1. Orthovoltage vs. megavoltage x-rays. (AL) External beam radiation sources: Orthovoltage radiotherapy: 200-500 kv range The radiation from orthovoltage units is referred to as x-rays, generated by bombarding a metallic target (tungsten) with high-energy electrons. This is a relatively low energy radiation source; typically operated at 250 kv. Maximum dose is deposited at the skin surface and dose falls to 90% at ~2 cm of depth in the tissue. As a result the acute effects to the skin can be severe. It is difficult to treat deep-seated tumors due to the limitations of the radiation tolerance of the overlying tissues; the skin dose becomes prohibitively large when adequate doses are to be delivered to deep-seated tumors. Additionally, there is differential absorption of dose in bone versus soft tissue and there is some risk of bone damage or necrosis. Orthovoltage irradiation is primarily suited for treatment of superficial tumors that do not involve adjacent bone. Applications include primarily skin tumors, and nasal cavity tumors after cytoreductive surgery. Orthovoltage units are operated at a relatively short source-to-skin distance (usually 50 cm) limiting the size of the treatment field; the field size is defined by the use of different sized/shaped attachments or cones (rectangular, circular, slanted). Orthovoltage units are relatively inexpensive machines, relatively easy to repair and maintain, and less shielding and space is required for operation. Megavoltage: 1-25 MV External beam therapy is commonly delivered via a medical linear accelerator or Cobalt- 60 unit. The introduction of these megavoltage units ushered in the modern age of radiation therapy. These units deposit the maximum dose beneath the surface; therefore, external beam therapy is considered skin sparing. Photons traverse the entire tissue thickness but deposit less dose as the depth increases. Many modern linear accelerators are also capable of producing electrons, which can be used for treatment situations where the limited depth penetration of these particles is useful, such as in cases where the contralateral parotid gland must be spared (see the image below). Cobalt-60 With cobalt-60 units gamma rays are emitted from a radioactive source with a 5.26-year half-life (the half-life is the time required for an isotope to decay to half of its original strength). The dose rate is constantly decreasing as the source decays and the source needs to be changed every 5 years. A typical teletherapy 60 Co source is a cylinder of diameter ranging from 1.0 to 2.0 cm and is positioned in the cobalt unit with its circular end facing the patient. Both isocentric (Theratron 780) and column-mounted (Eldorado 8)
units exist. The advantage of isocentric machines is that the patient is positioned only once for the treatment and then the source located in the head of the machine is rotated around the patient. The 60 Co source emits radiation constantly (as opposed to linear accelerators or orthovoltage units) and the source must be shielded when the machine is in the off position. The 60 Co source decays to 60 Ni with the emission of b particles (E max = 0.32 MeV and two photons per disintegration of energies 1.17 and 1.33 MeV (average energy 1.25 MeV); the gamma rays constitute the useful treatment beam. With an average energy of 1.25 MeV, there are a number of advantages of cobalt 60 over orthovoltage. There is greater penetrability for more deeply seated tumors due to the higher energy. There is uniform dose deposition in bone and soft tissue (versus orthovoltage). There is a dose build-up region such that maximum dose is not deposited until 0.5 cm below the skin surface resulting in what is termed a "skin-sparing" effect (there is less skin reaction than with orthovoltage). Treatment of superficial tumors and potential tumor cells in surgical incisions requires the placement of a tissue-equivalent material (bolus; superflab; wet gauze) over the site to allow dose build-up to occur and maximum dose deposition at the skin level. In this setting there will then be loss of the skin-sparing effect and increased radiation reaction in the skin. The source-to-skin distance is typically 80 cm so larger field sizes are possible than with orthovoltage. This is one of the most reliable radiation therapy machines because of their mechanical and electric simplicity. A radioactive material license is required for operation. Also, there is a low level of exposure to radiation with a cobalt 60 unit due to a minor persistent leak of radiation from the source despite the shielding; time spent in the room should be limited to the extent that is possible. Collimators (two pairs of heavy metal blocks) are use to alter the field size. Other beam modifying devices include the use of lead blocks (preformed or custom made; placed on a tray that is located near the head of the machine and is between the radiation source and the patient) or wedges (used to differentially absorb the photon beam to provide more uniform dose distribution in the tumor and normal tissues; specifically in situations where there is a slope in the patients contour; the use of wedges requires computer planning). Because the cobalt-60 source is not a point source this results in what is known as the geometric penumbra (penumbra refers to the region at the edge of the radiation beam over which the dose rate changes rapidly). This is one disadvantage of cobalt-60 units compared to linear accelerators. The one disadvantage is in the treatment of tumors that overly critical normal tissues. For example treatment of tumors that overly the thorax or abdomen may result in unacceptable normal tissue toxicity and patient morbidity due to delivery of dose at depth in the tissue. In these instances it is more advantageous to use a linear accelerator with electron capability (see discussion below). Linear Accelerator (linac)
