Physics of Nuclear Medicine and Radiotherapy PDF

College of Medicine/Babylon University Dr.Entidhar j.Khamees physics of nuclear medicine and Physics of Radiotherapy Radioactivity Radioactivity is the property of some natural elements that have unstable nuclei which disintegrate to emit various rays (alpha [α],...

College of Medicine/Babylon University Dr.Entidhar j.Khamees physics of nuclear medicine and Physics of Radiotherapy Radioactivity Radioactivity is the property of some natural elements that have unstable nuclei which disintegrate to emit various rays (alpha [α], beta [β], or gamma [γ]).  The unit of radioactivity is the curie (Ci) which equal to 3.7 x 10 10 disintegrations per second. The (SI) unit of radioactivity is the Becquerel (Bq) which defined as one disintegration per second.  Radioactive element decays at a fixed rate commonly indicated by the half-life (𝑇1/2 ) which is the time needed for the half of the radioactive nuclei to decay.  Alpha particles are nuclei of helium atom, positively charged and can stop in few centimetres of air.  Beta particles are negatively charged and more penetrating than alpha particles and can be stopped by few millimetres of tissue.  Gamma-rays are very penetrating photons identical to X-rays photons but have much higher energies than X-rays used in diagnostic radiology.  Isotopes are the nuclei of a given element with different numbers of neutrons.  Radioisotopes are the radioactive isotopes of an element.  Nuclear medicine involves the administration of trace amounts of radioactive materials (radionuclides) that are used to provide diagnostic information in a wide range of disease. Radiopharmaceuticals Radionuclides used in nuclear medicine are called radiopharmaceuticals and emit either gamma- ray or positron when decay. Positrons (positive beta) are emitted by the disintegration of some man-made radionuclides. Therefore, the short half-lives of the most commonly used positron- emitting radionuclides require an onsite accelerator. The most often used radionuclides are Tc-99m as photon emitter and F-18 as positron emitter. The half-life of Tc-99m is 6h, which is optimal for most metabolic studies. Tc-99m decays to Tc-99 by emitting a gamma ray with an energy output of 140 keV. This energy is optimal for detection by scintillation detectors. Tc-99 itself has a half-life of 211100 years and is therefore of a negligible effect on the patient. F-18 is cyclotron produced and has a half-life of 110 minutes. It decays to stable O-18 by emitting a positron. College of Medicine/Babylon University Dr.Entidhar j.Khamees Nuclear Medicine Imaging There are two classes of nuclear medicine imaging techniques: 1. Single photon imaging Single photon imaging uses radionuclides that decay by gamma-ray emission. A two- dimensional image is obtained by taking a picture of the radionuclide distribution in the patient using gamma camera. In gamma camera a large crystal of sodium iodide (scintillation detector) is used to detect gamma ray. The crystal gives a tiny flash of visible light when hit by gamma photon. Then photomultiplier converts the flash into electrical signals which are analysed by a computer to construct an image. This result in a two- dimensional image with little depth information, but still be diagnostically useful. For the tomographic mode of single photon imaging (SPECT, Single Photon Emission Computed Tomography), data are collected from many angles around the patient. This allows cross-sectional images of the distribution of the radionuclide to be reconstructed, thus providing three-dimensional with the depth information missing from two- dimensional imaging. 2. Positron Imaging Positron imaging use radionuclides that decay by positron emission. The emitted positron has a very short lifetime and, following annihilation with an electron, simultaneously produces two high-energy photons that subsequently are detected by an imaging camera. Tomographic images are formed by collecting data from many angles around the patient, resulting in Positron emission tomography (PET) images. Measurement Example of clinical use Isotope Imaging modality Bone metabolism Metastatic spread of cancer 99𝑚 𝑇𝐶 planner Myocardial Coronary artery disease 99m Tc SPECT or planner perfusion 201 Tl Renal function Kidney disease 99m Tc planner Cerebral blood Neurologic disorders 99m Tc SPECT flow Thyroid function Thyroid disorders 123 I Planner Thyroid cancer 131 I Lung perfusion / Pulmonary embolism 99m Tc Planner ventilation Site of infection Detection of inflammation 111 In planner Glucose Cancer, neurological disorders 18 F PET metabolism and myocardial disease Myocardial Coronary artery disease 82 Rb PET perfusion Sequestered in Tumour localisation 67 Ga Planner tumours College of Medicine/Babylon University Dr.Entidhar j.Khamees Physics of Radiotherapy  Roentgen: It is the unit of exposure. It based on ionisation in air. It is only used for X-rays and gamma rays in air. (1 R= 2.58 × 10−4 C/kg).  Rad: It is the unit of absorbed dose. The rad defined as a 100 ergs/g. It is a radiation beam that gives 100 ergs of energy to 1 g of tissue. The rad can be used for any type of radiation in any material.  