Isotopes and Accelerators PDF

Isotopes and their application in biology and medicine. Particle accelerators. The text under the slides was written by Andrea Dóczy-Bodnár 2019 This instructional material was prepared for the biophysics lect...

Isotopes and their application in biology and medicine. Particle accelerators. The text under the slides was written by Andrea Dóczy-Bodnár 2019 This instructional material was prepared for the biophysics lectures held by the Department of Biophysics and Cell Biology Faculty of Medicine University of Debrecen Hungary https://biophys.med.unideb.hu 2 The slide summarizes the main aims and topics of the lecture. Although stable isotopes will also be mentioned briefly, the major focus of the lecture is the use of radioactive isotopes in medicine and biology. Taking into account their widespread usage, it is not possible to discuss all the aspects in detail. Selected examples will be given to demonstrate the applications of radioactive isotopes in cell and molecular biology research, although it should be mentioned, that where it is possible, radioactive methods are replaced by other techniques. Due to their diagnostic and – along with other ionizing radiations – therapeutic importance, medical applications will be discussed in more detail. Many of the radioactive isotopes of medical importance as well as high energy radiations used for radiotherapy are generated by particle accelerators; therefore, the operation principles of linear accelerators and cyclotrons will also be mentioned. 3 The slide shows topics covered in previous biophysics lectures and other courses (including high school) that are necessary for understanding this lecture. Textbook chapters related to the material being discussed are also listed. 4 Application of stable isotopes is based on the fact that their mass is different from the naturally most abundant form of the same element. If in a molecule the naturally occurring isotope is replaced by an other stable isotope of the same element (isotope replacement), the (physico)chemical, spectral and physical properties of the molecule could also be changed, which can be detected with the appropriate method (e.g. spectrocopy, mass spectrometry, density gradient centrifugation, etc.). The lighter the given element, the greater the effect is, since the relative mass difference is more significant for elements having low atomic mass. E.g. for the two stable isotopes of hydrogen, 1H and 2H (deuterium) the relative mass difference is 100%. In the case of heavy elements the effect of a few neutron difference is negligible. The term “isotope effect” refers to the (physico)chemical and spectral effects of isotope replacement. (Although in many sources the term “isotope effect” is used in a more general sense, alterations in physical properties are also included.) Detailed discussion of the effects of isotope replacement exceeds the frames of the Biophysics course, only a few examples will be shown. 5 The so-called kinetic isotope effect is the alteration of the rate of chemical reactions depending on which isotope of a certain element is present in a molecule. In the case of the primary isotope effect the rate-limiting step of the chemical reaction involves the chemical bond the replaced isotope participates in (e.g. C-H and C-D). The different mass – among others – modulates the frequency of molecular vibrations: for bonds involving the heavier isotope frequencies are lower. Since these bonds are more stable (the energy of the particles is more negative), greater activation energy is required for the transitional state to be formed. As a result the rate of the chemical reaction will be lower as well. For hydrogen these considerations are valid not only for covalent bonds, but the H-bond as well: deuterium forms a stronger H-bond than protium (1H). E.g. Using deuterium replacement the rate of those enzyme reactions can be slowed down, where H-bonds or covalent bonds of hydrogen are broken; therefore, it can be used for studying the mechanism of enzyme reactions. 6 The Meselson–Stahl experiment proved the semiconservative replication mechanism of DNA (i.e. two copies of double-stranded DNA are formed: one strand is from the previous generation, whereas the other one is newly synthesized in both copies) in 1958. E. coli bacteria were cultered in a medium supplemented with 15N-isotope containing NH4Cl (ammonium-chloride) as a nitrogen source. In nature 14N is the most abundant isotope, therefore DNA consists of this isotope as well. If cells are cultured for longer time in the presence of 15N, it practically replaces 14N in DNA. Density of 15N-containing DNA is greater than that of 14N-containing DNA. The difference is only 1%, but using density gradient centrifugation, the “normal” and “heavy” DNA can be separated. A fraction of cells was then cultured in 14NH4Cl-containing medium. In samples drawn after the first cell division, intermediate density DNA (between the density of “normal” and “heavy” DNA) could be found only. This observation excluded the conservative mechanism of DNA replication (the entire DNA of the daughter cell is newly made), since according to that model the expected result would be 50-50% of „heavy” and „normal” DNA. After the 2nd division the ratio of intermediate and “normal” density DNA was 50-50%, which was in accordance with the semiconservative replication model, but excluded the mosaic model (after replication the DNA is made of old and newly made parts as well), since (in 50%) the form consisting 14N only also appeared. According to the mosaic modell homogenous density DNA is expected, with a density approaching that of the “normal” DNA after more cell divisions. 