Beta Radiation
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《肿瘤学家》
LEARNING OBJECTIVES
After completing this course, the reader will be able to:
Describe the basic science behind radioactive beta decay.
Discuss the common clinically useful beta-emitting radioisotopes.
Explain why some beta emitters are better suited than others for certain specific clinical indications.
Shortly after Becquerel discovered radioactivity in 1896, Ernst Rutherford categorized the emitted radiation by the depth of penetration—alpha, beta, and gamma in increasing order of penetration. We now know that alpha radiation is really a stream of positively charged particles that are in fact helium nuclei and that gamma radiation is high-energy electromagnetic radiation (i.e., photons) emanating from atomic nuclei. Beta radiation is a result of the ejection of charged particles from radioactive nuclei, and the charged particles are in fact electrons. The ejected electrons can be conventional negatively charged electrons ("beta minus") or, in the case of "beta plus" decay, positively charged electrons, that is, positrons. Beta plus decay is becoming increasingly more important in oncology through positron-emission tomography (PET) imaging. When a positron, as the antimatter counterpart of the ordinary electron, encounters an electron, mutual annihilation occurs with the complete conversion of all mass into energy. The energy is released in the form of two photons of similar energy traveling 180 degrees apart. These easily detected gamma photons form the basis of PET imaging. Rather than x-ray photons from an external source traveling through the patient to form an image, as in computerized tomography, the image-generating photons in PET come from within the patient. Tumors, with their relatively higher metabolism, tend to accumulate the administered fluorodeoxyglucose (FDG) that is tagged with the positron-emitting fluorine-18 radioisotope, providing the selectivity useful in oncology.
Beta decay occurs when the atomic nucleus has an "imbalance" between the number of protons and neutrons required for optimal stability. If there are too many protons, there is excess positive charge, and the nucleus attempts to attain greater stability by either emitting a positron through beta plus decay or capturing an orbiting electron and combining this with a proton, thereby neutralizing it and creating a neutron. When there is a deficiency of protons (i.e., not enough positive charge), the nucleus can gain greater stability by emitting an electron via beta minus decay, thereby converting a neutron into a proton. Initial investigations suggested that beta decay violated the laws of conservation of energy and momentum because the emitted beta particles do not always come out with a discrete energy and momentum, as is the case with alpha and gamma emission. Rather than abandon these cherished laws of nature, physicists proposed that another undetected particle accompanies emission of the beta particle to conserve energy and momentum. The hypothetical particle would carry the energy and momentum that was "missing" from the ejected electron. Years after the proposal, these so-called neutrinos were detected, just as predicted by the laws of conservation of energy and momentum. In keeping with another law of physics (the law of conservation of lepton number), beta minus decay involves the emission of an antineutrino, whereas beta plus decay involves the emission of an ordinary neutrino.
In clinical oncology, beta radiation is becoming more important thanks to effective treatments using beta-emitting isotopes such as iodine 131 (131I), yttrium 90 (90Y), samarium 153 (153Sm), strontium 89(89Sr), and phosphorus 32(32P), among others. The recent success of radioimmunotherapy for non-Hodgkin’s lymphoma comes in the form of Y-90 ibritumomab tiuxetan (Zevalin®; Biogen Idec Inc., Cambridge, MA) and I-131 tositumomab (Bexxar®; GlaxoSmithKline, Philadelphia), which owe their main therapeutic effects to the attached beta-emitting radioisotopes. Whereas 90Y is a pure beta emitter, 131I also emits gamma radiation, permitting easier scintigraphic imaging. 153Sm, similarly, is both a beta and gamma emitter, allowing effective scintigraphy, while 89Sr and 32P, like 90Y, are pure beta emitters. In clinical practice, the most important determinant of effectiveness is the targeting of the radioisotope, but other factors to keep in mind include the energy of the beta radiation (which determines the range of the particles and depth of penetration) and the half-life of the isotope (which relates to the dose rate of the radiation). Both of these factors impact overall efficacy and toxicity (e.g., myelosuppression) and are important to consider when contemplating therapy with beta-emitting radioisotopes.
