What is radiation therapy?

What is radiation therapy?

Radiation therapy is used primarily to kill cancer cells, although it can also be used to treat noncancerous conditions such as trigeminal neuralgia or arteriovenous malformations. Radiation therapy may be delivered using different types of radiation such as:

• X-rays
• Gamma rays
• Charged particles

The radiation can be delivered internally or externally. Internal radiation therapy, also called brachytherapy, is when radioactive material is put in the body near the cancer cells. There is also systemic radiation therapy, where radioactive substances travel through the blood to get rid of cancer cells. External-beam radiation therapy is delivered with a machine outside the body.

How does radiation therapy help fight cancer?

Radiation therapy helps fight cancer by causing enough damage to the DNA of cancer cells to kill those cells or stop them from dividing. Destroyed cells are then broken down and disposed of by the body’s natural processes.

Can radiation therapy kill other cells?

Yes, it can also potentially hurt other healthy cells, which can lead to side effects.

This possibility is taken into account when your doctor plans your radiation therapy. It is known how much radiation normal tissue can safely be given for all parts of the body. This information is used by doctors to aid them in deciding where they should concentrate radiation during treatment and how much radiation can be delivered.

Why do patients receive radiation therapy?

There are two different reasons why a patient might receive radiation therapy. One is with an intent to remove the cancer by eliminating a tumor or preventing cancer recurrence. In these cases, radiation therapy can be used alone or in combination with surgery or chemotherapy.

The other is given with a palliative intent, which means the treatment is not meant to cure but instead to relieve the patient’s symptoms, possibly prolong life, and decrease the discomfort caused by cancer.

Here are some examples of palliative radiation therapy:

• Shrink tumors formed from cancer cells that have spread to the brain from another part of the body
• Shrink a tumor that is pressing on the spine or growing within a bone, a condition that can be painful
• Shrink a tumor near the esophagus, a condition that can prevent a patient from eating and drinking

Planned radiation therapy for each patient

Your radiation oncologist will create a treatment plan for you, beginning with simulation. This includes detailed imaging scans to show the exact location of your tumor and the normal areas that surround it. These kinds of scans include:

• Computed tomography (CT)
• Magnetic resonance imaging (MRI)
• Positron emission tomography (PET)
• Ultrasound scans

CT scans are most commonly used in treatment plans. A CT scan shows the inside of your body through images created by a computer that is linked to an X-ray machine.

It is crucial to make sure you are in the exact same position while you are being given treatment or scanned by a machine. This ensures the treatment is going to the exact place it needs to be every time.

Your doctor may make body molds, head masks, or other devices to make it easier for you to hold still. The care team also will mark your skin to help with positioning. 

Once simulation is over, your radiation oncologist will determine:

• The exact area that will be treated
• The total radiation dose that will be delivered to the tumor
• How much dose will be allowed for the normal tissues around the tumor
• The safest paths (angles) for the radiation delivery

Prescribing the proper doses

Physicists and dosimetrists work with your radiation oncologist to design the exact details of your radiation plan. Once the plan is approved, your radiation oncologist will authorize the start of treatment. Checks are performed before your first treatment and throughout your treatment, which may happen over several weeks, and will be done to make sure the treatments are being delivered as planned.

Radiation doses are measured with a unit called a gray (Gy). A gray is the amount of radiation energy absorbed by one kilogram of human tissue. It takes different dosages to kill different types of cancer.

It is possible for radiation to damage some types of normal tissue more easily than others. For example, the reproductive organs such as the testicles and ovaries are more sensitive to radiation than bone. All of this information is taken into account when planning your treatment. 

Each part of the body has a maximum amount of radiation it can safely receive within a lifetime. This is why if an area of the body has been treated with radiation before, you may not be able to be given radiation therapy to that spot again. It may be possible for another area to be treated if the distance between the two is big enough.

A small amount of normal tissue is included with the area selected for treatment, this is done for two reasons:

• To reduce the risk of the tumor coming back from cancer cells that may have spread to the normal tissue surrounding the tumor
• To account for body movements, breathing, and normal organ movements within the body; this can change the location of a tumor between your treatments

How is the type of radiation chosen?