7 Urea (carbamide) breathing test is a fast diagnostic method which is used to examine Helicobacter pylori infection, responsible for gastric ulcer. The bacterium produces an enzyme with urease activity, which hydrolyzes urea to carbon dioxide and ammonia, therefore, locally neutralize the acidic pH of the stomach in order to promote survival of the bacterium. Carbon dioxide produced in the process is transported to the lungs via blood circulation and then is exhaled. If the patient is given urea, which consists of 13C instead of 12C, the most abundant isotope of carbon in nature, then based on the 13CO content of 2 exhaled air we can conclude the presence of H. pylori infection. 13C content of the exhaled air can be determined using mass spectrometry or infrared spectroscopy. (IR spectroscopy can be used to study the vibrational levels of molecules. Vibrational energy levels of the C-O bond will be different, depending on whether the “normal” 12C or the “heavy” 13C is present in the molecule.) The advantage of 13C is that this isotope is non-radioactive. It should be noted, however, that the radioactive version of the method can also be used: in this case 14C-labeled urea is used and the 14CO2 content is determined by activity measurements. (Reminder: 14C is a negative -decaying isotope.) 8 The slide lists the application fields of radioactive isotopes in biology and medicine. In the case of diagnostic imaging methods using radioactive isotopes we only discuss the major criteria of selecting the appropriate isotope; the methods themselves will be the discussed in detail in a later lecture. In the case of radiotherapeutic approaches the applications of ionizing radiations will be discussed in general independent of the radiation source (whether it is anisotope or not) (e.g. X-ray therapy, proton- and electron therapy). The usage of radioactive isotopes is either based on the detection of the emitted radiation or the ionizing/destructive effect of that radiation. 9 In the direct form of RIA two antibodies (the capture and detecting antibodies), both specific for the analyte (i.e. the molecule to be analyzed) are used, which recognize non-overlapping epitopes, therefore, can bind to the analyte at the same time. The steps of the method are the following: (1) The capture antibody is attached to the bottom of a dish. (2) The sample containing the analyte is added to the dish. After incubation for a certain time, the unbound sample is removed. (3) The detecting antibody labeled with a radioactive istope is added. This antibody will bind to the molecules trapped by the capture antibody in the previous step. The larger the amount of the analyte in the sample, the more is bound by the capture antibody. As a consequence, more detecting antibody can bind to it. After some incubation time the excess of detecting antibody is removed. (4) The radioactivity measured is directly proportional to the amount of analyte caught by the capture antibody, i.e. the concentration of the original sample. A possible variation of the method, when the detecting antibody is not radioactive. In this case after incubation with the detecting antibody (3/a.) a radioactively labeled secondary antibody is applied, which recognizes and binds to the detecting antibody (3/b.). After removal of the excess antibody the radioactivity is measured (4). Activity is determined by the number of bound secondary antibodies, which depends on the amount of detecting antibody, i.e. on the concentration of the analyte. 10 In the case of indirect or competitive RIA after the attachment of the capture antibody to the bottom of the dish (step #1) the mixture of the analyte-containing sample and the radioactive version of the same analyte in known amount is added (step #2). The capture antibody can bind both versions of the analyte, they are competing with each other for the binding sites. The higher the concentration of the analyte, the less radioactive version can be captured and vice versa. The measured radioactivity (step #3) therefore will be inversely proportional to the amount of analyte in the unknown sample. 