Option I – Biomedical Physics

Option I – Biomedical Physics 3. Radiation in medicine

Biological effects of radiation and dosimetry • We are all exposed in our daily lives from natural or artificial sources, or because of special conditions in the workplace. • Natural sources include radon, gamma rays from the earth, unstable isotopes found in food and cosmic rays. • Artificial sources include the exposure to radiation for medical reasons, radiations from building materials, radiations form nuclear weapons testing, and accidents in nuclear power plants. • Workers in research labs and radiology personnel in hospitals are exposed to radiation as a result of their work.

Biological effects of radiation and dosimetry • The energy that is required to break a molecular bond is of the order of 1eV. • Radiations that deposit energies of this order, or more, are therefore bound to alter the molecular structure of the matter they travel through. • It is now generally accepted that any amount of radiation, no matter how small, is harmful, and that in dealing with radiation, the following general rules must be followed: • Keep as far as possible from the source. • Keep the exposure as short as possible. • Use shielding whenever possible

Biological effects of radiation and dosimetry • The absorption of even a single photon of energy by a single molecule of a living organism may not be harmless. • The absorbed photon may have the effect of ejecting and electron from its molecule and this may imply that the structure of the molecule changes. • For example, if the ejected electron was crucial in the chemical bonds holding the molecule together, then the molecule may break apart. • Enzymes may not operate correctly if the molecule is altered in any way. • It is also known that irradiation of water produces very reactive free radicals.

Biological effects of radiation and dosimetry • As most living organisms contain water, the production of these free radicals induces changes in the chemical structure of the surrounding molecules with biological implications. • Such reactions include:

Biological effects of radiation and dosimetry • The damage from a few molecules from radiation will not, in general, result in loss of function for the cell, but if the amount of radiation received is large, then the cell may not be able to recover its function. • By contrast, damage to a gene will, in general, affect the function of the cell. • Radiation is particularly damaging to the bone marrow, which is responsible for producing blood cells, and seriously impairs the human immune system. • Thus, leukemia and other forms of cancer are common results of exposure to radiation.

Biological effects of radiation and dosimetry • To understand the effect of radiation on living mater we must first define appropriate quantities to measure the amount of radiation received. Absorbed dose is defined as the amount of energy E absorbed by a unit mass of the irradiated material: Its unit is joule per kilogram (J kg-1) and that is defined as the gray (Gy). Another unit still in use is the rad: 1 Gy = 1 J kg-1 = 100 rad

Dose Equivalent The damaged produced by radiation is not only proportional to the absorbed dose but also depends on the type of radiation used. Thus we define the dose equivalent to be where Q is a dimensionless number that characterizes various different radiations. It is called the quality factor of the radiation. The unit of H is the same as that for absorbed dose (J kg-1) but to distinguish the two quantities we use the sievert (Sv) for the dose equivalent. • Another unit still in use is the rem (standing for radiation equivalent man: 1 Sv= 1 J kg-1 = 100 rem

Biological effects of radiation and dosimetry • The quality factor is related to the relative biological equivalent which is defined as We will take Q and RBE to be the same

Dose Equivalent • To get an idea of the meaning of the sievert, note that a dose equivalent or more than 100 Sv results in death in a few days due to massive damage to large number of cells. • With and amount of 10 Sv symptoms of radiation sickness (nauseam diarrhoea, vomiting) begin to appear a few hours after exposure and death follows in a few weeks. • In radiation therapy, amounts up to 3 Sv are administered but patients have survived much larger amounts. • A typical chest X-ray examination gives 0.1 mSv and a flight at a height of 8 km gives 2μSv per hour.

Biological effects of radiation and dosimetry • Generally, the danger increases with the amount of dose equivalent and the probability of cancer increases by 1% for every sievert of radiation received. • The International Commission of Radiological Protection recommends that on a yearly basis: • A person working with radioactive materials should not be exposed to more than 50 mSv; • Other adults should not be exposed to more than 5 mSv; • Children should not be exposed to more than 0.5 mSv.