Your radiation oncologist will prescribe a type of radiation therapy to you based on many different factors, such as:

• Type of cancer
• Size of the cancer
• Location of the cancer
• How close the cancer is to normal tissues that are sensitive to radiation
• How far into the body the radiation needs to travel
• Your general health and medical history
• Whether you will have other types of cancer treatment
• Age
• Medical conditions

Explaining the different types of radiation therapy

External-beam radiation therapy

Most often, external-beam radiation therapy is given in the form of X-ray beams, which are produced by a machine called a linear accelerator (LINAC). A LINAC has an X-ray beam shaping system, called a collimator, which creates beams of different shapes for treating targets of varying shapes and sizes. LINACS can also control the direction of radiation beams, aiming them in a way such that normal tissue can be avoided and the radiation beams are focused on the target. LINACs also have image guidance systems that produce pictures of the patient under treatment, ensuring the patient is set up on the treatment table properly.  

Usually, a patient will receive external-beam radiation treatments daily over a course of many weeks. Each patient has a different number of sessions depending on many factors, like the total radiation dose that will be delivered.

Intensity-modulated radiation therapy (IMRT)  

With IMRT, the collimators that control the radiation beam shape can move during the treatment. This allows the intensity of the radiation beams to be adjusted for each beam direction during treatment sessions, providing the ability to deliver complex radiation dose distributions that cover the tumor and avoid healthy normal tissue.

IMRT is planned in reverse, unlike other radiation therapies. The radiation oncologist will choose the doses that will be delivered to the tumor and tissues around it, then a computer program calculates the necessary number of beams and angles for the treatment plan.

The goal of IMRT is to reduce the radiation exposure to sensitive areas of normal tissue while increasing the radiation dose to the areas that need it.

Image-guided radiation therapy (IGRT)

With IGRT, images of the patient are obtained throughout the course of treatment using imaging systems integrated with the LINAC. Both two-dimensional and three-dimensional images can be used for this purpose. The images are used to adjust the position of the patient on the treatment table to ensure the radiation is delivered to the correct location in the body. The images are also used to check for changes inside the patient, such as tumor shrinkage, which can be accounted for by making a new treatment plan if needed.

Stereotactic radiosurgery (SRS)

This method delivers one or more high doses of radiation to a small tumor. It also uses highly precise patient positioning for tumor targeting. This reduces unnecessary damage to normal tissue while still being able to give a high dose of radiation to the tumor.

Because it is most effective in treating small tumors that have well-defined edges, SRS is most often used to treat brain or spinal tumors and brain metastases from other types of cancers. 

A head frame may be required when using SRS. The frame may establish a spatial reference for imaging and immobilizes the patient during the treatment to make sure the dose of radiation is given precisely.

Stereotactic body radiation therapy (SBRT)

SBRT uses smaller radiation fields and higher doses to treat tumors that are found outside the brain, usually in one to five treatment sessions. These kinds of tumors may move while the patient is breathing so special imaging and delivery methods may be needed in order to treat them.

Generally, SBRT is given in more than one dose and is used to treat small, isolated tumors, including those located in the lung and liver.

Patients who are candidates for SBRT will usually undergo a four-dimensional CT (4D-CT) study, which produces a set of images that show how the patient’s internal anatomy moves while the patient is breathing. The 4D-CT images are used to figure out how much the tumor moves in order to create a treatment plan that takes the motion into account. Gated radiation therapy may be used to restrict how much the tumor moves while the radiation beam is on, which involves turning the radiation beam on and off at different times during the patient’s breathing cycle.

Internal radiation therapy

Internal radiation techniques used to treat cancer include

• Interstitial brachytherapy
• Intracavitary brachytherapy

The radioactive isotopes in brachytherapy are sealed in tiny pellets that are then placed in the patient with delivery methods like needles or catheters.  The isotopes decay naturally and as they do they release radiation that damages surrounding cancer cells.

After a few weeks or months, the isotopes in the pellets completely decay and will no longer emit radiation. The pellets do not cause damage to the body if they are left inside.

Internal radiation therapy can be delivered as a low- or high-dose-rate treatment.