11 In the case of radioactive tracing a radioactive element or the radioactive version of a biologically competent molecule (e.g. sugar, amino acid, DNA base, etc.) is added to a cell culture or a living organism. The radioactive element/molecule behaves in the same way as the non-radioactive counterpart, therefore, biological processes involving the given element/molecule can be followed by detecting the emitted radiation (e.g. following DNA synthesis by measuring incorporation of radioactive thymidine). The labeled biological or pharmacological molecules are called radioactive tracers (research) or radiopharmaceuticals (medicine). The method was pioneered by George (György) Hevesy, a Hungarian-born Nobel laurate (1943). The bottom part of the slide lists some commonly used radioactive tracers in biology and medicine. 12 Autoradiography is the detection of radioactive molecules on the basis of their emitted radiation. The method is frequently used in separation technigues (e.g. chromatography, electrophoresis). Ionizing radiation is recorded by a film or photosensitive detector placed on the blotting membrane or the chromatogram. Figure 1/a shows the 2D chromatogram of the (purified and digested) protein fraction of E. coli bacteria cultured in the presence of glucose and 14CO2. The cells synthesize amino acids from glucose and carbon-dioxide, which could be detected after separation by chromatography measuring the radioactivity of incorporated 14C. In the case Figure 1/b, the medium contained excess amount of non-radioactive threonine as well. Threonine could enter the cell, where it competes with the newly synthesized radioactive threonine; as a result, the spot of threonine is missing from the chromatogram. Isoleucine is also missing implying that synthesis of isoleucine is from threonine, not directly from glucose and CO2. Figure 2 shows examples for autoradiographic tracing of protein phosphorylation. In vivo measurements are performed in the presence of 32P-containing -ATP, whereas in vitro investigations use 32P-orthophosphate. In both cases radioactive phosphate group is added to the proteins upon phosphorylation. Proteins are separated by gel electrophoresis and then detecting radioactivity (black lanes in the figure) reveals which proteins were phosphorylated. Figure 2/A shows the result of an in vitro kinase assay, whereas Figure 2/B shows the phosphorylation pattern of proteins in cells treated with cholera toxin (CTX). 13 Autoradiography can be combined with light or electron microscopy. Samples labeled radioactively and prepared for microscopy are covered by a layer of photoemulsion or a film. Darkening evoked by radiation in the photosensitive substance can be localized within the cell using the microscopic image. As an example Palade’s classical experiment is shown, which proved the intracellular transport pathway of secreted proteins (1964). In this experiment 3H-containing leucine was injected into guinea pigs and their pancreas took it up for synthesizing secreted proteins. After a short time (a few minutes) the first injection was followed with the addition of excess amount of non-radioactive leucine. In certain time points some guinea pigs were sacrificed and their pancreas was removed. In the presence of radioactive leucine, the newly made proteins became radioactive as well; therefore, their intracellular localization could be identified combining autoradiography and electron microscopy. The excess amount of non-radioactive leucine given afterwards prevented incorporation of radioactive leucine into the newly synthesized proteins (the non-radioactive form had a higher chance to be incorporated): proteins made after the 2nd injection were non-radioactive and did not interfere with the observation of radioactive ones. In his experiment Palade proved that secreted proteins made in the rough ER are transported to the Golgi complex and then to the extracellular space by vesicles. The method used by Palade is called pulse-chase technique (for explanation see next slide). 14 The method used by Palade is called pulse-chase technique. The short labeling with the radioactive isotope (the molecule containing it) is called pulse. Subsequent addition of excess amount of the non-radioactive version of the same molecule is called chase. Proteins synthesized before injection of radioactive leucine are “invisible” for autoradiography. After injection of radioactive leucine, the newly made proteins incorporate 3H-leucine, which makes them „visible”. Addition of excess amount of non-labeled amino acid stops incorporation of radioactive leucine and so the further synthesis of radioactive proteins. As a result, when samples are drawn at different time points, only proteins made during the short incubation period with 3H-leucine will be detected. If we do not add non-radioactive leucine after a short time (only pulse), the cell would continue synthesizing radioactive proteins for a more prolonged period, which would make it impossible to follow the route of proteins within the cell. 15 This slide and the following one show the steps of Palade’s experiment in more detail. Since the major steps have already been discussed, the detailed description will be omitted here. 16 The samples were studied with transmission electron microscopy (TEM) combined with autoradiography. It should be noted, that samples were treated with osmium-tetroxide, which is the salt of a heavy element and binds to biological membranes; therefore, makes the recognition of membrane bound cellular compartments possible in the TEM images. 