Gray (Gy): It is the international (SI) unit of dose. 1Gy=1J/kg. Since a joule is 107ergs, a gray equals 100 rad.  Rem (rad equivalent man): it is the unit used for the quantity dose equivalent. The dose equivalent is equal to dose multiplied by radiation quality factor. Principles of Radiation Therapy  The basic principle of radiation therapy is to maximise damage to the tumour while minimising damage to normal tissue. This is generally accomplished by directing a beam of radiation at the tumour from several directions so that the maximum dose occurs at the tumour.  Ionising radiation tears electrons off atoms to produce positive and negative ions. It also breaks up molecules; the new chemicals formed are of no use to the body and can be considered a form of poison.  Factors that determine how much radiation is required are the type of radiation, the type of cell, and the environment of the cell.  Some types of radiation are more effective in killing cells or have a higher relative biological effect (RBE). The RBE is defined as the ratio of the number of grays of 250 kVp X-rays needed to produce a given biological effect to the number of grays of the test radiation needed to produce the same effect.  The quality factor (QF) is related to the relative biological effect (RBE). RBE for a particular radiation is often different for different types of cell, while the QF is arbitrarily defined to be a constant for a particular radiation. RBE is usually used in radiation therapy, while QF is used for radiation protection purpose.  LD50 (lethal dose for 50%): It is the quantity of radiation that will kill half of the organisms in a population. This quantity is sometimes modified to include the time factor. For example, the amount of radiation that will kill 50% of the organisms in 30 days is called the LD50(30). College of Medicine/Babylon University Dr.Entidhar j.Khamees  The cells irradiated in the presence of oxygen were much easier to be killed than cells of the same type irradiated without oxygen. Hyperbaric oxygen tanks are therefore developed for radiotherapy.  Treatment planning: It is the process of determining the best combination of radiation beams and their orientation. Radiotherapy Procedures Radiotherapy procedures fall into two main categories: 1. External beam radiotherapy In external beam radiotherapy the radiation source is at a certain distance from the patient and the target within the patient is irradiated with an external radiation beam. Most external beam radiotherapy is carried out with photon beams, some with electron beams and a very small fraction with more exotic particles such as protons, heavier ions or neutrons. There are two origins of photon beams: 1.γ-rays which originate from radioactive nuclei such as 60𝐶𝑜 which emits penetrating gamma rays of about 1.25MeV energy. 2.X-rays consist of bremsstrahlung photons and characteristic photons. X-rays are produced either in an X-ray tube (superficial or orthovoltage X-rays) or in a linear accelerator (megavoltage X-rays). 2. Brachytherapy  Brachytherapy is a term used to describe the short distance treatment of cancer with radiation from small, encapsulated radionuclide sources. This type of treatment is given by placing sources directly into or near the volume to be treated.  In brachytherapy the dose is delivered continuously, either over a short period of time (temporary implants) or over the lifetime of the source to a complete decay (permanent implants). Most common brachytherapy sources emit photons; however, in a few specialised situations beta or neutron emitting sources are used. There are two main types of brachytherapy treatment: a) ‫ بين الخاليا‬Intracavitary, in which the sources are placed in body cavities close to the tumour volume. It is always temporary, of short duration. b) ‫داخل الخلية‬Interstitial, in which the sources are implanted within the tumour volume. It may be temporary or permanent. College of Medicine/Babylon University Dr.Entidhar j.Khamees  The physical advantage of brachytherapy treatments compared with external beam radiotherapy is the improved localised delivery of dose to the target volume of interest (tumour).  The disadvantage is that brachytherapy can only be used in cases in which the tumour is well localised and relatively small.  In a typical radiotherapy department about 10–20% of all radiotherapy patients are treated with brachytherapy. Radiation Protection in Radiotherapy  Since the radiation therapy area of a hospital contains intense radiation sources, it is typically surrounded by concrete walls about 0.5m thick.  To protect individuals who might inadvertently enter the room during a treatment, the door has a switch that turns off the machine when the door is opened.  Also the radiation sources themselves must be adequately shielded.  The most serious radiation hazards in radiotherapy involve internal radiation sources. In this situation three simple ways to reduce the staff radiation: 1. Minimise the time near the radiation source. 2. Maximise the distance to the source. 3.Use shielding where possible.