressing issues of protection against neutron radiation, the specifics of its interaction with matter must be taken into account. Fast neutrons must be slowed down. Light nuclei (water, paraffin) are used for this purpose. Slow neutrons are then absorbed as a result of radiative capture in materials made of boron or cadmium. Since the capture process is accompanied by the emission of a gamma quantum, lead must be used as a shielding material. Thus, protection against neutrons is a complex construction.
Neutron capture therapy. The method was first proposed by Locher in 1936. This method uses a flux of slow neutrons obtained from nuclear reactors. For differentiated irradiation with a maximum effect in the tumor and a minimum in normal tissues, it is necessary to saturate the tumor with elements characterized by a large cross-section for capturing slow neutrons. Such elements are boron (10B) and lithium (6Li). However, these elements do not possess tumor tropism. To enrich the tumor with them, the different diffusion rates from the bloodstream into tissues are used (i.e., these elements pass slowly from the blood into the brain, but enter tumor tissue considerably faster). It has been established that 30 minutes after intravenous administration of boron compounds, its concentration in a brain tumor is 4-5 times higher than in normal tissue. And it is precisely at this time that irradiation should be carried out. The concentration of boron and lithium in muscle tissue is very high, and therefore neutron capture therapy cannot be used for tumors of the trunk and limbs. This method is applicable only for brain tumors.
It is carried out in 31 centers around the world that have neutron generators. It is used for RT of radioresistant tumors, bone sarcoma, and soft tissue sarcoma. The therapeutic effect is achieved only at the cost of radiation damage.
Remote (teletherapy) RT can be carried out in static and moving modes.
In static irradiation, the radiation source is fixed motionless relative to the patient. Shielding blocks and lead grids are used to change the field of action of the radiation beam.
In the moving method of irradiation, the radiation source moves along an arc relative to the patient's body. There are: S circular irradiation (rotation angle of 360°); s pendular, or sector, irradiation (swing angle less than
360°).
Indications for these types of moving irradiation are small tumors located in the area of the central and sagittal plane of the patient's body (i.e., deeply located). These include tumors of the head and neck, bronchopulmonary lymph nodes, tumors of the esophagus, rectum, bladder, etc.
-S Eccentric (shawl-shaped) irradiation - the swing radius
+forms a certain deviation angle with the central beam. It is used, for example, in the treatment of metastatic lesions in the ribs, in irradiation of the spleen, i.e., organs located close to the surface of the patient's body.
Contact methods of irradiation are RT techniques in which the source of ionizing radiation is located at a distance of less than 30 cm from the irradiated object. The following types of contact RT are distinguished:
-S application RT;
S intracavitary irradiation;
•S interstitial RT.
The main feature of the dose field in all contact RT methods is the rapid drop in dose rate with increasing distance from the source, already within the first centimeter, which makes it possible to create a high radiation dose in the pathological focus with a steep drop in dose rate beyond its boundaries. This feature is an advantage of the method, since the normal tissues surrounding the tumor are subjected to minimal irradiation.
In application RT sources of ionizing radiation are placed directly on the surface of the patient's body without disrupting tissue integrity. The radiation source is a radiating surface of various shapes, sizes and curvature. Currently, beta-applicators containing Sr90 and Y90 are used (for treating ophthalmological diseases); gamma-applicators contain Co60 preparations and are special mask-molds that model the shape of the irradiated area (for treating superficially located neoplasms: skin cancer, lip cancer, recurrences of breast cancer, etc.). Application RT is carried out over 5-10 days, with daily procedures lasting several hours.
Intracavitary irradiation is performed by inserting the radiation source into natural cavities (oral cavity, uterus, esophagus, rectum) or artificially formed ones (postoperative wound, etc.). Originally, in practice, an applicator, usually already loaded with radioactive sources, was placed on the patient. This led to irradiation of the staff during this procedure; in their haste, the sources were positioned worse than they should have been. This technique has now been replaced by the sequential insertion method (afterloading), in which an empty holder or guide channel for the sources is first inserted into the patient's body, after which their position is checked radiographically. Only after confirming that the position is correct is the patient moved to an isolated room with the necessary shielding, where the radioactive sources are then inserted. For performing intracavitary RT, there is a series of remote afterloading (hose-type) devices of various designs that make it possible to position the sources near the tumor in an automated manner and carry out targeted irradiation.
Interstitial RT. In addition to inserting closed radioactive sources into the patient's body cavities, needles, granules and wires containing radioactive sources can be inserted directly into tumors or placed on tumor surfaces. They are arranged according to geometric schemes calculated so that the target volume is irradiated comparatively uniformly. It is possible to thread the tumor with radioactive nylon threads carrying emitting granules (Co60), or tantalum wire; injection implantation of colloidal radionuclide solutions (Au198) is also used. In interstitial RT the radiation source remains in the tumor or in the patient's body tissues throughout the entire treatment process. In internal irradiation, organotropic radionuclides or labeled compounds, which are selectively absorbed by the tumor or other pathologically altered tissues, are administered orally, intramuscularly or intravenously.
