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Sharmaine Galon

on 12 February 2015

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Electron Beam Production
Target Definition

As with photon beam treatments, the first step in the initiation of electron therapy is to determine accurately the target to be treated. All available diagnostic, operative and medical information should be consulted to determine the extent and the final planning target volume (PTV) with appropriate margins to be treated before simulation and placement of the electron fields is initiated.

Three main type of radiation used

1. Gamma rays
2. Xray
3. Electron beam

Why Electron?
Electrons have been used in radiotherapy since the early 1950’s.
Delivers a reasonably uniform dose from the surface to a specific depth, after which dose falls off rapidly, eventually to a near zero value
Using electron electron beams allows disease within approximately 6cm of the surface to be treated effectively, sparing deeper normal tissues

Electron beam therapy is the choice for skin and shallow target in the body. Electron therapy or Electron Beam Therapy (EBT) is a kind of external beam radiotherapy where electrons are directed to a tumor site
Electron beam therapy is commonly performed using a medical linear accelerator.
Other machines can also be used in electron beam radiation therapy like Tomotherapy, betatrons, microtrons.

What is Electron Beam
Radiation Therapy?

Two types of EBRT

Treatment Schedule

Clinical Scenarios
Intracavitary Irradiation
Intracavitary radiation is performed for treatment of intraoral or intravaginal areas of the body. Aditionally, IORT can be considered an intracavitary electron technique.
It is used in the treatment of orallesions presenting in the floor of the mouth, tongue,soft palate, and retromolar trigone.
For all intracavitary irradiation, specially designed treatment cones are required. In addition, an adapter to attach the cone to the linear accelerator has to be available.
Intraoperative Irradiation
Either a dedicated linear accelerator room that can meet the requirements of operating room sterile conditions or new mobile electron linacs that can be transported to a shielded OR need to be used.
Total Limb Irradiation
Treatment of the entire periphery of the body extremities (e.g melanoma ,lymphoma, Kaposi’s sarcoma ) can be accomplished using electron fields spaced uniformly around the limb.
Electrons offer a technique of delivering a uniform dose while sparing deep tissues and structures which are uninvolved.


Total skin electron treatments are employed in the management of myeosis fungoides.

The first requirement for total skin electron treatment is a uniform electron field largeenough to cover the entire patient in a standing position from head to foot and in the right to left direction.

This is accomplished by treating the patient at an extended distance (410 cm), angling the beams superiorly and inferiorly, and using a large sheet of plastic (3/8-inch thickness acrylic at 20 cm from the patient surface) to scatter the beam.


Replacement of the posterior photon field with a high energy electron field can reduce reatly the exit dose to the upper thorax region, especially the heart, and the lower digestive tract. This is especially important for pediatric patients and results in reductions of both acute and late complications.

The lateral photon fields are rotated through an angle to match the divergence of the posterior electron field.

The superior field edge of electron field “el” is not moved during the treatment but the inferior border of the photon fields is shifted 9 mm farther the junction location.

One-third of the photon treatments are delivered with the inferior border of the two photon fields coincident with the electron field edge. The next one-third of the photon treatments are delivered with the edge of one photon field 9mm superior to the electron field edge and the edge of the second photon field moved 9mm inferiorly to the electron field edge. The final one third of the photon treatments are delivered with the edges of the photon fields reversed from their previous position.

The angle of the two electron fields are rotated by an angle to account for the divergence of each of these electron fields and to produce a common field edge.

Electron Arc Therapy
Electron are therapy that is useful in treating postmastectomy chest wall.

It is more useful in barrel chested women, where tangent beams can irradiate too much lung.

There are three levels of collimation in electron therapy: the primary xray collimators, a shaped secondary Cerrobend insert, and skin collimation.

Future Directions

Electron therapy can be expected to become more sophisticated in the future as the enthusiasm for intensity-modulated radiation therapy will carry into electron therapy.
Advances in electron dose calculations, methods for electron-beam optimization, and availability of electron multileaf collimators will enable the practice of intensity modulated and energy-modulated electron therapy.
Intracavitary Irradiation
(Note: Differentiate Spot treatment from TSEB Therapy)
Spot treatment

• Usually given 4 times a week over 3 to 4 weeks.
• You should plan on being in the department for 60 to 90 minutes for each treatment.