DISCLOSURE OF POTENTIAL CONFLICTS OF INTEREST
The author indicates no potential conflicts of interest.(James S. Welsh)
After completing this course, the reader will be able to:
Describe the basic science behind radioactive beta decay.
Discuss the common clinically useful beta-emitting radioisotopes.
Explain why some beta emitters are better suited than others for certain specific clinical indications.
Shortly after Becquerel discovered radioactivity in 1896, Ernst Rutherford categorized the emitted radiation by the depth of penetration—alpha, beta, and gamma in increasing order of penetration. We now know that alpha radiation is really a stream of positively charged particles that are in fact helium nuclei and that gamma radiation is high-energy electromagnetic radiation (i.e., photons) emanating from atomic nuclei. Beta radiation is a result of the ejection of charged particles from radioactive nuclei, and the charged particles are in fact electrons. The ejected electrons can be conventional negatively charged electrons ("beta minus") or, in the case of "beta plus" decay, positively charged electrons, that is, positrons. Beta plus decay is becoming increasingly more important in oncology through positron-emission tomography (PET) imaging. When a positron, as the antimatter counterpart of the ordinary electron, encounters an electron, mutual annihilation occurs with the complete conversion of all mass into energy. The energy is released in the form of two photons of similar energy traveling 180 degrees apart. These easily detected gamma photons form the basis of PET imaging. Rather than x-ray photons from an external source traveling through the patient to form an image, as in computerized tomography, the image-generating photons in PET come from within the patient. Tumors, with their relatively higher metabolism, tend to accumulate the administered fluorodeoxyglucose (FDG) that is tagged with the positron-emitting fluorine-18 radioisotope, providing the selectivity useful in oncology.
Beta decay occurs when the atomic nucleus has an "imbalance" between the number of protons and neutrons required for optimal stability. If there are too many protons, there is excess positive charge, and the nucleus attempts to attain greater stability by either emitting a positron through beta plus decay or capturing an orbiting electron and combining this with a proton, thereby neutralizing it and creating a neutron. When there is a deficiency of protons (i.e., not enough positive charge), the nucleus can gain greater stability by emitting an electron via beta minus decay, thereby converting a neutron into a proton. Initial investigations suggested that beta decay violated the laws of conservation of energy and momentum because the emitted beta particles do not always come out with a discrete energy and momentum, as is the case with alpha and gamma emission. Rather than abandon these cherished laws of nature, physicists proposed that another undetected particle accompanies emission of the beta particle to conserve energy and momentum. The hypothetical particle would carry the energy and momentum that was "missing" from the ejected electron. Years after the proposal, these so-called neutrinos were detected, just as predicted by the laws of conservation of energy and momentum. In keeping with another law of physics (the law of conservation of lepton number), beta minus decay involves the emission of an antineutrino, whereas beta plus decay involves the emission of an ordinary neutrino.
In clinical oncology, beta radiation is becoming more important thanks to effective treatments using beta-emitting isotopes such as iodine 131 (131I), yttrium 90 (90Y), samarium 153 (153Sm), strontium 89(89Sr), and phosphorus 32(32P), among others. The recent success of radioimmunotherapy for non-Hodgkin’s lymphoma comes in the form of Y-90 ibritumomab tiuxetan (Zevalin®; Biogen Idec Inc., Cambridge, MA) and I-131 tositumomab (Bexxar®; GlaxoSmithKline, Philadelphia), which owe their main therapeutic effects to the attached beta-emitting radioisotopes. Whereas 90Y is a pure beta emitter, 131I also emits gamma radiation, permitting easier scintigraphic imaging. 153Sm, similarly, is both a beta and gamma emitter, allowing effective scintigraphy, while 89Sr and 32P, like 90Y, are pure beta emitters. In clinical practice, the most important determinant of effectiveness is the targeting of the radioisotope, but other factors to keep in mind include the energy of the beta radiation (which determines the range of the particles and depth of penetration) and the half-life of the isotope (which relates to the dose rate of the radiation). Both of these factors impact overall efficacy and toxicity (e.g., myelosuppression) and are important to consider when contemplating therapy with beta-emitting radioisotopes.
DISCLOSURE OF POTENTIAL CONFLICTS OF INTEREST
The author indicates no potential conflicts of interest.(James S. Welsh)