Radiation therapy is the use of high-energy beams such as electrons or protons to kill cancer cells. The goal of radiotherapy is to kill cancer cells without damaging healthy tissues. During radiotherapy, a dosage of radiation is used to damage the DNA within the cancer cells. The DNA is the genetic material controlling the behavior of cells. DNA damage is achieved directly or through the creation of charged particles (free radicals). When their DNA is damaged, cancerous cells die and are broken down and naturally removed from the body. While normal cells may be destroyed by radiation, they usually repair themselves after treatment.
How is radiation delivered?
Radiation therapy is commonly administered in two ways, internally and externally. Internal radiation therapy (brachytherapy) involves placing a radioactive source, such as radiation seeds, inside the body. The radiation source is surgically placed near the affected tissue or area. External radiation therapy (external beam radiotherapy) uses a machine placed outside the body to direct high-energy beams at the cancer. Depending on the size, type, stage and location of the cancer, one or a combination of techniques may be used to deliver the radiation.
External beam radiation therapy (EBRT)
External beam radiation therapy (EBRT) is the delivery of high-energy beams through the skin to a tumor and its immediate surrounding. The beams are generated outside the body using a linear accelerator and then directed at the tumor site. High-tech software is used to plan and control the radiation dose according to tumor size, shape and location, ensuring that the high-energy radiation dosage is precisely deposited at the tumor site, destroying cancer cells while sparing normal tissues. To minimize adverse effects, treatments are given 5 days a week (Monday through Friday) for several weeks, ensuring that enough radiation is delivered to damage tumor cells while allowing healthy cells time to recover. With EBRT, no radiation source is placed inside the body.
Why is external beam radiation therapy performed?
EBRT is the most commonly used method of treating cancer. It is a safe and effective way of treating breast cancer, colorectal (bowel) cancer, esophageal cancer, head and neck cancer, lung cancer, prostate cancer and brain tumors. When applied, the goal is typically to eliminate the tumor or to prevent tumor recurrence. External beam radiotherapy also can be performed before or after surgery to shrink a tumor and reduce its size before operation, or to prevent the tumor from recurring after surgery. EBRT also can be a palliative treatment in patients with advanced-stage tumors or cancers that have widely spread to other parts of the body. When applied as palliative treatment, radiation is used to relieve symptoms instead of curing the cancer.
Types of external beam radiation therapy
Prior to treatment with external beam radiation, the doctor will choose the right type of technique and machine to deliver the radiation to the cancer site. Different types of external beam radiotherapy include:
1. Three-dimensional conformal radiotherapy (3D-CRT)
Since tumors are usually irregular, coming in different sizes and shapes, a precise technique that delivers radiation dose according to the size and shape of a tumor is necessary. The three-dimensional conformal radiotherapy (3D-CRT) is the use of computers and special imaging during radiotherapy planning to map out the location, size and shape of a tumor and then to deliver a radiation dosage that matches the shape of the tumor as closely as possible. As a result, healthy tissue is spared and adverse effects are minimized. 3D-CRT uses magnetic resonance imaging (MRI) scans, computer-assisted tomography (CT) scans and positron emission tomography (PET) scans to produce detailed, three-dimensional images of tumors and surrounding tissues. With the images, radiation oncologists can then tailor radiation beams to match the shape and size of tumors.
2. Intensity-modulated radiation therapy (IMRT)
IMRT (intensity-modulated radiotherapy) is a special form of 3D-CRT which delivers hundreds of small beams of varying intensities into the body from different angles. The radiation beam is broken down into smaller beams (beamlets), with the intensity of each beamlet adjusted individually. By controlling the intensity and quantity of radiation, IMRT ensures that a higher radiation dose is delivered to the tumor site without damaging surrounding tissues. IMRT was initially used for neck and head cancers and prostate cancers, but is currently used for almost all cancers. It is very precise, safe and effective.
3. Image-guided radiation therapy (IGRT)
Image-guided radiotherapy (IGRT) is the use of imaging techniques such as CT scan, ultrasound or X-ray imaging during treatment to ensure better accuracy and precision. The images enable radiotherapy treatment teams to visualize fine margins and small changes in tumor size, shape and location and to make the requisite adjustments when administering the radiation. Since tumors can change slightly between treatments due to movements such as breathing or organ filling, image-guided radiotherapy ensures that cancer cells are accurately targeted while healthy tissues are spared.
4. Stereotactic radiotherapy
Also called stereotactic ablative radiotherapy (SABR), stereotactic radiotherapy is a newer, more innovative technique for delivering radiation beams to tumor sites with greater precision. The accuracy and precision is often achieved through a very secure immobilization, such as head frames applied in treating brain tumors. The cyberknife is an example of stereotactic radiotherapy which delivers very thin low-dose beams of radiation to targeted sites from different angles.
Stereotactic radiotherapy is often given in a single dose to the tumor site with the help of highly accurate planning and imaging. This is called radiosurgery or stereotactic radiosurgery (SRS). A head frame is applied to ensure the patient remains completely still. In other cases, radiation doses can be delivered across several sessions (3-8) to small tumors outside the spine or brain, a procedure called stereotactic body radiation therapy (SBRT).