Total Skin Electron Beam Therapy

• Given 2 times a week over 6 to 9 weeks.
• You should plan on being in the department for 60 to 90 minutes for each treatment.

After Treatment

• The Radiation Oncologist may want you to have “boost” treatments to areas of your skin that need more radiation.

• This includes the soles of the feet, perineum, scalp, skinfolds under the breasts, and on the stomach area.

• Six to 10 boost treatments are usually given over 2 to 3 weeks.
• Be in the department for 60-90 minutes for each treatment.

What to expect:

• You will check in at the reception desk and have a seat in the waiting room.

• Your therapists will tell you to change into a hospital gown. They will bring you into the treatment room and help you get into position.

• You will be in the treatment room for up to 30 minutes. Most of this time will be spent positioning you. The actual treatment only takes a few minutes.

• During TSEB therapy, you will be standing on a platform that rotates so that the entire surface of your skin can be treated from different angles.

• During spot treatment, you will be lying in the same position that you were in during your simulation.

• You will be asked to disrobe and will be given a disposable yellow gown to wear during your treatments.

• You may be given goggles to protect your eyes and put shields on your hands, feet, or both during some of your treatments.

• Once you are in position, your therapist will leave the room and begin the treatment.

• Breathe normally during your treatment, but do not move.

• If you are very uncomfortable and need help, tell your therapists. They can turn off the machine and come in to see you at any time, if necessary.

(Note: Explain simulation briefly)

•If you are having TSEB therapy, you will not have a treatment planning procedure. This is because the entire surface of your skin will be treated.
• If you are having spot treatment, you will first have a treatment planning procedure called a simulation.
Megavoltage electron beams represent an important treatment modality in modern radiotherapy, often providing a unique option in the treatment of superficial tumors.

Electron beam therapy is the choice for skin and shallow target in the body. Electron therapy or Electron Beam Therapy (EBT) is a kind of external beam radiotherapy where electrons are directed to a tumor site.
Spot Treatment- if 1 or more spots
Total Skin Electron Beam Therapy- if the entire surface of the skin is treated.

Electron beam therapy is used in the treatment of:
Superficial tumors like cancer of skin regions, or total skin (e.g. mycosis fungoides)
Diseases of the limbs (e.g. melanoma and lymphoma)
Nodal irradiation
Cancer of the skin - eyelids, nose, ear, scalp, and limbs
Cancer of the upper respiratory and digestive tract – floor of mouth, soft palate, retromolar trigone, salivary gland
Cancer of the breast - chest wall irradiation following mastectomy
Cancer in other sites - retina, orbital, spine (craniospinal irradiation)
Pancreas and other abdominal structures (intraoperative therapy)
Cervix (intracavitary irradiation)

May also be used to boost the radiation dose to the surgical bed after mastectomy or lumpectomy. For deeper regions intraoperative electron radiation therapy might be applied.
Generation of Electron Beams in a Linear Accelerator

Medical linear accelerators (linacs) are cyclic accelerators which accelerate electrons to kinetic energies from 4 MeV to 25 MeV using non-conservative microwave RF fields.

Components of Modern Linacs
Five Major and Distinct Sections of the Machine:

Gantry stand or support
Modulator cabinet
Patient support assembly (Treatment couch)
Control console

Source of electrons
Also called as “electron gun”
It contains a heated filament cathode and a perforated grounded anode
Injection System
The microwave radiation, used in the accelerating waveguide to accelerate electrons to the desired kinetic energy, is produced by the RF power generation system which consists of two major components:
1. RF power source
- is either a magnetron or a klystron
- both are devices using electron acceleration and deccelaration in vacuum for the production of high power RF fields.

2. Pulse modulator
- produces the high voltage, high current, short duration pulses required by the RF power source (magnetron or klystron) and the injection system (electron gun)

RF Power Generation System
Waveguides are evacuated or gas-filled metallic structures of rectangular or circular cross-sections used in transition of microwaves.
There are 2 types:
RF power transmission waveguides
Accelerating waveguides

Accelerating Waveguide
Electron Beam Transport System
Bending magnets are used in linacs operating at energies above 6 MeV where the accelerating waveguides are too long for straight-through mounting.
The accelerating waveguide is usually mounted parallel to the gantry rotation axis and the electron beam must be bent to be able to exit through the beam exit window.
Three systems for electron bending have been developed: 90° bending, 270° bending, 112.5° bending.
Linac Treatment Head
The linac head contains several components, which influence the production, shaping, localizing, and monitoring of the clinical photon and electron beams.