• Low-dose-rate: cancer cells are given continuous radiation at a low dose over a period of days to months.
• High-dose-rate: a robotic machine that is attached to delivery tubes placed inside the patient’s body guides one or more radioactive sources into or near the tumor. The sources are then removed at the end of the session. This treatment can be delivered in one or more sessions.

The placement of sources can also be temporary or permanent:

• Permanent: a type of low-dose-rate brachytherapy for which radiation sources are implanted within the patient’s body. The source pellets remain implanted after the radiation has been delivered and have been shown not to cause harm to the patient.
• Temporary: this type of brachytherapy can be either a low-dose-rate or a high-dose-rate treatment. The tubes or other carriers that are used to deliver the radiation and the sources are removed after treatment.

Brachytherapy can be used alone or added to external-beam radiation therapy to increase the amount of radiation to a tumor while still sparing the normal tissue surrounding it.

Systemic radiation therapy

If your doctor were to choose this method you will either swallow or receive an injection of a radioactive substance, such as radioactive iodine.

Radioactive iodine is generally used to treat some types of thyroid cancer because thyroid cells naturally take up radioactive iodine.

For other types of cancers, a monoclonal antibody or special targeted molecule is used that has the radioactive substance attached that is then concentrated by the antibody’s or compound’s ability to concentrate selectively at the tumor tissue. The two travel together through the blood, finding and destroying tumor cells.

Through clinical trials, radiation oncologists are testing other systemic radiation therapy drugs.

When will a patient get radiation therapy?

You will receive radiation before, during, or after surgery. Sometimes patients are given radiation therapy by itself, without other treatments. Some patients receive radiation and chemotherapy at the same time. Determining when you receive radiation therapy depends on the type of cancer you have and the goal of your treatment.

• Pre-operative or neoadjuvant radiation is given before surgery.
• Intraoperative radiation therapy (IORT) is given during surgery and is often delivered as brachytherapy.
• Post-operative or adjuvant radiation therapy is given after surgery.

There are side effects that can come from radiation therapy being given after some types of complicated surgery, so radiation oncologists may elect to have radiation given before surgery in those cases.

Chemoradiation or radiochemotherapy is the combination of chemotherapy and radiation therapy. This combination has the potential to kill more cancer cells for certain types of cancer and also has the potential to cause more side effects.

Possible side effects from radiation therapy

There are both acute (immediate) and chronic (late or ongoing) side effects that come with radiation therapy. Acute side effects arise during treatment and chronic side effects can occur months or sometimes years after treatment is ended. The side effects that develop depend on many factors, such as

• Area of the body being treated
• The dose that is given per day
• The total dose that is given
• The patient’s general medical condition
• Other treatments that are given at the same time

Damage to rapidly dividing normal cells in the area that is being treated is what causes acute radiation side effects. Examples of these kinds of side effects include:

• Skin irritation or damage at regions exposed to the radiation beams
• Damage to the salivary glands or hair loss when the head and neck area is treated
• Urinary problems when the lower abdomen is treated

Acute side effects usually disappear after your treatment ends. But some, like salivary gland damage, have long-term consequences.

A very general side effect of radiation therapy that every patient experiences is fatigue. When areas within the abdomen are treated, nausea with or without vomiting is a common side effect. There are medications your doctor may prescribe to prevent or treat your nausea and vomiting during your treatment.

Late side effects are possible and can include:

• Damage to the bowels
• Memory loss
• Fibrosis
• Infertility

Talk with your radiation oncologist about the likelihood of these late side effects based on your condition and type of therapy.

It is rare, but a second cancer can be caused by radiation exposure. The second cancers that develop depend on the part of the body that was treated with radiation. For example,  if a young girl’s chest were treated for Hodgkin lymphoma she would have a greater risk of developing breast cancer as an adult. Most commonly, the risk of a second cancer is the highest in those who were treated for cancer as children or adolescents.

Chemotherapy drugs, genetic risk factors, and lifestyle factors can also play a role in whether or not you experience late side effects. Before your radiation oncologist will suggest radiation therapy as a part of your treatment, they attentively evaluate the known side effects and risks against the possible benefits for you. The benefits may include:

• Relief of symptoms
• Shrinking a tumor
• Potential cure

Radiation oncologists use their own clinical experience and access to the results of hundreds of clinical trials to decide on the best course of treatment for you and your condition.