5. Proton beam therapy
This is an advanced form of external beam radiotherapy in which protons are used instead of X-rays. Proton beam therapy is preferred for tumors located near vital organs (such as tumors adjacent to the brainstem) and in young patients where long-term adverse effects of radiotherapy such as intellectual development delay, hormonal imbalances and secondary cancers need to be minimized. Proton therapy is offered only by a few specialized centers.
6. Neutron beam radiotherapy
Neutron beam radiotherapy is a highly specialized form of radiotherapy used to treat radio-resistant tumors. It is often recommended for tumors that are difficult to kill through conventional X-ray radiotherapy because neutrons give higher biologic impact on cells than X-rays and other kinds of radiations. When used carefully, neutron beam radiotherapy is invaluable in certain situations. The treatment is available in just a few specialized centers.
Who is involved in external beam radiotherapy?
External beam radiotherapy is given in the hospital radiotherapy department. Administration of external beam therapy is planned and completed by a treatment team that includes a radiation oncologist, dosimetrist, radiation therapist, medical physicist, specialist nurse and a specialist radiographer. The oncologist is the doctor who evaluates the patient and recommends the right treatment or combination of treatments. Working with the dosimetrist and medical physicist, the oncologist determines the area to apply the radiation, the dosage to deliver and the techniques to use. The dosimetrist and physicist then develop detailed treatment calculations and the necessary quality assurance measures for the treatment. The radiation therapists are well-trained technologists who deliver daily radiation treatments, while a specialist radiographer and specialist nurse help with treatment planning, patient care and controlling adverse side effects.
What happens before treatment?
1. Simulation and planning session
A week or two prior to treatment, your treatment team will have a simulation and planning session to make certain that the radiation therapists know the exact location, position, shape and size of your tumor and are sure to aim radiation at the tumor without damage to the surrounding healthy tissues. During simulation and planning:
(a) You will be placed in a treatment position, on a CT scanner.
(b) Pads, masks and other immobilization devices will be used to hold you still in a specific position.
(c) Images will be taken while you are in the treatment position. You will have a computerized tomography (CT) scan and where possible magnetic resonance imaging (MRI) scans.
(d) Using the images, the radiation oncologist will determine tumor volume and extent of spread and outline areas that should be treated, marking them out in the treatment planning images.
(e) The radiation oncologist, dosimetrist and medical physicist will use a special computer program to work out the radiation dose to be delivered to the tumor in each treatment session.
(f) You will get 3 or more small marks (tattoos) on your skin to help define the precise areas that will receive radiation.
(g) Marker seeds also may be placed in the targeted tumor or organ.
(h) The radiographers will discuss with you the procedure and what you are expected to do throughout.
2. Equipment used
Radiation oncologists use cobalt machines or linear accelerators to deliver external beam radiotherapy. The equipment uses electricity to create radioactive beams. During treatment, the machine will not touch you and you will not feel anything, but you may experience slight discomfort or pain after treatment. The machine is operated by a highly-trained technologist called radiation therapist, who works according to the overall treatment plan as created by the radiation oncologist.
3. Radiotherapy fractions
The recommended full dose of radiation is typically divided into several small doses known as fractions. Administration of small doses enables healthy cells to recover and repair between treatments. The fractions are delivered in a series of treatment sessions making up a whole radiotherapy course. When radiation is used only to relieve symptoms (palliative radiotherapy), the fractions are fewer and may even be just one treatment.
4. Course of treatment
When planning for your treatment, the oncologist takes into account the type of cancer, position of the tumor in the body, any previous treatment and your general health. Therefore, treatment is tailored perfectly to your needs. When radiotherapy is aimed at curing cancer, the course of treatment will last between 1 and 6 weeks, though it may be longer. Daily treatment sessions will run for 5 days a week (Monday through Friday), allowing for rest over the weekends. However, depending on your condition, you may have a different treatment plan, such as treatment 2-3 times a day, or 3 days a week.
What happens during treatment?
Just before treatment, you will be asked to change into a gown and guided into the treatment room where you will be placed on a treatment couch in the same position as during the simulation session. The radiation therapist will position you carefully using immobilization devices, alignment lasers and the marks placed on your body during simulation. Imaging is used to confirm the accuracy of the setup before the therapist turns on the linear accelerator. Once turned on, the accelerator will deliver beams in one or more angles and for as long as necessary. Treatment begins as soon as the machine starts to revolve. You are required to remain still until the session is over, typically for 10-20 minutes including the time taken to put you into position.
It is safe to be around other people during the course of external beam radiotherapy. The radiation does not remain in the body so you will not give it off. You also will be able to go about your routine activities during your treatment course without difficulties. After treatment is completed, your doctor will recommend a series of follow-up examinations, such as physical check-ups, imaging procedures, blood tests and other tests. These follow-up visits will give you the chance to discuss any side effects with you doctor. Remember, follow-up visits are necessary even if your cancer is completely cured.