The important components found in a typical head of a fourth or fifth generation linac include:
1. Several retractable x-ray targets
2. Flattening filters and electron scattering foils (also called scattering filters)
3. Primary and adjustable secondary collimators
4. Dual transmission ionization chambers
5. Field defining light and range finder
6. Optional retractable wedges
7. Optional multileaf collimator (MLC)

In a typical modern medical linac, the electron beam collimation is achieved with two or three collimator devices:
1. Primary collimator
2. Secondary movable beam-defining collimators
3. Multileaf collimator (MLC)
Clinical electron beams also rely on electron beam applicators (cones) for beam collimation
Beam Collimation
Dose Monitoring System

Most common dose monitors in linacs are transmission ionisation chambers permanently imbedded in the linac clinical photon and electron beams to monitor the beam output continuously during patient treatment.
Most linacs use sealed ionisation chambers to make their response independent of ambient temperature and pressure.
The customary position of the dose monitor chambers is between the flattening filter or scattering foil and the photon beam secondary collimator.

The main requirements for the ionization chamber monitor are as follows:

1. Chambers must have a minimal effect on clinical photon and electron radiation beams.

2. Chamber response should be independent of ambient temperature and pressure (most linacs use sealed ionisation chambers to satisfy this condition).

3. Chambers should be operated under saturation conditions.
Interaction of Electrons with Absorbing Material

Electron entering a material interacts as a negatively charged particle with electric fields of specimen atoms.
These interactions are classified in to two different types.

1. Elastic interactions: In this case no energy is transferred from electron to sample. As result electron leaving the sample still has the original energy.
2. Inelastic interactions: The energy of the incident electron is transferred to the sample atoms. Hence, after the interaction electron energy is reduced.

Elastic Interaction

Elastic interactions deflects the electron beam along new trajectory, causing them to spread laterally.

A strong elastic scatter very near to the nucleus may result in beam electron leaving the specimen via back scattering, called Backscattered electrons (BSE).
Probability of elastic scattering
- Increases strongly with atomic number, as heavier atoms have much stronger positive charge at nucleus
- Decreases as electron energy increases.

Inelastic Interaction

An inelastic collision, in contrast to an elastic collision, is a collision in which kinetic energy is not conserved. In collisions of macroscopic bodies, some kinetic energy is turned into vibrational energy of the atoms, causing a heating effect, and the bodies are deformed.
With the inelastic scattering, beam electrons loose energy to specimen atoms in various ways.
Can produce ionization, Bremsstahlung or a secondary electron.
This is done to make sure that:

• Your treatment site is mapped out correctly.
• You get the right dose of radiation.
• The dose of radiation to nearby tissue is as small as possible.

Guidelines during simulation:

• Do not apply ointments, creams, lotions, talcum powders, alcohol, deodorants, anti-perspirants, perfumes, make-up or after-shave lotions in the treatment area unless prescribed by your physician. These products may intensify a skin reaction.

• Do not wear earrings or necklaces.

• Eat and drink as you normally would.

• Wear comfortable clothes.

• You will be lying still for a long time. This is uncomfortable for some patients. If you think it will be for you, take acetaminophen or your usual pain medication 1 hour before your appointment.

• If you think you may get anxious during your procedure, speak with your radiation oncologist about whether medication may be helpful.

During your simulation:

• Your therapists will take pictures of your skin and mark up the area(s) to be treated with a felt marker.
• This will take about 2 to 4 hours.
• The position that you are in during your simulation will be the same position you will be in for your spot treatments.

Side Effects:

• Patients who get spot treatment usually have minor side effects that involve the skin, hair, and nails in the area being treated.

• Patients who get TSEB therapy usually have side effects that involve all of the skin, hair, and both fingernails and toenails.

• Your skin will become: red, dark, dry, irritated (similar to sunburn), sore around your lips if your treatment is on your face.

• The redness and irritation will get better after your treatment is done but your skin in the treated areas will be drier than the usual.