Biological effects of radiation and dosimetry • ICRP’s recommendations for protection also include short-term limits of exposure of: • No more than 10 μSv per hour for γ rays at a distance of 10 cm; • No more than 50 μSv per hour for β particles, also at a distance of 10 cm from the source; • Particle sources with activity larger than 40 kBq (40000 decays per second) should be avoided. • The amounts of dose equivalent from various sources in everyday life depends on the location and elevation above sea level.

Amounts of radiation received per year by an average person living in a modern city at sea level • The amount from cosmic rays increases by about 0.2 mSv for every 1 km of elevation

Exposure A related quantity is exposure, X, which is defined as the total amount of produced charges q due to ionization in a mass m of air: • Ionisation means that positive and negative charges are produces. • In defining exposure we measure only the positive charges. • The unit for exposure is coulomb per kilogram (C kg-1). • Thus, ionisation of exposure 50 C kg-1 means that in 1 kg of air 50 C of positive or negative charge is produced as a result of ionisation by a particular radiation.

Exposure • The exposure rate is defined as the exposure per unit time, which is the amount of charge produced in 1 kg or air per second. • There is a connection between exposure and absorbeddose. • This is because in order to produce one ion in air an energy of approximately 34 eV is needed. • Thus, let the exposure of a given radiation be E (in C kg-1). • This radiation produces:

Exposure • Each electron requires an energy of 34 eV and thus the energy absorbed (absorbed dose) is:

Exposure • This is the connection between absorbed dose in air and exposure. • For materials other than air, we must take into account the fact that it takes different energies to produce an ion. • Thus, the relationship between exposure and absorbed dose is: • where f is a factor that depends on the material and the photon energy. • For muscle, f is approximately 40, independent of the photon energy. For bone, f drops from 150 at very low photon energy to about 40 for energies up to 1 MeV. • For higher energies, f stays at about 40.

Radiation therapy • Radiation is harmful to healthy cells, but it can also be harmful to malignant, cancerous cells. • Radiation can thus be used to destroy such cells. • In radiation therapy, X-rays or gamma rays can be directed in very narrow beams at the collection of cancerous cells to destroy them. In order to minimize the damage of the surrounding normal cells, tissue and bone it is necessary to direct the beam at the tumour from a number of different angles. • This assumes that the tumour is well localized and it is only in this case the radiation therapy can be effective. • The energies of X-rays and gamma rays used in cancer treatment vary from 200 keV to 5 MeV.

Radiation therapy • The tumour may also be injected or implanted with radioactive material whose radiation is release within the tumour, thus destroying it. • Radium, which is an α emitter, is commonly used in this way. • A related technique is to give the patient a quantity of a radioactive isotope of iodine, I-131, which, once in the blood stream, will accumulate in the thyroid gland. • Its radiation can then kill cancerous cells in that gland. • An isotope of gold, Au-198, collects in the lungs, destroying cancerous cells in the fluids that line the lungs.

Physical and biological half-life • A radioactive isotope decays according to an exponential decay law, so that the number of nuclei that are undecayed after time t is given by or equivalently • Where the physical decay constant λp is related to the physical half-life through

Physical and biological half-life • Note that by differentiating this equation we obtain: • The activity of the isotope (no. decays per second) obeys a similar exponential law • Where the initial activity is given by λpN0. here and in the decay formula above, N0 stands for the number of radioactive nuclei initially present. • When a radioactive isotope is taken in by a patient, however, its activity falls off faster that the law above implies.

Physical and biological half-life • This is because the number of isotope nuclei in the body decreases not only by radioactive decay but also because some of the radioactive material is removed from the body as waste (sweat, urine, etc.), in other words, by biological decay. • If we assume that the biological removal of the isotope obeys and exponential law as well, with a decay constant λ, then in a time Δt the number of nuclei removed is. • Where λE is the effective decay constant, which represents both physical decay and biological decrease of nuclei in the body.

Physical and biological half-life • Thus, we may define effective, physical and biological half-lives from these constants and hence • In other words, physical half-life is the time for half of the radioactive nuclei to decay away and biological half-life is the time for half of the radioactive nuclei to be removed from the body by biological processes. • The effective half-life is the time for half of the radioactive nuclei to be removed by both decay and biological removal.

Option I – Biomedical Physics