Linear accelerators utilize x-rays (also referred to as photons) or electron beams. Linear accelerators use high-frequency electromagnetic waves to accelerate charged particles, ie, electrons, to high energies through a tube; the electrons can be extracted from the unit and used for the treatment of superficial lesions; or they can be directed to strike a target to produce high-energy x-rays for treatment of deep-seated tumors. The energy is higher and varies depending on the machine specifications with a range of 4-25+ MeV (Note: 4 MV machines do not have electron capability; this typically requires a 6 MV machine or higher energy). With the higher energy there is an even greater skin-sparing effect with maximum dose deposited at a depth related to the energy of the photons (see Table 1). The source-to-skin distance is 80-100 cm, and the relatively large source-to-skin distance allows treatment of large fields. It is also possible to treat large volume tumors more uniformly due to the depth dose characteristics. The relatively small focal spot limits the penumbra of the beam, and results in a relatively sharper edge to the treatment field. High output from the machine shortens the treatment time for individual patients and allows treatment of a larger number of patients per day. Linear accelerators can potentially be equipped with a multileaf collimator allowing the shape of the field to match the shape of the target. Multileaf collimators consist of a large number of pairs of narrow rods with motors that drive the rods in or out of the treatment field thus creating the desired field shape. Units without multileaf collimators have two sets of jaws that can move independently but basically allow the formation of a square or rectangular field, and further modification of the field requires the use of lead blocks. TABLE 1 : Depths at Which the Dose is 100%, 80%, and 50% of the Maximum Dose for Common Photon Energies : PHOTON BEAM DEPTH (cm) VERSUS ENERGY PERCENTAGE OF MAXIMUM DOSE 100 % 80 % 50% 230 kv 0 3.0 6.8 60 Co 0.5 4.7 11.6 4 MV 1.0 5.6 13.0 6 MV 1.2 6.8 15.6 10 MV 2.0 7.8 19.0 25 MV 3.0 10.2 21.8 Electron beams are used to treat superficial lesions. In human radiation therapy centers approximately 15% of patients are treated with electrons at some time during their therapy. Electron beam dosimetry is different from megavoltage. The percent depth doses fall off rapidly. The range (in cm) of electrons in tissue is approximately one half of their energy in million electron volts (MeV). For example, 12 MeV electrons have a range of about 6 cm. Electrons lose about 2 MeV of energy for each centimeter in tissue traversed. Normally the 80 or 90% depth isodose curve is used to encompass the target volume. The 80% isodose curve lies at a depth (in cm) of tissue that is about one third of the electron energy (MeV). In general, higher energy electron beams exhibit a higher surface dose than lower energy electron beams. With lower energy electron beams (below 15 MeV)
there is a significant skin sparing effect and if tumors involve the skin it may be necessary to add bolus to increase the skin dose. The use of electrons requires cones for collimation; electrons scatter readily in air so the beam collimation must extend as close as possible to the skin surface of the patient; electron cones of variable sizes attached to the collimator and extending to the patient's skin surface are used to collimate the electron beams; secondary beam shaping can be accomplished by adding lead cutouts at the end of a cone. TABLE 2 : Depths at Which the Dose is 100%, 80%, 50% and 10% of the Maximum Central-Axis Dose for Various Electron Beam Energies ELECTRON BEAM DEPTH (cm) VERSUS PERCENTAGE OF MAXIMUM CENTRAL-AXIS DOSE ENERGY 100% 80% 50% 10% 6 MeV 1.4 2.0 2.4 2.9 9 MeV 2.0 3.1 3.6 4.4 16 MeV 3.0 5.7 6.6 7.9 20 MeV 1.9 6.9 8.3 10.1 Note : A comparison of the above table to that for various photon energies (Table 1) demonstrates the extent to which the dose drops off in tissue with electrons as opposed to photons. This allows the treatment of tumors over the thorax or abdomen while minimizing the dose delivered the underlying critical normal tissues (e.g., lung, heart, and intestinal tract). 2. Describe the Photoelectric effect and Compton effect in ionizing radiation. (AL) In the Compton effect, the gamma rays are scattered from the outer electrons of the atoms, transferring energy to the electrons and in the process reducing the energy of the gamma ray. If enough energy is supplied during scattering, the outer electron will be removed from the atom, leaving an ion and giving rise to a free electron. Compton effect involves interaction with outer electrons that are bound more loosely. This effect is related to electron density and, therefore, results in much more uniform tissue absorption than lower energy photons. In radiation therapy, Compton effect predominates; therefore, the contrast observed on therapy port films is inferior to diagnostic radiographs.