17 Medical imaging techniques based on the application of radioactive isotopes will be discussed later during the Biophysics course. In this lecture the major criteria which should be considered to select the proper isotope are summarized. The main aim of imaging techniques is to gain information from inside the body. Therefore only those isotopes are suitable where - or X-ray photons are generated in the decay process (see the table), since only these radiations are capable of reaching the detector outside the body. Accordingly, isotopes decaying with -emission, +-emission (here the two -photons formed in the annihilation process are detected) or K-capture are appropriate. As we could see previously, ionizing density of - and -particles are significantly greater than that of - or X-ray photons with the same energy and so their path is much shorter, i.e. these particles are not able to reach the detector. In addition, the radiation damage in the surrounding tissues is much greater as well. The physical half-life of the isotope is also an important factor: if it is too short, the examination cannot be performed. If it is too long, too large activity should be injected in order to get the number of decay events necessary for imaging. Isotopes with a physical half-life of a couple of hours (or occasionally a couple of days) are the most optimal. 18 (cont’d) The number of decays during a medical examination can be calculated as the product of the original amount of radioactive nuclei, the decay constant and the duration of the examination (Eq. 1). (This calculation is valid if the duration of the examination is significantly shorter than the physical half-life. Since an examination typically lasts for 20-30 minutes, for isotopes with a half-life of a couple of hourswe can use this simplified calculation). The N× product is called activity (unit of measurement: Becquerel, Bq); in practice the activity value is used to characterize the radioactive samples. Activity is the number of decays in one second. Taking into account the relationship between the decay constant and the half-life of the isotope (T1/2=ln2/), it can be seen that the longer the half-life, the greater the amount of radioactive isotope to be injected is in order to get the same number of decays (i.e. same activity). (The duration of the examination is limited; it cannot be increased infinitely.) The biological half-life of the isotope (the radipharmaceutical/carrier) is also critical, especially in the case of kinetic measurements (Eq. 2). It should match the time scale of the measurement. If the biological half-life is too short, the isotope is eliminated from the body before the measurement could be carried out. It should be noted that the physical half-lives of positron-emitting isotopes used in PET (positron emission tomography) are usually rather short, therefore special care is required when working with them (see PET lecture). 19 The slide explains the importance of physical half-life using two isotopes with significantly different half-lives: 99mTc, with an approximate half-life of 6h, and a hypothetical isotope with a half-life of 600 hours. Let’s suppose that 100 MBq activity should be injected in order to detect the appropriate number of decay events during the examination lasting for 22.5 minutes. Taking into account the definition of activity (see previous slide) and the relationship between the decay constant and the half-life, it can be shown that in order to get the same activity a 100× greater amount of isotope should be injected from isotope having a longer half- life than from 99mTc. Using the equation of radioactive decay, the number of decays during the examination can be calculated as follows: Ndecay=N0-N02-t/T, where the second term shows the number of undecayed nuclei after time “t”. The number of decays are the same for both isotopes (as it could be expected, since the same activity was injected), however for the longer half-life isotope only 0.04% of the nuclei disintegrated, i.e. practically the number of radioactive isotopes hardly changed due to the decay. In the case of Tc 4% of the original amount decayed during imaging, i.e. this fraction of the injected radioactivity contributed usefully to making the image. The isotope with the longer half-life will irradiate the patient for a longer time. 20 The slide shows examples for isotopes of medical importance. The most frequently used isotope is the metastable technetium-99 (99mTc). The generation of this isotope will be presented on the next slide. Isotopes used in PET will be mentioned in the lecture discussing this imaging technique. Supplementary information: Isotopes of iodine are mainly used in the diagnostics of thyroid gland and kidneys. Following oral administration iodine isotopes are encriched in the thyroid gland and then excreted via the kidneys. 