+All of the listed RT methods are used in three main functional units of the radiology departments of an oncology dispensary: for remote (teletherapy) RT, for working with closed radiation sources, and for working with open liquid radionuclides. Each of these units has its own specifics of work, shielding, and patient care, as well as special equipment and apparatus.
Different tumors react differently to irradiation, since they have different histological natures and degrees of cell differentiation, and contain different amounts of oxygen and actively proliferating cells at different stages of the mitotic cycle. It is essentially these parameters that determine the radiosensitivity of a tumor, which is undoubtedly taken into account when deciding on individual indications for RT.
Upon irradiation, suppression of cell division is observed in the tumor. As the dose increases, an ever greater number of cells lose their ability to reproduce. The number of pathological mitoses initially increases. However, cells that continue to reproduce die after a number of divisions as a result of chromosomal aberrations (structural rearrangements of the cell nucleus's chromosomes) and gene mutations associated with damage to nucleoproteins and DNA, which exercises primary control over all the most important processes of the organism's vital activity and serves as the keeper of hereditary information.
At the same time, there is a proliferation of granulation tissue rich in capillaries, consisting of epithelioid and lymphatic cells, histiocytes, plasma cells and fibroblasts. As the granulation tissue grows, the mass of tumor cells is broken up into separate islets and noticeably diminishes. Under the influence of irradiation, changes occur in the blood vessels of the tumor itself and of the surrounding tissues. These are expressed as endophlebitis and proliferating endarteritis with hyalinization of the arteriolar walls. Obliteration of small vessels disrupts the nutrition of the tumor, which leads to its dystrophy. With a sufficient dose, the death of tumor cells is completed and the granulation tissue is transformed into scar tissue.
Different parts of the same tumor also react differently to irradiation. Three zones of differing radiosensitivity can be distinguished within a tumor: a zone of active proliferation near the vascular spaces (the most sensitive), a zone with low proliferative activity (less sensitive), and a radioresistant zone of spontaneous necrosis. In addition, there is a difference in the radiosensitivity of cells that are in different phases of mitosis or unequally saturated with oxygen.
Thus, tumor regression under the influence of RT occurs due to:
1) the direct death of the most radiosensitive tumor cells (interphase death). The number of such cells is small;
2) disruption of the reproduction processes (mitotic death). The time of tumor regression is related to the duration of the mitotic cycle of this type of tumor cell and the number of generations that developed before reproduction completely stopped;
3) the reaction of the surrounding normal tissues (decreased vascularization of the tumor, proliferation of connective tissue and
etc.).
The difference in radiosensitivity between a malignant tumor and the tissue surrounding it is defined as the therapeutic radiosensitivity interval - the radiotherapeutic interval. The greater the radiotherapeutic interval, the easier it is to achieve destruction of the tumor elements while preserving the viability of the surrounding tissues. The radiotherapeutic interval can be increased by changing the irradiation conditions (varying the dose, rhythm and time of irradiation), the degree of tissue oxygen saturation, by introducing various chemical compounds into the patient's body, etc.
The use of RT in the treatment of malignant tumors is indicated and can be successful when there is a real possibility of irradiating the entire zone of spread of the tumor process - the primary tumor, the zone of subclinical dissemination, and the area of regional metastasis. Partial irradiation of a tumor accelerates the growth of its unirradiated part and of its metastases.
Methods of modifying radiosensitivity
Various methods of modifying tissue radiosensitivity are used to increase the effectiveness of RT. The following techniques for increasing the radiosensitivity of tumors are currently used: S Oxygen therapy (use of an oxygen mask during the
irradiation session)
s Oxygen-radiotherapy (irradiation in a hyperbaric chamber) S Hypoxic radiotherapy (inhalation of a hypoxic gas mixture) s Thermoradiotherapy (use of microwave (SHF) or administration of pyrogenal) s Hyperglycemia (administration of large doses of glucose) s Polyradiomodification (combination of various techniques).
Types of radiation treatment
S Radical - cure (ionizing radiation acts on the primary tumor and on the presumed zones of lymphogenous metastasis).
s Palliative - prolongation of life (to halt tumor growth, reduce its size).
s Symptomatic - elimination of individual symptoms aggravating the patient's condition (pain, superior vena cava compression syndrome, etc.).