• You will lose hair on your whole body (scalp, eyebrows, under your arms, and pubic hair) but will begin to grow back in 3 to 6 months after the treatment.

• Your nails will fall off in the areas being treated. As your old nails fall out, new ones will be growing in underneath.

Field Shaping and Collimation

lead or low-melting-point lead alloy. (lipowitz)
millimeters - to stop primary electrons is given by.
Patient Shielding

A variety of shielding can be placed close to, on or inside the patient.
External shields:

External shields can be placed over most body surfaces

Internal shields these are thin shields that are places within a body cavity.
Depth Dose

major attraction of the electron beam irradiation is the shape of the depth dose curve.

region of more or less uniform dose followed by a rapid dropoff of the dose offers a distinct clinical advantage over the conventional xray modalities.

the depth in centimeters at which electrons deliver a dose to the 80% to 90% isodose level, is equal to approximately one-third to one-fourth of the electron energy in MeV

most useful treatment depth, or theraputic range, of electron is given b the depth of the 90% depth dose.
Energy Dependence of Depth Dose

the percentage depth dose increases as the energy increases
the percentage depth dose increases as the energy increases
depth dose variatons with SSD are ussually insignicant. Differences in the depth dse resulting from inverse square effect are small because electron do not penetrate that deep (6cm or less in the therapeutic region) and because the significant growth of penumbra width with SSD restricts the SSD in clinical practice to typicallt 115cm or less
Dose Distribution in patient
the ideal irradiation condition is for electron beam to be incident normal to a flat surface with underlying homogeneous soft issues. the dose distributiob for this condition similar to that for a water phantom described previously. this scenario
What is radiation therapy?

It is the use of high energy rays usually x-ray and similar rays
(such as electrons) to treat disease. It works by destroying cancer cells in the area that’s treated.

Main Beam-forming Components of a Modern Linac
Injection system
RF Power generation system
Accelerating waveguide
Auxillary system
Beam transport system
Beam collimation and beam monitoring system

Consists of several services which are not directly involved with electron acceleration,
yet they make the acceleration possible and
the linac viable for clinical operation.

Auxillary Sytem
The linac auxiliary system comprises four systems:
1. Vacuum pumping system producing a vacuum pressure of ~10-6 tor in the accelerating guide and the RF generator .
2. Water cooling system used for cooling the accelerating guide, target, circulator, and RF generator.
3. Optional air pressure system for pneumatic movement of the target and other beam shaping components.
4. Shielding against leakage radiation.

The majority of higher energy linacs, in addition to providing single or dual photon energies, also provide electron
beams with several nominal electron beam energies in the range from 6 to 30 MeV.
To activate an electron beam mode both the target and flattening filter of the x-ray beam mode are removed from the electron beam.

Two techniques are available for producing clinical electron beams from electron pencil beams:
1. Pencil beam scattering
2. Pencil beam scanning

Elastic Interaction
Elastic interactions deflects the electron beam along new trajectory, causing them to spread laterally.

A strong elastic scatter very near to the nucleus may result in beam electron leaving the specimen via back scattering, called Backscattered electrons (BSE).

Probability of elastic scattering
- Increases strongly with atomic number, as heavier atoms have much stronger positive charge at nucleus
- Decreases as electron energy increases.

The rate of energy loss for radiative interactions (bremsstrahlung) is approximately proportional to the electron energy and to the square of the atomic number of the absorber.

Radiative losses is more efficient for higher energy electrons and higher atomic number materials.

When a beam of electrons passes through a medium the electrons suffer multiple scattering, due to Coulomb force interactions between the incident electrons and predominantly the nuclei of the medium.

As the electron beam traverses the patient, its mean energy decreases and its angular spread increases.

The scattering power of electrons varies approximately as the square of the atomic number and inversely as the square of the kinetic energy. For this reason high atomic number materials are used in the construction of scattering foils used for the production of clinical electron beams in a linac.
Electron Beam Radiation Therapy
Galon, Sharmaine
Alba, Alea Mae
Albano, Darren
Banate, Vanessa Lace Blancaflor, Lawrence
Buensalida, Adriane
Campañas, Leianngiezl
Cruz, Mary Grace
Sarmiento, Christine
Paliza, Lou Grace
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