131I is a negative -decaying isotope, where -photons used for imaging are emitted by the daughter nucleus (124Xe). Since - particles are harmful for the surrounding tissues, usage of 131I is mainly restricted to radiotherapy of thyroid, where imaging modality is necessary to check the efficiency of the therapy. For pure diagnostic purposes iodine-123 is used, which decays by electron capture to excited state tellurium-123. Excited state 123Te nucleus gets rid of the excess energy by emitting -photons, which could be detected by a gamma camera or SPECT instrument. (Iodine-123 is generated in cyclotrons by bombarding 124Xe with high energy protons.) Similar to 99mTc, 113mIn and 133mXe are metastable excited state nuclei („m” stands for metastable). In this case the excited state of the nucleus is significantly different from the ground state, therefore de-excitation (i.e. emission of - photons) takes place much slower, than for the non-metastable excited states. As a result, the half-life of metastable nuclei is significantly longer than non-metastable ones. 21 99mTc is the product of the --decay of 99Mo. It releases its excess energy by emitting - photons. The relatively long half-life of metastable 99Tc (6 hours) allows the separation of the 99mTc emitting pure -radiation from the undecayed 99Mo and its subsequent use in diagnostic imaging. The instrument where the isotope is generated is called “technetium generator”. Due to its relatively long half-life (66 hours) 99Mo can be stored for longer time. (99Mo is derived from Uran-235 by bombardment with neutrons.) What happens in the technetium generator? Molybdenum is used in the form of molybdate which is bound to Al2O3 columns. Upon the negative beta-decay pertechnate is produced, which has only a single negative charge contrary to the molybdate with two negative charges; therefore, it binds to the column less tightly and can be eluted (washed down) with appropriate salt solution. 22 Using radioactive isotopes, the volume of body compartments (e.g. blood plasma, total body water, ect.) can also be determined. In static measurements the volume is the only question, whereas dynamic (time-dependent) studies can be used for determining lifetime or kinetics as well. In the examinations isotopes with known activity are introduced to the body. After a certain time or – in dynamic measurements – at different, well-determined time points samples are drawn and their activity is measured. The slide lists the most important applications and the isotope used in them. The figure shows an example regarding determination of the volume of blood plasma. After injection the radioactive substance is distributed in the body according to its distribution volume, in our example in the blood. After a sample is drawn and its activity is measured the distribution volume can be calculated using a ratio pair, where the volume and activity of the sample as well as the injected, original activity is used. Supposing even distribution of the radioactive substance in the whole volume, the ratio of the activity of the sample to the injected activity is equal to the ratio of the sample volume to the total distribution volume. The equation used in the example is valid if the half-life of the isotope is much longer than the duration of the measurement, i.e. the number of decays is negligible. If this requirement is not fulfilled, the activity detected for the sample will be lower not only due to the distribution of the isotope, but because of the decays as well, therefore we get false results. 23 Radiotherapy is a primary or adjuvant (supplementary) treatment of tumors, where the pathological tissues are destroyed by ionizating radiation. The use of ionizing radiation in tumor therapy is based on the fact that tumor tissues are more radiosensitive than their healthy counterparts. The radiation source can be located either inside or outside the body, depending on the type of radiation used. (-)- and -decaying isotopes are used inside the body. As we could see previously, the characteristic range/penetration depth of (-)- and -particles is rather short (a few millimeters and 10-100 m in biological tissues, respectively), therefore, these isotopes can only be applied topically (locally). The graph shows the relative depth-dose curves of α- and β(-)-radiation having the same energy (the absorbed dose deposited by the radiation beam into a medium plotted against the penetration depth). For (-) particles the absorbed dose (therefore the damaging effect) is practically constant* until a given penetration depth is reached. At the same time, most of the energy of -particles is absorbed at the Bragg-peak, and as a consequence, the damaging effect is the greatest here. (*Supplementary