RT of malignant tumors can be used as an independent method of treatment or be one of the stages of combined treatment. Irradiation can be combined with surgery, chemotherapy and hormone therapy. Combined RT is a combination of two or more RT methods (remote gamma-therapy + intracavitary therapy, etc.).
Radiation therapy in combination with surgical intervention is used in three different variants:
1. Preoperative RT, i.e., carried out before surgery.
Tasks: ^ destruction of the most radiosensitive cells and reduction of the
viability of the remaining tumor elements; ^ elimination of inflammatory phenomena in the tumor and around it;
obliteration of small vessels, leading to reduced vas1cularization of the stroma and, consequently, to a reduced risk of dissemination of tumor cells, as well as the conversion of
tumors that are on the verge of operability into an operable state.
They use remote irradiation (remote
gamma-therapy or high-energy bremsstrahlung radiation),
intracavitary gamma-therapy and combined irradiation. Fractionation regimens:
S standard fractionation. Single focal dose - 2 Gy, 5 fractions per week, total dose - 45-50 Gy. Surgical treatment 2-3 weeks later (after radiation reactions subside);
s large fractionation. Single focal dose - 4-10 Gy, number of fractions - from 1 to 5, total dose - 20-30 Gy. Indication - confidence in the operability of the tumor (if not - standard fractionation). Surgery should be planned no later than 72 hours after the end of RT (since the effect on tumor cells is sublethal);
S use of large daily fractions in a multi-fractionation regimen. Single focal dose - 2 Gy twice a day, total dose - 20-24 Gy.
2. Intraoperative irradiation (irradiation into the wound). It is used, in particular, in the treatment of pancreatic tumors. During the operation, a dose of about 15-20 Gy is delivered in a single session to the tumor bed (the use of fast electrons is preferable). This is followed by postoperative remote RT up to the required total dose. Intraoperative interstitial RT may also be used.
3. Postoperative RT. Its purpose is to consolidate the effect of surgical treatment and reduce the risk of local or regional recurrence and distant metastases. Remote irradiation is usually used (gamma-therapy, high-energy bremsstrahlung radiation, fast electrons).
Techniques: S standard fractionation. Single focal dose - 2 Gy,
total dose - 50 Gy; S multi-fractionation regimen. Single focal dose - 1.2 Gy twice a day (4-hour interval).
Indications for RT of tumor diseases
The indication for RT is the presence of a histologically verified malignant tumor (cytological verification is sometimes possible). RT is used as an independent method of treatment or in various combinations with other treatment methods for treating radiosensitive tumors of practically all locations. In 60% of cases it is used with radical intent, and in 40% with palliative intent.
Contraindications to RT of malignant tumors
Sharp weakening of the body's resistance (cancer
cachexia)
Radiation sickness s Severe decompensated diseases of the cardiovascular,
respiratory systems, liver and kidneys s Mental illnesses s Tuberculosis
At present, two ways of increasing the effectiveness of radiation therapy have emerged. First of all, this is the continuing improvement of radiotherapeutic equipment. In the 1950s, X-ray therapy machines were replaced by machines for remote gamma-therapy, which has by now reached a high degree of sophistication.
However, the availability of only one type of radiation - gamma quanta with an energy of about 1.25 MeV, the need for periodic replacement of radionuclides, and the radiation hazard to personnel working with radionuclides dictate the need to introduce new equipment. The most acceptable and accessible devices for widespread use at the current level of equipment manufacturing are electron accelerators, which make it possible to a much greater extent to realize the basic principle of radiation therapy: to concentrate the dose in the pathological focus as much as possible while minimizing the dose to the surrounding normal tissues. In the future, wider introduction of proton accelerators and neutron generators is also possible.
At the same time, the technology of contact intracavitary and interstitial irradiation is being improved through the creation of a series of devices with sequential automated insertion of guide wires and radiation sources, which can be moved during the irradiation session to form an individual dose field.
In addition, special X-ray equipment (simulators), computed tomography scanners and planning stations for pre-irradiation topometry have been developed. This makes it possible to precisely determine the boundaries of the target to be irradiated and to create optimal irradiation programs, which places radiation therapy among the high-precision disciplines.
+The second way of increasing the effectiveness of radiation therapy is associated with the introduction into clinical practice of the achievements of modern radiobiology, thanks to which it has become possible to control the radiosensitivity of tumor and normal tissues. The use of radiomodifying agents has begun, i.e., various physical and chemical factors capable of weakening the radiation susceptibility of normal tissues or enhancing the radiosensitivity of the tumor. In addition, mathematical models of the optimal irradiation rhythm are being developed for tumors with different biological characteristics.-
Часть 1 6. Devices and systems for exposure to X-ray and radioisotope radiation.
Часть 2 Contact methods of irradiation - 6. Devices and systems for
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