Orthovoltage vs Megavoltage

8/20/2019 Orthovoltage vs Megavoltage

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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 ametallic 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 effectsto 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 bonedamage 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 areoperated 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 relativelyinexpensive 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 isuseful, 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-yearhalf-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 teletherapy60

Co source is a cylinder of

diameter ranging from 1.0 to 2.0 cm and is positioned in the cobalt unit with its circularend 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 rotatedaround the patient. The

60Co 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. The60

Co source decays to60 Ni with the emission of b particles (Emax =

0.32 MeV and two photons per disintegration of energies 1.17 and 1.33 MeV (averageenergy 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 thehigher 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-skindistance 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 mechanicaland 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 andthe 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. Forexample 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: 4MV 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 theshape 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 twosets 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 theMaximum Dose for Common Photon Energies :

DEPTH (cm) VERSUS

PERCENTAGE OF MAXIMUM DOSE

PHOTON BEAM

ENERGY

100 % 80 % 50%

230 kV 0 3.0 6.860

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 centersapproximately 15% of patients are treated with electrons at some time during their

therapy. Electron beam dosimetry is different from megavoltage. The percent depth dosesfall 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 dosethan 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'sskin 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% ofthe Maximum Central-Axis Dose for Various Electron Beam Energies

DEPTH (cm) VERSUS PERCENTAGE OF MAXIMUM

CENTRAL-AXIS DOSE

ELECTRON

BEAM

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 7.920 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 ionizingradiation. (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 thegamma 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 diagnosticradiographs.

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 theelectron. 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 originalgamma 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 areduced 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 strikesan 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 productioninteraction.

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 intercellularfluid 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 effectis 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 toradiation 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 specializedand 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 radiosensitivethan 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 theradiosensitivity 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 ovariesSkin, particularly the portion around hair follicles

Lining of blood vessels

Lining of liver and adrenal glandsOther tissues - including bone, muscle and nerves, in that order (least

radiosensitive)

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Orthovoltage vs Megavoltage