information: As it was discussed previously, electrons can change their direction easily due to their small mass. Therefore, after entering the tissue, a fraction of electrons is scattered back leading to the slightly reduced absorbed dose at the beginning of the graph. ) 24 There are two major approaches used for selective targeting of tumors by radioisotopes applied internally: (1) Targeted radionuclide therapy: the radioisotope is delivered to the tumor by a carrier (e.g. antibody, receptor ligands, etc.), which recognizes and binds to tumor-specific target structures. Alternatively, the radioisotope may accumulate in cancer cells by some physiological mechanisms characteristic of neoplasia. These approaches has the potential to eliminate both primary tumors and metastasis simultaneously, including malignant cell populations undetectable by diagnostic imaging. Both β(-)- and α-decaying isotopes are used as radiation sources in targeted radionuclide therapy. Some examples, including the radionuclide used and the mechanism of selective targeting, are provided on the slide. (2) Brachytherapy: the radiation source is placed onto/into or near the tumor in a capsule (or other types of applicators). β(-)-emitting isotopes comprise one of the major groups of brachytherapeutic radiation sources. With a few exceptions, α-emitting isotopes are not used in brachytherapy due to their much more limited penetration depth. The slide includes the types of tumors where brachytherapy is most common, and some common radiation sources are also mentioned. The most common tumor types where brachytherapy is applied along with selected examples of radiation sources can be found on the slide. 25 The other major group of radiation sources in brachytherapy utilizes -radiation (accompanying other types of decay processes) or characteristic X-ray (generated in electron capture processes) to destroy the tumor. When designing therapy, it is important to consider that due to their much longer penetration depth, - or X-ray photons will reach other, “non- target” tissues as well. In addition, radiation exposure of the environment could be significantly higher than for β(-)- or α-emitting sources. 26 In most cases the radiation source is outside the body. The slide shows the basic principles of electron and proton radiation therapies and their characteristic depth-dose curves. Both electron and proton radiations are generated in accelerators (see later). The absorption mechanism of high energy electrons making up electron radiation is exactly the same as that of --particles. Energy deposit and – as a result – the damaging effect hardly changes after entering the body until a certain penetration depth. At the same time – contrary to --decay – the energy spectrum of electron radiation is discrete. Furthermore, the energy and so the penetration depth of the radiation can be adjusted by changing the accelerating voltage (see the graph). Linear accelerators used in the medical practice can accelerate electrons up to approx. 20 MeV, with a penetration depth approx. 7 cm. Accordingly, electron therapy is suitable for the treatment of near-surface tumours. The aborption mechanism of proton radiation is similar to that of -particles. However, similar to electron radiation, the energy and penetration depth of proton radiation can be adjusted. An important advantage of proton radiaton compared to electron radiation is the existence of Bragg-peak (see the graph); i.e. with proper adjustment of the parameters, it is possible to focus the absorbed dose on the tumor and spare the normal tissues along the path as much as possible. The disadvantage of this approach is that it is very costly. To reach the right depth (10-20 cm), protons need to be accelerated to several hundred MeV, which requires huge accelerators. 27 -radiation and high-energy X-ray are also used in radiotherapy, the source is outside the body. -radiation is produced using Co-60 isotopes, which decay emitting negative - particles to 60mNi isotopes. The metastable Ni-60 emits (in one or two steps, depending on the excited state the -decay led to) -photons to release excess energy. The energy of - photons is quantized, determined by the decay process. The depth-dose of -radiation decreases nearly exponentially with the penetration depth (top graph); therefore, in the case of tumors located at greater depth from the surface, the majority of radiation is absorbed by normal tissues preceding the tumor. Accordingly, selective irradiation of tumors is not possible with a single radiation source. High energy X-ray beams (higher energies than -energies produced in Co-60 therapy!) are generated in X-ray tubes or accelerators. The energy spectrum is continous, the lower energy photons are filtered out. Its energy and so the penetration depth can be adjusted by changing the accelerating voltage. Similar to - radiation, the depth-dose decreases with the penetration depth nearly exponentially (bottom graph). Due to the secondary electrons (- and X-ray photons are indirectly ionizing radiations!), the maximal absorbed dose is not right at the body surface, but somewhat deeper in the tissue. The greater the energy of radiation, the deeper the position of the maximum absorbed dose is in the body. This can be exploited to protect the skin and to some extent the underlying tissues from radiation damage. Energies needed for the irradiation of 28 tumors located at greater depths are so high that they cannot be ensured by Co-therapy, where the maximal dose is close to the body surface. Therefore, high energy X-rays are more often applied. 