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How Radiation Therapy Works for Cancer Treatment
Radiation Oncology

How Radiation Therapy Works for Cancer Treatment

| 08 June 2026

 

Cancer treatment has evolved significantly over the decades, and today, patients have access to highly advanced, targeted therapies that were unimaginable just a generation ago. Among the most powerful tools in modern oncology is radiation therapy, a treatment that uses precisely directed high-energy rays or particles to destroy cancer cells and shrink tumours. At Jaslok Hospital, our dedicated oncology team combines clinical expertise with cutting-edge technology to deliver radiation therapy that is both effective and as comfortable as possible for the patient.

If you or a loved one has been diagnosed with cancer and radiation therapy has been recommended, understanding how this treatment works can help ease anxiety and support informed decision-making.

 

What Is Radiation Therapy?

Radiation therapy, also called radiotherapy, is a medical treatment that uses high doses of radiation to kill cancer cells or slow their growth. Radiation works by damaging the DNA inside cancer cells, making it impossible for them to divide and multiply. While healthy cells in the surrounding area may also be affected, they have a much stronger ability to repair themselves compared to cancer cells.

Radiation therapy is used to:

  • Cure early-stage cancers by completely eliminating tumour cells
  • Control cancer growth when a cure may not be possible
  • Reduce symptoms (palliative care), such as relieving pain caused by a tumour pressing on nerves or bones
  • Shrink tumours before surgery (neoadjuvant therapy) to make them easier to remove
  • Destroy remaining cancer cells after surgery (adjuvant therapy) to prevent recurrence

Radiation therapy may be used alone or in combination with surgery, chemotherapy, immunotherapy, or hormone therapy, depending on the type and stage of cancer.

 

Types of Radiation Therapy

There is no single form of radiation therapy. Oncologists at Jaslok Hospital evaluate each patient's diagnosis, tumour location, size, and overall health to determine which type is most appropriate.

 

1. External Beam Radiation Therapy (EBRT)

This is the most common form of radiation therapy. A machine called a linear accelerator (LINAC) directs beams of high-energy X-rays, protons, or electrons at the tumour from outside the body. The machine rotates around the patient, targeting the tumour from multiple angles while minimising exposure to surrounding healthy tissue.

Subtypes of EBRT include:

  • 3D Conformal Radiation Therapy (3D-CRT): Uses 3D imaging to shape radiation beams to match the contours of the tumour.
  • Intensity-Modulated Radiation Therapy (IMRT): A more advanced form of 3D-CRT that varies the intensity of the radiation beam, delivering higher doses to the tumour while sparing adjacent organs.
  • Stereotactic Radiosurgery (SRS) and Stereotactic Body Radiation Therapy (SBRT): Deliver very high, precise doses of radiation to small tumours in a limited number of sessions. Often used for brain tumours, lung cancer, and spine tumours.
  • Image-Guided Radiation Therapy (IGRT): Uses real-time imaging during treatment to account for any movement (such as breathing) and ensure maximum precision.
  • Volumetric Modulated Arc Therapy (VMAT): Delivers radiation continuously as the machine rotates 360° around the patient, making treatment faster and highly precise.

 

2. Internal Radiation Therapy (Brachytherapy)

In brachytherapy, radioactive material is placed directly inside or very close to the tumour. This can be done temporarily (for a few minutes to a few days) or permanently, depending on the type of cancer. Brachytherapy is commonly used for cervical cancer, prostate cancer, breast cancer, and head and neck cancers. Because the radiation source is so close to the tumour, very high doses can be delivered with minimal impact on surrounding healthy tissue.

 

3. Systemic Radiation Therapy

In this approach, radioactive substances are swallowed or injected into the bloodstream. The most well-known example is radioactive iodine (RAI), used to treat thyroid cancer. Because thyroid cells naturally absorb iodine, the radioactive form concentrates in thyroid tissue and destroys cancer cells throughout the body.

 

How Is Radiation Therapy Planned?

One of the most critical phases of radiation treatment is the planning process, known as simulation. This step ensures that radiation is delivered accurately to the right area every single time.

 

Step 1: Imaging and Mapping

Before treatment begins, the patient undergoes CT scans, MRI scans, or PET scans. These detailed images allow the oncology team to precisely locate the tumour, understand its size and shape, and identify nearby critical structures such as the spinal cord, lungs, heart, or kidneys that must be protected.

 

Step 2: Treatment Planning

Using specialised software, a radiation oncologist and medical physicist work together to design the treatment plan. This involves determining the optimal beam angles, the total dose of radiation required, and the number of sessions (called fractions) over which that dose will be delivered. The goal is to maximise the dose to the tumour while minimising it to healthy tissue.

 

Step 3: Simulation and Positioning

The patient lies in the same position they will be in during treatment. Immobilisation devices, such as a custom-fitted mesh mask for head and neck cancers, may be created to ensure the patient stays perfectly still during every session. Small marks or tattoos may be placed on the skin as reference points for alignment.

 

Step 4: Verification

Before the first treatment session, the plan is verified using test images and often a dry run to ensure everything is precisely calibrated.

 

What to Expect During a Radiation Therapy Session

Many patients feel anxious before their first radiation session, but the procedure itself is painless and non-invasive.

  • You will be positioned on the treatment table in the same way as during simulation.
  • The radiation therapist will align the machine to the reference marks on your skin.
  • The machine will deliver radiation for a few minutes. During this time, you must remain as still as possible.
  • The therapist monitors you from an adjacent room via cameras and an intercom.
  • The entire session typically lasts 15 to 30 minutes, though the actual radiation delivery is only a few minutes.

Most patients receive radiation therapy five days a week for several weeks, depending on the treatment plan. The total number of sessions is carefully calculated to deliver the right cumulative dose.

 

Side Effects of Radiation Therapy

Radiation therapy is a localised treatment, which means side effects are generally limited to the area being treated. Side effects vary depending on the location, dose, and duration of treatment.

Common side effects include:

  • Fatigue, the most frequently reported side effect
  • Skin changes in the treatment area, such as redness, dryness, or peeling
  • Hair loss in the treated region (not necessarily the whole body)
  • Nausea (if the abdomen or brain is being treated)
  • Difficulty swallowing (if the throat area is being treated)
  • Urinary changes (if the pelvis or prostate is being treated)

Most side effects are temporary and resolve after treatment ends. The experienced oncology team at Jaslok Hospital closely monitors patients throughout treatment and provides supportive care to manage side effects effectively.

 

Who Needs Radiation Therapy?

Radiation therapy is used to treat a wide range of cancers, including:

  • Breast cancer
  • Prostate cancer
  • Lung cancer
  • Brain tumours
  • Head and neck cancers
  • Colorectal cancer
  • Cervical and uterine cancers
  • Skin cancer
  • Lymphoma

It is also increasingly used alongside immunotherapy and targeted therapies as part of multimodal treatment strategies. Your oncologist will determine whether radiation therapy is right for you based on the cancer type, stage, location, and your overall health.

 

Related: Foods That Help Reduce Radiation Side Effects

 

Radiation Therapy at Jaslok Hospital

At Jaslok Hospital, we understand that a cancer diagnosis changes lives. Our Department of Radiation Oncology is equipped with state-of-the-art technology and staffed by a multidisciplinary team of radiation oncologists, medical physicists, radiation therapists, and nurses, all working together to deliver the safest, most effective treatment possible.

We are committed to treating every patient with compassion, precision, and the very latest advances in radiation oncology. From the first consultation through to follow-up care, you are supported every step of the way.

If you have questions about radiation therapy or would like to consult with our oncology team, contact Jaslok Hospital today to schedule an appointment.