In the photoelectric effect, one of the inner electrons of the atom absorbs the energy of the gamma ray, and is ejected from the atom, again leaving a positively charged ion and a
free electron. Following this, it is often the case that one of the outer electrons falls down to fill the vacancy. As a consequence, an X-ray is emitted from the atom. Photoelectric effect involves the interaction of the photon with the tightly bound inner electrons and is proportional to the cube power of the absorbing matter's atomic number. This interaction is responsible for the different radiographic densities seen on diagnostic radiographs. Compton scattering is the predominant method of interaction for mid-energy range photons. As Figure RP-1-4 indicates, in Compton scattering a medium energy gamma interacts with an orbiting electron near the nucleus imparting some of its energy to the electron. When this occurs, the electron that absorbs the energy leaves the atom to form an ion pair, and, because it has a significant amount of kinetic energy, produces ionization the same as a beta particle does. In addition, because the energy of the original gamma photon was not all absorbed the lower energy photon continues on to cause other interactions. Therefore, the eventual result of a Compton scattering reaction is that a mid energy range photon results in the production of an ion pair, and the photon continues at a reduced energy to undergo another interaction. The photoelectric effect is the predominant method of interaction for low energy range photons. As Figure RP-1-4 indicates, the photoelectric effect, a low energy photon strikes an electron. If the photon has the same energy as the binding energy of the electron (the energy that holds the electron in its orbit), the photon will give all its energy to the electron and disappear. The electron is knocked out of the electron shells, forming an ion pair. Therefore, in the photoelectric effect reaction, the photon disappears and an ion pair is formed. (The photoelectric effect is applied in light meters used in photography.) Not all types of photons can undergo all three types of photon interactions. For example, visible light is a photon, but it does not have enough energy to cause a pair production interaction. 3. Effects of radiation on a cellular level. (AL) Direct cell damage DNA is damaged via direct damage to the nucleus Indirect cell damage radiation strikes the cytoplasm surrounding the nucleus rather then the nucleus itself. The cytoplasm is compose primarily of water and is the intercellular fluid described in the previous section. When radiation interacts with a water molecule, certain free radicals can be formed. The free radicals are chemically reactive, and they can cause the cell to become chemically imbalanced; the result is cell damage. The effect is caused indirectly, the chemical changes brought about by the formation of the free radicals are what ultimately cause the cell damage. If the damage is so great that the cell cannot repair itself, the result is the same as in direct cell damage, the cell dies.
RADIOSENSITIVITY OF CELLS Each type of cell responds to radiation in a different manner. This relative response to radiation exposure is called radiosensitivity. It follows that cells that are highly radiosensitive are easily damaged by radiation and cells that are less radiosensitive are more difficult to damage with radiation.what makes a cell more or less radiosensitive? One factor is the function of the cell. If a cell has only one function, it is very specialized and is less radiosensitive. Nerve cells, as an example, are among the least radiosensitive cells in the body because of their singular function. The reproduction capacity of the cell also affects it's radiosensitivity. If a cell is multiplying rapidly, it is more radiosensitive than a cell that is not multiplying. The important factor affecting cell radiosensitivity is it rate of replication. In the late eighteen hundreds and early nineteen hundreds, two French scientists, Bergionne and Tribondeau conducted experiments exploring the radiosensitivity of cells. RP-1 The Biological Effects of Ionizing Radiation They found that cell radiosensitivity varied indirectly as the degree of differentiation and directly as the rate of multiplication. This has become known as the Law of Bergionne and Tribondeau. Radiosensitivity of Adult Body Cells and Tissues Lymphoid tissue, particularly lymphocytes (most radiosensitive) Immature blood cells found in bone marrow Cells lining gastro-intestinal canal Cells of the gonads-testes more sensitive than ovaries Skin, particularly the portion around hair follicles Lining of blood vessels Lining of liver and adrenal glands Other tissues - including bone, muscle and nerves, in that order (least radiosensitive)