29 As we could see, selective irradiation of tumors with a single source is not possible for - radiation. Due to the exponential decrease of the depth-dose, the majority of the radiation is absorbed by normal tissues preceding the tumor, therefore these tissues will also be damaged. The radiation damage of normal tissues can be even higher than that of the tumor, since the tumor gets a significantly decreased radiation dose. The solution is to apply irradiation from more directions. Although the intensity (and therefore the absorbed dose) of the individual beams decreases significantly by the time they reach the tumor, their effect is added up, therefore the actual dose deposited in the tumor will be much higher despite the attenuated intensity of the individual beams. The graphs demonstrate that increasing the number of directions, the actual dose in the center (location of the tumor) is getting higher as well. Above a certain number of directions, the depth-dose will be maximal at the location of the tumor. (See the last graph, where the irradiation was made from 8 different directions.) In the case of high energy X-ray the location of the maximal depth-dose can be adjusted by changing the energy of radiation, therefore tumors can be targeted without significant damage of the preceding tissues. However, tissues behind the tumor can still be damaged; therefore, irradiation from more directions is also applied in X-ray therapy to ensure more precise treatment with fewer side effects. 30 1. Rotating irradiation: During this procedure the radiation source is rotated around an axis that passes through the target tissue, so despite the actual direction of the radiation source, the target is always irradiated. Healthy tissues are irradiated only for a short period of time, i.e. only while they are in the path of radiation. The figure shows the typical design of the device. The irradiation head can contain a whole linear accelerator as well. In the lateral direction collimators are used to reduce the width of the beam, therefore the aspecific radiation damage is reduced. The operation principles of collimators will be mentioned in the lecture discussing -camera and SPECT. 2. Irradiation from different directions with more radiation sources: In the case of gamma- knife instead of rotating a single radiation source, multiple radiation sources are applied from different directions simultaneously. Approx. 200 sources (usually 60Co isotope) are placed on the surface of a hemisphere. The beams are directed to a common point, the center of the sphere, where the tumor should be located. The patient has a special frame fixed to his/her head which helps in targeting. The movement of the patient together with the treatment table is controlled remotely. Due to its construction, the gamma-knife is suitable for the treatment of brain tumors. The individual beams are weak, therefore do not damage healthy tissues. The dose is powerful enough to destroy the target positioned in the intersection of the beams. 31 High energy radiations (e.g. proton-, electron- and other particle radiations, high energy X- ray) for radiotherapy and many of the isotopes of medical importance (e.g. β+-decaying isotopes for PET) are generated in particle accelerators, where high kinetic energy („fast”) charged particles are produced using electric field. In the case of linear accelerators, the path of the particle is a straight line, whereas for circular or cyclic ones it is circular. Here cyclotrons as typical examples of cyclic accelerators will be discussed in more details. The principle of acceleration is the same in both types of accelerators and identical with the principles discussed previously (e.g. electron microscopy, X-ray tube): the charged particle is accelerated in an electric field by the electric force. Direction of the force/acceleration depends on the charge of the particle (1). Kinetic energy and so the speed are determined by the applied voltage and the charge (2). In circular accelerators a magnetic field is also applied. As it was discussed previously (see mass spectrometry), if a moving, charged particle enters a magnetic field, it is forced to a circular path by the magnetic Lorentz force (3a and 3b). Although the particle is accelerated in the magnetic field as well, since only the direction of its velocity vector changes, the kinetic energy/speed remains constant. The magnetic Lorentz force is the product of centripetal acceleration and the mass of the particle (4) (Newton’s 2nd law). Rearranging the equation (4 and 5) we can see that the angular frequency () is independent of the radius of the circle. The siginificance of this will be discussed later. 32 The slide explains the basic operation principles of linear accelerators and the steps of the acceleration process using the proton as an example. The electrodes are hollow cylinders placed after each other in a straight line. Electric field is present only between the ion source and the first electrode and between the subsequent electrodes. Inside the electrodes there is no electric field, therefore, the kinetic energy of the particle does not increase here, acceleration only takes place between the electrodes. The polarity of the field applied between the electrodes changes when the particle is inside, therefore when the particle leaves an electrode it can continue its motion toward the next one. The accelerating voltage is constant, therefore the extent of kinetic energy gained by the particle between two subsequent electrodes is always the same. The overall kinetic energy gained by the particle is determined by the number of acceleration steps (see the equation). Since the kinetic energy and therefore the speed of the particle increases continually in the subsequent acceleration steps, the length of the electrodes should be increased along the tube so that the particle leaves the actual electrode in synchrony with the polarity change of the electric field. (This way the electric generator can operate at constant frequency.) The electrodes are in vacuum, since collisions with air particles would make the acceleration process impossible. 33 An example of linear accelerators used in the medical practice are shown on the slide. Using this instrument, electrons could be accelerated e.g. for the generation of high energy X-ray used in radiotherapy. 34 In physics research much greater energies are required than in medicine, therefore, linear accelerators are significantly longer. Depending on the accelerated particle and the required energy, these accelerators could be even several kilometer long. 35 Electrodes in cyclotrons are D-shaped (also called “dees”). As in linear accelerators the electric field is present only in the space between the electrodes, but not inside them. Inside the electrodes a magnetic field is applied. Particles arriving at the center of the cyclotron are accelerated toward the appropriate electrode, i.e. gain kinetic energy due to the electric field. The particle then enters the electrode perpendicular to the direction of the magnetic field and is forced to a circular path by the magnetic field. Since there is no electric field inside the electrode, the speed does not increase further. When the particle reenters the space between the electrodes, the polarity of the electric field changes, so the particle is accelerated toward the other electrode and enters it. Due to the newer acceleration, its speed and so the radius of its path will be greater. (The overall path travelled by the particle after several acceleration steps looks like a spiral.) As it was shown previously, the angular frequency and so the period of circular motion (see Eq.) are independent of the radius (i.e. the actual speed of the particle). As a consequence, the particle stays in the electrode for the same duration, regardless the actual acceleration step; i.e. the electric generator controlling the polarity change of the electric field can operate at constant frequency. 36 (cont’d) This consideration is valid only if the particle’s mass is constant throughout the acceleration process. At high energies, however, the period increases due to the relativistic mass increase. As a result, synchronization between the frequency of the particle and that of the electric generator is abolished, the particle cannot be accelerated further. To overcome the problem, either the frequency of the electric generator or the magnetic field strength is changed. The former is performed in synchro-cyclotrons, while the latter option is used in synchrotons. 37 The slide shows the steps of acceleration in a cyclotron using a proton as an example. The slide contains all the necessary information. In addition, the principles were already discussed (see previous slide), therefore no further explanation is added here. The kinetic energy gained by the particle is determined by the number of acceleration cycles (see equation), as it was already shown for linear accelerators. The accelerating voltage is constant; therefore, between the two electrodes the kinetic energy of the particle is increased with the same amount of energy in all the acceleration steps. 38 The first image shows the cyclotron constructed by Ernest O. Lawrence who invented and patented the first cyclotron (1932). The other two photos show a cyclotron magnet and a cyclotron used in medical practice, respectively. 39 40